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from the constants-aren't-variables-won't dept.
https://www.phoronix.com/scan.php?page=news_item&px=Linux-Kills-The-VLA:
VLAs [(Varable Length Arrays)] 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.
[...] 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. :)"
[*] KeesCook.
Have you ever used VLAs? Removed them? Why?
(Score: 4, Informative) by pe1rxq on Monday October 29 2018, @03:52PM
Not precise.....
You now ended up with a variable lenght array of integer pointers instead of integers...
Different example:
void foo_with_alloc(size_t array_size)
{
int *my_array = malloc(sizeof(int) * array_size);
/* do foo */
free(my_array);
}
And replace it with:
void foo_with_vla(size_t array_size)
{
int my_array[array_size];
/* do foo */
}
The differences are that malloc uses heap while vla uses the stack.
The big danger with the malloc version is that you forget the free at the end (or any other 'ends' if foo has multiple exit points)
The big danger with the vla version is that your stack can get really big if you don't know what you are doing.