The Linux Kernel Is Now VLA-Free: A Win For Security, Less Overhead & Better For Clang
With the in-development Linux 4.20 kernel, it is now effectively VLA-free... 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:
- 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.
- 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.
- Arguably most importantly is there can be security implications from VLAs around the kernel's stack usage.
Linus Torvalds has also expressed his displeasure in the past over VLA usage with comments like "USING VLA'S IS ACTIVELY STUPID! It generates much more code, and much _slower_ code (and more fragile code), than just using a fixed key size would have done."
Particularly over the past several cycles there has been code eliminating the kernel's usage of VLAs and that has continued so far for this Linux 4.20~5.0 cycle. There had been more than 200 spots in the kernel relying upon VLAs but now as of the latest Linux Git code it should be basically over.
Kees Cook sent out the pull request today for VLA removal that now enables the "-Wvla" compiler flag to emit a warning should any variable-length array be found within the kernel's C code. That will help prevent new code from inadvertently using VLAs and also spot any lingering kernel code still relying upon this behavior.
Kees Cook wrote that there still might be a few places where VLAs could be found, "there may be a couple more VLAs hiding in hard-to-find randconfigs, but nothing big has shaken out in the last month or so in linux-next. We should be basically VLA-free now! Wheee. :)"