Messages in this thread | | | Date | Fri, 10 Sep 2021 19:19:50 -0700 | From | Kees Cook <> | Subject | Re: [PATCH 00/10] raise minimum GCC version to 5.1 |
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On Fri, Sep 10, 2021 at 04:40:37PM -0700, Nick Desaulniers wrote: > commit fad7cd3310db ("nbd: add the check to prevent overflow in > __nbd_ioctl()") > > raised an issue from the fallback helpers added in > > commit f0907827a8a9 ("compiler.h: enable builtin overflow checkers and add fallback code") > > Specifically, the helpers for checking whether the results of a > multiplication overflowed (__unsigned_mul_overflow, > __signed_add_overflow) use the division operator when > !COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW. This is problematic for 64b > operands on 32b hosts. > > Also, because the macro is type agnostic, it is very difficult to write > a similarly type generic macro that dispatches to one of: > * div64_s64 > * div64_u64 > * div_s64 > * div_u64
Given that it's all compile-time type-aware goo, this isn't so bad. The gist[1] you linked off the bug report is pretty close. Needs some bikeshedding. ;)
> Raising the minimum supported versions allows us to remove all of the > fallback helpers for !COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW, instead > dispatching the compiler builtins. > > arm64 has already raised the minimum supported GCC version to 5.1, do > this for all targets now. See the link below for the previous > discussion.
That said, I'd much prefer raising the minimum GCC -- no one appears to actually be building on 4.9 -- there are close to 200 errors (neé warnings) on x86_64 allmodconfig there currently.
-Kees
[1] https://gist.github.com/nickdesaulniers/2479818f4983bbf2d688cebbab435863
-- Kees Cook
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