Avoid O_CLOEXEC to allow building on older Linux (RHEL5)

[plopresti]

Summary

See https://github.com/google/sanitizers/issues/1253.

Small patch to enable compilation on (ancient) Red Hat Enterprise Linux 5.

[kcc]

Vitaly, please review (and land if ok).

We don't call fcntl() anywhere else in the sanitizer run-time.
Can anything go wrong if we do?

[vitalybuka]

compiler-rt/lib/sanitizer_common/sanitizer_posix.cpp

355

I guess idea was to avoid leaking fd as results of data race.
We use this function from random places, so race is quite possible.
Probably this the leak is not very critical however I'd prefer to we use O_CLOEXEC where possible.
Could you update the patch to use fcntl only when O_CLOEXEC is not available?

[plopresti]

Use O_CLOEXEC if it exists.

[plopresti]

Tried to keep this clean and to avoid duplicating code on the #if - #else path. Let me know if you see a better way to do it...

[arichardson]

compiler-rt/lib/sanitizer_common/sanitizer_posix.cpp

358

This is causing a build failure for me:

libRTSanitizerCommon.test.nolibc.x86_64.a(sanitizer_posix.cpp.o): In function `__sanitizer::GetNamedMappingFd(char const*, unsigned long, int*)':
/local/scratch/alr48/cheri/upstream-llvm-project/compiler-rt/lib/sanitizer_common/sanitizer_posix.cpp:358: undefined reference to `fcntl'

Should there be an internal_fcntl wrapper? Or maybe a internal_set_cloexec() to avoid having to add a varargs syscall wrapper?

compiler-rt/lib/sanitizer_common/sanitizer_posix.cpp

 #if SANITIZER_LINUX && !SANITIZER_ANDROID && !SANITIZER_GO
 int GetNamedMappingFd(const char *name, uptr size, int *flags) {
   if (!common_flags()->decorate_proc_maps || !name)
     return -1;
   char shmname[200];
   CHECK(internal_strlen(name) < sizeof(shmname) - 10);
   internal_snprintf(shmname, sizeof(shmname), "/dev/shm/%zu [%s]",
                     internal_getpid(), name);
+  int o_cloexec = 0;
+#if defined(O_CLOEXEC)
+  o_cloexec = O_CLOEXEC;
+#endif
   int fd = ReserveStandardFds(
-      internal_open(shmname, O_RDWR | O_CREAT | O_TRUNC | O_CLOEXEC, S_IRWXU));
+      internal_open(shmname, O_RDWR | O_CREAT | O_TRUNC | o_cloexec, S_IRWXU));
   CHECK_GE(fd, 0);
+  if (!o_cloexec) {
+    int res = fcntl(fd, F_SETFD, FD_CLOEXEC);
+    CHECK_EQ(0, res);
+  }
   int res = internal_ftruncate(fd, size);
   CHECK_EQ(0, res);
   res = internal_unlink(shmname);
   CHECK_EQ(0, res);
   *flags &= ~(MAP_ANON | MAP_ANONYMOUS);
   return fd;
 }
 #else