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