How Your Compiler Can Compromise Application Security

jfruh writes "Most day-to-day programmers have only a general idea of how compilers transform human-readable code into the machine language that actually powers computers. In an attempt to streamline applications, many compilers actually remove code that it perceives to be undefined or unstable — and, as a research group at MIT has found, in doing so can make applications less secure. The good news is the researchers have developed a model and a static checker for identifying unstable code. Their checker is called STACK, and it currently works for checking C/C++ code. The idea is that it will warn programmers about unstable code in their applications, so they can fix it, rather than have the compiler simply leave it out. They also hope it will encourage compiler writers to rethink how they can optimize code in more secure ways. STACK was run against a number of systems written in C/C++ and it found 160 new bugs in the systems tested, including the Linux kernel (32 bugs found), Mozilla (3), Postgres (9) and Python (5). They also found that, of the 8,575 packages in the Debian Wheezy archive that contained C/C++ code, STACK detected at least one instance of unstable code in 3,471 of them, which, as the researchers write (PDF), 'suggests that unstable code is a widespread problem.'"

Comments

TFA does a poor job of defining what's happening

[istartedi]

If my C code contains *foo=2, the compiler can't just leave that out. If my code contains if (foo) { *foo=2 } else { return EDUFUS; } it can verify that my code is checking for NULL pointers. That's nice; but the questions remain:

What is "unstable code" and how can a compiler leave it out? If the compiler can leave it out, it's unreachable code and/or code that is devoid of semantics. No sane compiler can alter the semantics of your code, at least no compiler I would want to use. I'd rather set -Wall and get a warning.

Inflammatory Subject

[Imagix]

This is complaining because code which is already broken is broken more by the compiler? The programmer is already causing unpredictable things to happen, so even "leaving the code in" still provides no assurances of correct behaviour. An example of how the article is skewed:

Since C/C++ is fairly liberal about allowing undefined behavior

No, it's not. The language forbids undefined behavior. If your program invokes undefined behavior, it is no longer well-formed C or C++.

Re:Inflammatory Subject

[Murdoch5]

You're right, there should never be undefined behavior or clueless development. If things are getting compiled out of the code then you clearly don't know enough about the compiler and language. I love when developers blame things like pointers and memory faults instead of the misuse of these by bad programming.

Re:TFA does a poor job of defining what's happenin

[Anonymous Coward]

An example of "unstable code":

char *a = malloc(sizeof(char));
*a = 5;
char *b = realloc(a, sizeof(char));
*b = 2;
if (a == b && *a != *b)
{
        launchMissiles();
}

A cursory glance at this code suggests missiles will not be launched. With gcc, that's probably true at the moment. With clang, as I understand it, this is not true -missiles will be launched. The reason for this is that the spec says that the first argument of realloc becomes invalid after the call, therefore any use of that pointer has undefined behaviour. Clang takes advantage of this, and defines the behaviour of this to be that *a will not change after that point. Therefore it optimises if (a == b && *a != *b) into if (a == b && 5 != *b). This clearly then passes, and missiles get launched.

The truth here is that your compiler is not compromising application security – the code that relies on undefined behaviours is.

Re:News flash

[war4peace]

I would also like to understand what's the definition of "unstable code".

Do compilers really remove this?

[Todd Knarr]

I haven't heard of any compiler that removes code just because it contains undefined behavior. All compilers I know of leave it in, and whether it misbehaves at run-time or not is... well, undefined. It may work just fine, eg. dereferencing a null pointer may just give you a block of zeroed-out read-only memory and what happens next depends on what you try to do with the dereferenced object. It may immediately crash with a memory access exception. Or it may cause all mounted filesystems to wipe and reformat themselves. But the code's still in the executable. I know compilers remove code that they've determined can't be executed, or where they've determined that the end state doesn't depend on the execution of the code, and that can cause program malfunctions (or sometimes cause programs to fail to malfunction, eg. an infinite loop in the code that didn't go into an infinite loop when the program ran because the compiler'd determined the code had no side-effects so it elided the entire loop).

I'd also note that I don't know any software developers who use the term "unstable code" as a technical term. That's a term used for plain old buggy code that doesn't behave consistently. And compilers are just fine with that kind of code, otherwise I wouldn't spend so much time tracking down and eradicating those bugs.

Re:Do compilers really remove this?

[seebs]

gcc's been doing this for ages. We had a new compiler "break" the ARM kernel once. Turns out that something had a test for whether a pointer was null or not after a dereference of that pointer, and gcc threw out the test because it couldn't possibly apply.

Re:TFA does a poor job of defining what's happenin

[Nanoda]

What is "unstable code" and how can a compiler leave it out?

The article is actually using that as an abbreviation for what they're calling "optimization-unstable code", or code that is included at some specified compiler optimization levels, but discarded at higher levels. Basically they think it's unstable due to being included or not randomly, not because the code itself necessarily results in random behaviour.

Re:Null pointer detection at compile time

[Zero__Kelvin]

"For example, if a pointer is passed to a function, is the function allowed to dereference it without first checking it for NULL?"

Of course it is, and it is supposed to be able to do so. If you were an embedded systems programmer you would know that, and also know why. Next you'll be complaining that languages allow infinite loops (again, a very useful thing to be able to do). C doesn't protect the programmer from himself, and that's by design. Compilers have switches for a reason. If they don't know how it is being built or what the purpose of the code is then they can't possibly determine with another program if the code is "unstable".

Re:x86 memory model is to blame?

[Myria]

Do you think most-all exploits are down to the defective x86 segmented memory architecture.

I think those who coded for the SNES or Apple IIGS in C would disagree with blaming the x86 exclusively =)

Re:News flash

[Mitchell314]

Code with a finite half-life. Sometimes radiates when it decays. The byproducts tend to be hazardous to health, and most cause symptoms such as headaches, tremors, Carpal Tunnel Syndrome, and Acute Induced Tourette Syndrome. Handle with care. The Daily WTF has an emergency hotline if you or somebody you know has been exposed to unsafe levels of unstable code.

-Wall

[Spazmania]

If I set -Wall and the compiler fails to warn me that it optimized out a piece of my code then the compiler is wrong. Period. Full stop.

I don't care what "unstable" justification its authors gleaned from the standard, don't mess with my code without telling me you did so.

Re:-Wall

[Anonymous Coward]

If the compiler finds two constants it can combine then I've usually made a mistake in my code...

Or it inlined a function for you. Or you indexed at a constant index (perhaps 0) into a global array. Or any number of other things that can arise naturally and implicitly.

The compiler has a setting where it doesn't "mess with your code" -- it's called -O0.

Re:-Wall

[Spazmania]

If I've set -Wall, I want a warning about "*a=1 is useless code." If the compiler optimizes it away without that warning, I'm going to cry about it sooner or later because there's a bug in my code. If I had meant *a=(*b)+1 I would have written it that way.

Re:-Wall

[lgw]

If a compiler finds itself able to remove my if statement then either it's wrong or far more likely I made a mistake.

You do realize modern optimized object code lacks any straightforward relationship to the source? It can be quite a puzzle sometimes when debugging through the binary. The way instruction pipelining works makes good object code look quite odd sometimes. The instructions corresponding to one line of source might be scattered and mixed with the object from the next 20 lines, depending on what different parts of the CPU are going to be busy doing, and when the result will be needed.

Why would you want your object to map directly to your source, unless you specifically want debuggable object instead of optimized object?

For that matter, do you ever write cross-platform code with #ifdefs? Sometimes blocks can be removed at compile time on one platform and not another, and sometimes it's not obvious that's going to be the case (especially when those #ifdefs are in library code, so that "function call" is inline on one platform but not another).

Re:-Wall

[Your.Master]

The specific case where you literally wrote exactly that snippet is warnable and is obviously incorrect, and I agree that case could be a warning, but that doesn't lead to your general conclusion at all since it's just a trivial case.

That null-check could be inlined code, or code in a macro, both of which can also appear in contexts which truly need the null check. In neither case was the if (!tun) in the original source code, so first off it's hard to even emit a sensible warning, and secondly there's no good reason to believe that case is wrong. You're just using a general purpose function that validates its inputs.

That effectively means you either have function inlining (or the good parts of link-time-code-generation), or you cannot use any functions that do input validation.

Re:-Wall

[lgw]

OK, sounds like a feature request that some compiler vendor might take you up on. But eliminating the conditional is so common that most people wouldn't want that spew. Would you also want a warning that "x *= 8" was optimized into a shift left instruction on one platform, and three add instruction on another? That "(x > 30)" was optimized into a rotate instruction some platform? Heck, half your lines of code won't have 1-for-1 mappings between C operators and opcodes in the object - why the focus on conditionals?

Really small EXE mystery solved

[Tablizer]

many compilers actually remove code that it perceives to be undefined or unstable

No wonder my app came out with 0 bytes.

PC Lint anyone?

[ArcadeNut]

Back in the day when I was doing C++ work, I used a product called PC Lint (http://www.gimpel.com/html/pcl.htm) that did basically the same thing STACK does. Static Analysis of code to find errors such as referencing NULL pointers, buffer over flows, etc... Maybe they should teach History at MIT first...

Re:PC Lint anyone?

[EvanED]

Don't worry, the authors know what they're doing.

Just because PC Lint could find a small number of potential bugs doesn't mean it's a solved problem by any means. Program analysis is still pretty crappy in general, and they made another improvement, just like tons of people before them, PC Lint before them, and tons of people before PC Lint.

Re:News flash

[Cryacin]

So that's why you have to restart your computer. Gets rid of dangerous radiation from weapons grade baloneyum decay.

IBM had a tool to do this for a long time already

[PolygamousRanchKid]

It's a pretty cool critter, but I don't know if they actually sell it as a product. It might be something that they only use internally:

http://www.research.ibm.com/da/beam.html

http://www.research.ibm.com/da/publications/beam_data_flow.pdf

Yes compilers really do this

[Anonymous Coward]

Yes it leads to real bugs - Brad Spengler uncovered one of these issues in the Linux kernel in 2009 and it led to the kernel using the -fno-delete-null-pointer-checks gcc flag to disable the spec correct "optimisation".

Re:TFA does a poor job of defining what's happenin

[dgatwood]

Another, more common example of code optimizations causing security problems is this pattern:

int a = [some value obtained externally];
int b = a + 2;
if (b < a) {
// integer overflow occurred ...
}

The C spec says that signed integer overflow is undefined. If a compiler does no optimization, this works. However, it is technically legal for the compiler to rightfully conclude that two more than any number is always larger than that number, and optimize out the entire "if" statement and everything inside it.

For proper safety, you must write this as:

int a = [some value obtained externally];
if (INT_MAX - a < 2) {
// integer overflow will occur ...
}
int b = a + 2;

Re:TFA does a poor job of defining what's happenin

[Cryacin]

YOU SUNK MY BATTLESHIP!

Re:Need new compiler features

[MRe_nl]

"But in our enthusiasm, we could not resist a radical overhaul of the system, in which all of its major weaknesses have been exposed, analyzed, and replaced with new weaknesses".

Bruce Leverett, Register Allocation in Optimizing Compilers

Re:TFA does a poor job of defining what's happenin

[Spikeles]

The TFA links to the actual paper. Maybe you should read that.

Towards Optimization-Safe Systems:Analyzing the Impact of Undefined Behavior

struct tun_struct *tun = ...;
struct sock *sk = tun->sk;
if (!tun)
return POLLERR; /* write to address based on tun */

For example, when gcc first sees the dereference tun->sk, it concludes that the pointer tun must be non-null, because the C standard states that dereferencing a null pointer is undefined [24:6.5.3]. Since tun is non-null, gcc further determines that the null pointer check is unnecessary and eliminates the check, making a privilege escalation exploit possible that would not otherwise be.

Re:TFA does a poor job of defining what's happenin

[complete loony]

"What every C programmer should know about undefined behaviour" (part 3, see links for first 2 parts).

For example, overflows of unsigned values is undefined behaviour in the C standard. Compilers can make decisions like using an instruction that traps on overflow if it would execute faster, or if that is the only operator available. Since overflowing might trap, and thus cause undefined behaviour, the compiler may assume that the programmer didn't intend for that to ever happen. Therefore this test will always evaluate to true, this code block is dead and can be eliminated.

This is why there are a number of compilation optimisations that gcc can perform, but which are disabled when building the linux kernel. With those optimisations, almost every memory address overflow test would be eliminated.

The paper gives examples

[AdamHaun]

The article doesn't summarize this very well, but the paper (second link) provides a couple examples. First up:

char *buf = ...;
char *buf_end = ...;
unsigned int len = ...;
if (buf + len >= buf_end)
  return; /* len too large */

if (buf + len < buf)
  return; /* overflow, buf+len wrapped around */ /* write to buf[0..len-1] */

To understand unstable code, consider the pointer overflow check buf + len < buf shown [above], where buf is a pointer and len is a positive integer. The programmer's intention is to catch the case when len is so large that buf + len wraps around and bypasses the first check ... We have found similar checks in a number of systems, including the Chromium browser, the Linux kernel, and the Python interpreter.

While this check appears to work on a flat address space, it fails on a segmented architecture. Therefore, the C standard states that an overflowed pointer is undefined, which allows gcc to simply assume that no pointer overflow ever occurs on any architecture. Under this assumption, buf + len must be larger than buf, and thus the "overflow" check always evaluates to false. Consequently, gcc removes the check, paving the way for an attack to the system.

They then give another example, this time from the Linux kernel:

struct tun_struct *tun = ...;
struct sock *sk = tun->sk;
if (!tun)
  return POLLERR; /* write to address based on tun */

In addition to introducing new vulnerabilities, unstable code can amplify existing weakness in the system. [The above] shows a mild defect in the Linux kernel, where the programmer incorrectly placed the dereference tun->sk before the null pointer check !tun. Normally, the kernel forbids access to page zero; a null tun pointing to page zero causes a kernel oops at tun->sk and terminates the current process. Even if page zero is made accessible (e.g. via mmap or some other exploits), the check !tun would catch a null tun and prevent any further exploits. In either case, an adversary should not be able to go beyond the null pointer check.

Unfortunately, unstable code can turn this simple bug into an exploitable vulnerability. For example, when gcc first sees the dereference tun->sk, it concludes that the pointer tun must be non-null, because the C standard states that dereferencing a null pointer is undefined. Since tun is non-null, gcc further determines that the null pointer check is unnecessary and eliminates the check, making a privilege escalation exploit possible that would not otherwise be.

The basic issue here is that optimizers are making aggressive inferences from the code based on the assumption of standards-compliance. Programmers, meanwhile, are writing code that sometimes violates the C standard, particularly in corner cases. Many of these seem to be attempts at machine-specific optimization, such as this "clever" trick from Postgres for checking whether an integer is the most negative number possible:

int64_t arg1 = ...;
if (arg1 != 0 && ((-arg1 < 0) == (arg1 < 0)))
  ereport(ERROR, ...);

The remainder of the paper goes into the gory Comp Sci details and discusses their model for detecting unstable code, which they implemented in LLVM. Of particular interest is the table on page 9, which lists the number of unstable code fragments found in a variety of software packages, including exciting ones like Kerberos.

Re:Fix the C standard to not be so silly

[seebs]

Pretty sure the embedded systems guys wouldn't be super supportive of this, and they're by far the largest market for C.

And I just don't think these are big sources of trouble most of the time. If people would just go read Spencer's 10 Commandments for C Programmers, this would be pretty much solved.

Re:TFA does a poor job of defining what's happenin

[EvanED]

The first mistake was using signed integers. unsigned integers always have well-defined overflow (modulo semantics), which means it's easier to construct safe conditionals

Not in C and C++ they don't. The compiler is allowed to perform that optimization with either signed or unsigned integers.

Meanwhile, THEIR code is sketchy

[belphegore]

Checked out their git repo and did a build. They have a couple sketchy-looking warnings in their own code. A reference to an undefined variable; storing a 35-bit value in a 32-bit variable...

lglib.c:6896:7: warning: variable 'res' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
lglib.c:6967:10: note: uninitialized use occurs here
plingeling.c:456:17: warning: signed shift result (0x300000000) requires 35 bits to represent, but 'int' only has 32 bits [-Wshift-overflow]

Re:Meanwhile, THEIR code is sketchy

[belphegore]

I probably got in early enough to grab the code before they go slashdotted. Looks like it's also on github, here:

https://github.com/xiw/stack

Know your C

[gmuslera]

Somewhat this made me remember that slideshow on Deep C. I only know that i don't know nothing of C, after reading it.

Re:TFA does a poor job of defining what's happenin

[lgw]

No, the compiler is allowed to to anything it damn well pleases wherever the standard calls behaviou "undefined". One of my favorite quotes ever from a standards discussion:

When the compiler encounters [a given undefined construct] it is legal for it to make demons fly out of your nose

Nasal demons can cause code instability.

It's time

[countach]

It really should be time that 99.9% of the code written ought not to be in languages that have undefined behaviour. It's time we all use languages which are fully defined.

Having said that, if something in code is undefined, and the compiler knows it, then it should generate an error. Very easily solved. If this STACK program is so clever, it should be in the compiler, and it should be an error to do something undefined.

OK, before somebody else points it out...

[istartedi]

My statement is contradictory. I recommended a course of action for undefined behavior, while maintaining that Clang is wrong for documenting a course of action for undefined behavior.

My understanding of "undefined behavior" in the C spec is that it means "anything can happen and the programmer shouldn't rely on what the compiler currently does". Of course, in the real world *something* must happen. If a 3rd party documents what that something is, the compiler is still compliant. It's the programmer's fault for relying on it.

OTOH, if the behavior was "implementation defined" then the compiler authors can define it. If they change their definition from one rev to another without documenting the change, then it's the compiler author's fault for not documenting it.

In other words:

undefined -- programmer's fault for relying on it.
implemenation defined -- compiler's fault for not documenting it.

Re:OK, before somebody else points it out...

[DeKO]

There are actually 3 categories:

• Implementation Defined: the implementation (compiler, standard library, execution environment) has to document what happens. Code relying on this is not portable.
• Unspecified: the implementation can choose to do what makes sense, and not tell you. Even reverse-engineering and relying on what you found out, is unreliable. The actual address returned by malloc is unspecified; is it aligned? Does it always grow in value if nothing was free-ed? You shouldn't even care about this detail, so the standard leaves it unspecified.
• Undefined Behaviour: you wrote something that doesn't make sense, if you get lucky the compiler/standard library/operating system will react in a sensible way, but the standard says it's not the implementation's fault you get something wrong as a result. Things like reading variables before initializing them.

Diagnosing UB can be too demanding from the implementation, so the standard doesn't even require it. How would you diagnose incorrect usage of realloc? Add run-time checks? Write a special rule in the compiler so it knows about realloc? Extend the language with metadata? What if realloc is hidden behind a user-defined function? At some point you have to stop, otherwise you could even solve the halting problem.

Re:TFA does a poor job of defining what's happenin

[Spazmania]

If I tell the compiler to give me warnings, it detects a code whose behavior is undefined in the standard, but then fails to issue a warning then the compiler is broken. If it goes on to make a fancy assumption about the undefined behavior instead of letting it fall through to runtime as written then it's doubly broken.

Re:News flash

[foobar bazbot]

I would also like to understand what's the definition of "unstable code".

Unstable code is code such that, when you make an arbitrarily small change, you end up rewriting the entire thing.

Stable code, by contrast, is code such that when you make an arbitrarily small change, the code ends up being restored to its original state, or perhaps engaging in a bounded oscillation, where you and another coder keep changing it back and forth with every release.

Re:TFA does a poor job of defining what's happenin

[lgw]

, fucked up computer languages allow "undefined code", ie. C / C++.

Every language has some undefined behavior (and there are libraries with undefined behavior in every language), except maybe ADA.

Java leaves a wide area undefined when it comes to multi-threaded code.

Python has the same, plus it inherits some undefined behaviors from C.

C/C++ leaves a wide are undefined to support oddball system architectures. For example, if you have some memory that only can store floating point numbers, and some general-purpose memory, the address ranges might overlap - that's why pointer subtraction is undefined unless within an array. In practice most programmers can treat all memory as one contiguous byte array, but on special-purpose hardware you can still use C. Most of C's undefined behavior comes from the much wider variety of system architectures when C was young, but can still be useful for embedded systems.

Re:Null pointer detection at compile time

[EvanED]

Of course it is, and it is supposed to be able to do so.

Actually no, you're not, or you're programming in Some-C-Like-Language and not C. In C, dereferencing a NULL pointer is always undefined behavior, and compilers are allowed (though presumably very unlikely to on embededd platforms) to make transformations based on that assumption, such as the following:

void f(int * p) {
  int x = *p;
  if (p == NULL) {
    g();
  }
}

C compilers are allowed to optimize away the null check and subsequent call to g(), and if you rely them not, you're relying on behavior that's not guaranteed by the standard.

Re:News flash

[ShanghaiBill]

Didn't RTFA because this is /., but I'd guess that it's code that works now but is fragile under a change of compiler, compiler version, optimization level, or platform.

Yes, you didn't RTFA, because your definition actually makes sense. TFA defines "unstable code" as code with undefined behavior. TFA also claims that many compilers simply DELETE such code. I have never seen a compiler that does that, and I seriously doubt if is really common. Does anyone know of a single compiler that does this? Or is TFA just completely full of crap (as I strongly suspect)?

Re:TFA does a poor job of defining what's happenin

[CODiNE]

That reminds me of this gem:Overflow in sorting algorithms

That little bug just sat around for a few decades before anyone noticed it.

Quick summary: (low + high) / 2
May have an overflow which is undefined behavior. Really every time we add ints it's possible. Just usually our values don't pass the MAX.

Re:News flash

[EvanED]

Yes, you didn't RTFA, because your definition actually makes sense. TFA defines "unstable code" as code with undefined behavior.

...and undefined behavior is exactly what causes the things I listed.

TFA also claims that many compilers simply DELETE such code. I have never seen a compiler that does that, and I seriously doubt if is really common.

You probably haven't used any desktop compilers.

Just a sampling:

• During MS's security push a decade ago, they discovered that the compiler was optimizing away the memset in code such as memset(password, '\0', len); free(password); that was limiting the lifetime of sensitive information, because the assignment to password in the memset was a dead assignment -- it was never read from (not actually undefined behavior, but it is an example of the compiler deleting unused code that was actually there for a purpose)
• I linked part 3 of this series to you in another response, but the first example in here discusses such an optimization that GCC did which removed security checks in the Linux kernel (see also this series -- look down at "A Fun Case Analysis")
• GCC has long turned on -fno-strict-aliasing because optimizations based on the strict aliasing assumption break the kernel (more precisely: code that violates the standard's strict aliasing rules was being "mis-"optimized), though I don't know if it led to security implications

Re:TFA does a poor job of defining what's happenin

[Old Wolf]

>a == b is not a use of the argument that has been invalidated

Yes it is. Evaluating the expression "a" causes undefined behaviour if "a" is
indeterminate. "a" is considered to no longer have a value, any attempt to
refer to its value causes UB. (It has the same status as a variable that has
been defined but not initialized, i.e. "int a;"

The only thing that can be done with "a" thereafter is to assign a new value to it
(or take its address, or do "sizeof a" .. can't think of any other exceptions)

Re:TFA does a poor job of defining what's happenin

[Old Wolf]

>The dereference is undefined, and therefore

Stop right here. Once undefined behaviour occurs, "all bets are off" as they say; the remaining code may have any behaviour whatsoever. C works like this on purpose , and it's something I agree with. It means the compiler doesn't have to insert screeds of extra checks , both at compile-time and run-time.

There are plenty of other languages you can use if you want a different language definition :)

These bugs exist even *without* signed integers!

[Myria]

The first mistake was using signed integers.

The problem is C's promotion rules. In C, when promoting integers to the next size up, typically to the minimum of "int", the rule is to use signed integers if the source type fits, even if the source type is unsigned. This can cause code that seems to use unsigned integers everywhere break because C says signed integer overflow is undefined. Take the following code, for example, which I saw on a blog recently:

uint64_t MultiplyWords(uint16_t x, uint16_y)
{
    uint32_t product = x * y;
    return product;
}

MultiplyWords(0xFFFF, 0xFFFF) on GCC for x86-64 was returning 0xFFFFFFFFFFFE0001, and yet this is not a compiler bug. From the promotion rules, uint16_t (unsigned short) gets promoted to int, because unsigned short fits in int completely without loss or overflow. So the multiplication became ((int) 0xFFFF) * ((int) 0xFFFF). That multiplication overflows in a signed sense, an undefined operation. The compiler can do whatever it feels like - including generate code that crashes if it wants.

GCC in this case assumes that overflow cannot happen, so therefore x * y is positive (when it's really not at runtime). This means the uint32_t cast does nothing, so is omitted by the optimizer. Now, the code generator sees an int cast to uint64_t, which means sign extension. The optimizer this time isn't smart enough to know again that it's positive and therefore can ignore sign extension and use "mov eax, ecx" to clear the high 32 bits, so it emits a "cqo" opcode to do the sign extension.

So no, avoiding signed integers does not always save you.

Re:TFA does a poor job of defining what's happenin

[russotto]

Quick summary: (low + high) / 2

I like this one, because it shows a very common weakness in high level languages.

In most machine languages, getting the average of two unsigned numbers up to UINT_MAX is absolutely trivial -- add the two, then shift right including the carry. The average of two signed numbers rounding to zero is a little more difficult (x86 makes it harder than it should be by not setting flags in a convenient manner), but still a few instructions.

In C? Assuming low and high are unsigned
(low >> 1) + (high >> 1) + (low & high & 1). Ick. The answer given in your article is inadequate; it gets you one more bit.

Of course, now we have 64 bit integers and the problem is solved ONCE AND FOR ALL.

But there are algorithms which need the average of 64-bit unsigned numbers too..
ONCE AND FOR ALL

Re:News flash

[gweihir]

That is not "unstable" or "undefined" code. There is already a word for it: dead code. In addition, any programmer worth his/her salt will make sure to define things like that as "volatile", i.e. tell the compiler that they might be accessed at any time from place the complier does not see. Which is exactly the security problem here. Don't blame compilers for programmer incompetence....

Re:News flash

[epee1221]

I have never seen a compiler that does that, and I seriously doubt if is really common.

I'm a bit depressed to find a /.er who's never seen GCC :-P
I once wrote an overflow check wrong -- I tried to write an `if' that would check whether the preceding operation on signed integers had overflowed. Overflow on signed integers is undefined behavior, so once it happens, it is legal for the program to do anything. "Anything" includes updating the variable with the overflowed value and then skipping the condition check, which is what GCC's output code did.

Re:TFA does a poor job of defining what's happenin

[russotto]

Yes, (low + high)/2 is well-defined for unsigned ints even when (low + high) > UINT_MAX. It's not the definition you want, though. The average of (UINT_MAX/2 + 1000) and (UINT_MAX/2 - 500) should not be 250.

Re:These bugs exist even *without* signed integers

[Animats]

The problem is C's promotion rules. In C, when promoting integers to the next size up, typically to the minimum of "int", the rule is to use signed integers if the source type fits, even if the source type is unsigned.

I know. C's handling of integer overflow is "undefined". In Pascal, integer overflow was a detected error. DEC VAX computers could be set to raise a hardware exception on integer overflow, and about thirty years ago, I rebuilt the UNIX command line tools with that checking enabled. Most of them broke.

In the first release of 4.3BSD, TCP would fail to work with non-BSD systems during alternate 4-hour periods. The sequence number arithmetic had been botched due to incorrect casts involving signed and unsigned integers. I found that bug. It wasn't fun.

C's casual attitude towards integer overflow is why today's machines don't have the hardware to interrupt on it. Ada and Java do overflow checks, but the predominance of C sloppyness influenced hardware design too much.

I once wrote a paper, "Type Integer Considered Harmful" on this topic. One of my points was that unsigned arithmetic should not "wrap around" by default. If you want modular arithmetic, you should write something like n = (n +1) % 65536;. The compiler can optimize that into machine instructions that exploit word lengths when the hardware allows, and you'll get the same result on all platforms.

Re:News flash

[maxwell demon]

While what you say is true, I think it's not what they mean. Instead what they mean is compilers taking advantage of undefined behaviour you didn't notice. The compiler is allowed to assume that undefined behaviour never happens, and optimize accordingly. The important point is that this can even affect code before the undefined behaviour would occur. For example, consider the following code, where undefined() is some code that causes undefined behaviour:

if (a>4)
{
  a=4;
  big=true;
  undefined();
}
else
{
  big=false;
}
assert(a<=4);

Now if a>4, the code inevitably runs into undefined behaviour, and therefore it may assume that a is not larger than 4 right from the start. Therefore it is allowed to compile the complete block to simply

big=false;

Note that even the assert doesn't help because the compiler "knows" it cannot trigger anyway, and therefore optimizes it out.

I think it is not hard to imagine how this can lead to security problems.

Another nice example (which I read on the gcc mailing list quite some time ago; not an exact quote though):

bool validate_passwd(char const* user)
{
  int tries = 0;
  char const* given_passwd = ask_password();
  char const* user_passwd = get_password(user);
  while (strcmp(given_password, user_password))
  {
    tries = tries++; /* undefined behaviour! */
    if (tries > 3)
      return false; /* allow only to try three times */
    printf("password not valid. Please try again.\n");
    given_passwd = ask_passwd();
  }
  return true;
}

Now if strcmp returns anything but 0, the code inevitably runs into undefined behaviour, therefore the compiler is allowed to assume that never happens, and therefore is allowed to optimize the code to simply

bool validate_passwd(char const* user)
{
  char const* given_passwd = ask_password();
  char const* user_passwd = get_password(user);
  return true;
}

So there goes your password security.

Re:News flash

[TheRaven64]

I have never seen a compiler that does that, and I seriously doubt if is really common. Does anyone know of a single compiler that does this?

The only compilers I know of that definitely do this are GCC, LLVM, ICC, Open64, ARMCC, and XLC, but others probably do too. Compilers use undefined behaviour to propagate unreachable state and aggressively trim code paths. There's a fun case in ARM's compiler, where you write something like this:

int x[5];
int y;
...
for (int i=0 ; i<10 ; i++)
y += x[i];

The entire loop is optimised away to an infinite loop. Why? Because accesses to array elements after the end of the array are undefined. This means that, when you write x[i] then either i is in the range 0-4 (inclusive), or you are hitting undefined behaviour. Because the compiler can do anything it wants in cases of undefined behaviour, it is free to assume that they never occur. Therefore, it assumes that, at the end of the loop, i is always less than 5. Therefore, i++ is always less than 10, and therefore the loop will never terminate. Therefore, since the body of the loop has no side effects, it can be elided. Therefore, the declarations of x and y are never read from in anything with side effects and so can be elided. Therefore, the entire function becomes a single branch instruction that just jumps back to itself.

If your code relies on undefined behaviour, then it's broken. A compiler is entirely free to do whatever it wants in the cases where the behaviour is undefined. Checking for undefined behaviour statically is very hard, however (consider trying to check for correct use of the restrict keyword - you need to do accurate alias analysis on the entire program) and so compilers won't warn you in all cases. Often, the undefined behaviour is only apparent after inlining, at which point it's difficult to tell what the source of the problem was.

Re:News flash

[Joce640k]

I'm more interested in how Linus is going to respond to a bunch of C++ programmers finding 32 bugs in his kernel.

Re:News flash

[Alioth]

In that vein, I tried:

while(1) {
      bar=bar++;
      if(bar > 3) {
            printf("bar = %d\n", bar);
            break;
      }
}

Under gcc (trying -O0 to -O3 and -Os), this code printed "bar = 4". Compiling the same code with clang resulted in an infinite loop.