Merge bitcoin/bitcoin#29815: crypto: chacha20: always use our fallback timingsafe_bcmp rather than libc's

2d1819455cb4c516f6cdf81c11e869a23dee3e6b crypto: chacha20: always use our fallback timingsafe_bcmp rather than libc's (Cory Fields)

Pull request description:

  Looking at libc sources, apple and openbsd implementations match our naive fallback. Only FreeBSD (and only x86_64) seems to [implement an optimized version](https://github.com/freebsd/freebsd-src/blob/main/lib/libc/amd64/string/timingsafe_bcmp.S).

  It's not worth the hassle of using a platform-specific function for such little gain.

  Additionally, as mentioned below, this is the only case outside of sha2 that requires an autoconf check, and I have upcoming PRs to remove the sha2 ones.

  Apple's [impl is unoptimized](https://opensource.apple.com/source/Libc/Libc-1244.1.7/string/FreeBSD/timingsafe_bcmp.c.auto.html).

  As-is [OpenBSD's impl](https://github.com/openbsd/src/blob/master/lib/libc/string/timingsafe_bcmp.c).

  Relevant IRC conversation with sipa:

  > \<cfields\> sipa: chacha20poly1305.cpp uses libc's timingsafe_bcmp when possible. But looking around at apple/freebsd/openbsd, I don't see any impl that doesn't use the naive implementation that matches our fallback...
  > \<cfields\> is there any reason to belive there's an optimized impl somewhere that we're actually hitting?
  > \<cfields\> asking because after cleaning up sha2, timingsafe_bcmp is the last autoconf check that remains in all of crypto. It'd make life easy if we could just always use our internal one.
  > \<cfields\> *all of crypto/
  > \<sipa\> cfields: let's get rid of the dependency then
  > \<sipa\> it's a trivial function
  > \<sipa\> and if we need it for some platforms, no real reason not to use it on all

  After the above discusstion, I did end up finding the x86_64-optimized FreeBSD impl, but I don't think that's all that significant.

ACKs for top commit:
  sipa:
    utACK 2d1819455cb4c516f6cdf81c11e869a23dee3e6b
  fanquake:
    ACK 2d1819455cb4c516f6cdf81c11e869a23dee3e6b
  TheCharlatan:
    ACK 2d1819455cb4c516f6cdf81c11e869a23dee3e6b
  theStack:
    ACK 2d1819455cb4c516f6cdf81c11e869a23dee3e6b

Tree-SHA512: b9583e19ac2f77c5d572aa5b95bc4b53669d5717e5708babef930644980de7c5d06a9c7decd5c2b559d70b8597328ecfe513375e3d8c3ef523db80012dfe9266

1 files changed, 0 insertions, 2 deletions

diff --git a/configure.ac b/configure.ac
index ba7e0dea82..d8b7159e27 100644
--- a/configure.ac
+++ b/configure.ac
@@ -968,8 +968,6 @@ AC_CHECK_DECLS([setsid])
 
 AC_CHECK_DECLS([pipe2])
 
-AC_CHECK_FUNCS([timingsafe_bcmp])
-
 AC_MSG_CHECKING([for __builtin_clzl])
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[ ]], [[
  (void) __builtin_clzl(0);