[Sanitizers] Guard FP_XSTATE_MAGIC1 usage by GLIBC version

[vchuravy]

Summary

Follow-up on https://reviews.llvm.org/D118970 FP_XSTATE_MAGIC1 is only available on glibc 2.27 and upwards

[MaskRay]

FP_XSTATE_MAGIC1 is only available on glibc 2.27 and upwards

Please make some verification before making a claim. The macro is defined in sysdeps/unix/sysv/linux/x86/bits/sigcontext.h in glibc's release/2.16/master branch.

commit 14adcbfc36fbec33594635a8d93834149cb8a8fa
Author: H.J. Lu
Date:   Mon Jun 4 12:40:46 2012

    Update x86_64 bits/sigcontext.h from kernel 3.4.0

I am not sure we want to support pre-2.16 systems.

compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.cpp

#if SANITIZER_GLIBC || SANITIZER_FREEBSD || SANITIZER_NETBSD || SANITIZER_MAC
   unsigned struct_fstab_sz = sizeof(struct fstab);
 #endif  // SANITIZER_GLIBC || SANITIZER_FREEBSD || SANITIZER_NETBSD ||
         // SANITIZER_MAC
 #if !SANITIZER_ANDROID
   unsigned struct_statfs_sz = sizeof(struct statfs);
   unsigned struct_sockaddr_sz = sizeof(struct sockaddr);
 
   unsigned ucontext_t_sz(void *ctx) {
-#    if SANITIZER_GLIBC && SANITIZER_X64
+#    if SANITIZER_GLIBC && SANITIZER_X64 &&  __GLIBC_PREREQ (2, 27)
     // See kernel arch/x86/kernel/fpu/signal.c for details.
     const auto *fpregs = static_cast<ucontext_t *>(ctx)->uc_mcontext.fpregs;
     // The member names differ across header versions, but the actual layout
     // is always the same.  So avoid using members, just use arithmetic.
     const uint32_t *after_xmm =
         reinterpret_cast<const uint32_t *>(fpregs + 1) - 24;
     if (after_xmm[12] == FP_XSTATE_MAGIC1)
       return reinterpret_cast<const char *>(fpregs) + after_xmm[13] -

compiler-rt/test/msan/Linux/signal_mcontext.cpp

 // RUN: %clangxx_msan -fsanitize-memory-track-origins=2 -O1 %s -o %t && %run %t 2>&1 | FileCheck %s
 
 #include <pthread.h>
 #include <sanitizer/msan_interface.h>
 #include <signal.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <ucontext.h>
 #include <unistd.h>
 
 void handler(int sig, siginfo_t *info, void *uctx) {
   __msan_check_mem_is_initialized(uctx, sizeof(ucontext_t));
-#if defined(__GLIBC__) && defined(__x86_64__)
+#if defined(__GLIBC__) && defined(__x86_64__) && __GLIBC_PREREQ(2, 27)
   auto *mctx = &static_cast<ucontext_t *>(uctx)->uc_mcontext;
   if (auto *fpregs = mctx->fpregs) {
     // The member names differ across header versions, but the actual layout
     // is always the same.  So avoid using members, just use arithmetic.
     const uint32_t *after_xmm =
         reinterpret_cast<const uint32_t *>(fpregs + 1) - 24;
     if (after_xmm[12] == FP_XSTATE_MAGIC1) {
       auto *xstate = reinterpret_cast<_xstate *>(mctx->fpregs);