Bug#1129746: glibc is not built with -fstack-clash-protection (despite it being in dpkg-buildflags)

Hi,

On 2026-03-05 14:25, Jann Horn wrote:
> Package: libc6
> Version: 2.42-13
> 
> My understanding is that dpkg-buildflags defaults include
> -fstack-clash-protection (see
> https://git.dpkg.org/cgit/dpkg/dpkg.git/commit/?id=11efff1bf), but
> somehow Debian's glibc appears to not be built with
> -fstack-clash-protection; there are unprotected large stack
> allocations in functions like ____strtold_l_internal,
> ____strtof128_l_internal, tempnam, tmpfile64, and more:

Good catch. Indeed glibc does not uses build flags from dpkg for 
multiple reasons:
- Some flags set by dpkg-buildflags are not compatible with glibc and 
  cause build failure of ld.so
- On many architectures the glibc source package builds also build 
  biarch packages (e.g. libc6-i386:amd64) which do not necessarily 
  support the same flags as the main build
- To add some complexity the main multiarch build and the biarch builds 
  (e.g. libc6:i386 and libc6-i386:amd64) should be built with consistent 
  flags to avoid issues when the two are mixed at runtime.

> Can you please also build glibc with -fstack-clash-protection?

Yep. I'll think if I can find a way for glibc to partly use 
dpkg-buildflags, otherwise I'll just hardcode that on amd64, just like 
it's already done for arm64.

Regards
Aurelien