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Kees Cook authored
In the effort to remove all VLAs from the kernel[1], it is desirable to build with -Wvla. However, this warning is overly pessimistic, in that it is only happy with stack array sizes that are declared as constant expressions, and not constant values. One case of this is the evaluation of the max() macro which, due to its construction, ends up converting constant expression arguments into a constant value result. All attempts to rewrite this macro with __builtin_constant_p() failed with older compilers (e.g. gcc 4.4)[2]. However, Martin Uecker, constructed[3] a mind-shattering solution that works everywhere. Cthulhu fhtagn! This patch updates the min()/max() macros to evaluate to a constant expression when called on constant expression arguments. This removes several false-positive stack VLA warnings from an x86 allmodconfig build when -Wvla is added: $ diff -u before.txt after.txt | grep ^- -drivers/input/touchscreen/cyttsp4_core.c:871:2: warning: ISO C90 forbids variable length array ‘ids’ [-Wvla] -fs/btrfs/tree-checker.c:344:4: warning: ISO C90 forbids variable length array ‘namebuf’ [-Wvla] -lib/vsprintf.c:747:2: warning: ISO C90 forbids variable length array ‘sym’ [-Wvla] -net/ipv4/proc.c:403:2: warning: ISO C90 forbids variable length array ‘buff’ [-Wvla] -net/ipv6/proc.c:198:2: warning: ISO C90 forbids variable length array ‘buff’ [-Wvla] -net/ipv6/proc.c:218:2: warning: ISO C90 forbids variable length array ‘buff64’ [-Wvla] This also updates two cases where different enums were being compared and explicitly casts them to int (which matches the old side-effect of the single-evaluation code): one in tpm/tpm_tis_core.h, and one in drm/drm_color_mgmt.c. [1] https://lkml.org/lkml/2018/3/7/621 [2] https://lkml.org/lkml/2018/3/10/170 [3] https://lkml.org/lkml/2018/3/20/845 Co-Developed-by:
Linus Torvalds <torvalds@linux-foundation.org> Co-Developed-by:
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