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The Linux Kernel Is Now VLA-Free: A Win For Security, Less Overhead and Better For Clang (phoronix.com)
With the in-development Linux 4.20 kernel, it is now effectively VLA-free. From a report: The variable-length arrays (VLAs) that can be convenient and part of the C99 standard but can have unintended consequences. VLAs allow for array lengths to be determined at run-time rather than compile time. The Linux kernel has long relied upon VLAs in different parts of the kernel -- including within structures -- but going on for months now (and years if counting the kernel Clang'ing efforts) has been to remove the usage of variable-length arrays within the kernel. The problems with them are:
1. Using variable-length arrays can add some minor run-time overhead to the code due to needing to determine the size of the array at run-time.
2. VLAs within structures is not supported by the LLVM Clang compiler and thus an issue for those wanting to build the kernel outside of GCC, Clang only supports the C99-style VLAs.
3. Arguably most importantly is there can be security implications from VLAs around the kernel's stack usage.
1. Using variable-length arrays can add some minor run-time overhead to the code due to needing to determine the size of the array at run-time.
2. VLAs within structures is not supported by the LLVM Clang compiler and thus an issue for those wanting to build the kernel outside of GCC, Clang only supports the C99-style VLAs.
3. Arguably most importantly is there can be security implications from VLAs around the kernel's stack usage.
Yes. Exactly that. It's allocating space for you. It figures out at run-time the length of your array rather than you having to do it by hand at compile-time. I didn't actually know of any security flaws this would lead to, but it stops debuggers from knowing details about calls so it obscured some information from me and pissed me off once.