[Support] Fix getMainExecutable on FreeBSD when called via an absolute path

[jrtc27]

Summary

On FreeBSD, absolute paths are passed unmodified in AT_EXECPATH, but
relative paths are resolved to absolute paths, and any symlinks will be
followed in the process. This means that the resource dir calculation
will be wrong if Clang is invoked as an absolute path to a symlink, and
this currently causes clang/test/Driver/rocm-detect.hip to fail on
FreeBSD. Thus, make sure to call realpath on the result, just like is
done on macOS.

Whilst here, clean up the old fallback auxargs loop to use the actual
type for auxargs rather than using lots of hacky casts that rely on
addresses and pointers being the same (which is not the case on CHERI,
and thus Arm's prototype Morello, although for little-endian systems it
happens to work still as the word-sized integer will be padded to a full
pointer). This also makes the code easier to follow, and removes the
confusing double-increment of p.

[jrtc27]

I guess the CHERI argument is a bit silly, because (a) you'll take a bounds fault trying to walk off the end of environ (b) CheriBSD is newer than 1300057 so this will just call elf_aux_info, but it's still nicer to use the right types.

[dim]

Yes, this looks good to me. Thanks for this fix!

llvm/lib/Support/Unix/Path.inc

#if defined(__APPLE__)
   }
 #elif defined(__FreeBSD__)
   // On FreeBSD if the exec path specified in ELF auxiliary vectors is
   // preferred, if available.  /proc/curproc/file and the KERN_PROC_PATHNAME
   // sysctl may not return the desired path if there are multiple hardlinks
   // to the file.
   char exe_path[PATH_MAX];
 #if __FreeBSD_version >= 1300057
-  if (elf_aux_info(AT_EXECPATH, exe_path, sizeof(exe_path)) == 0)
-    return exe_path;
+  if (elf_aux_info(AT_EXECPATH, exe_path, sizeof(exe_path)) == 0) {
+    char link_path[PATH_MAX];
+    if (realpath(exe_path, link_path))
+      return link_path;
+  }
 #else
   // elf_aux_info(AT_EXECPATH, ... is not available in all supported versions,
   // fall back to finding the ELF auxiliary vectors after the process's
   // environment.
   char **p = ::environ;
   while (*p++ != 0)
     ;
   // Iterate through auxiliary vectors for AT_EXECPATH.
-  for (; *(uintptr_t *)p != AT_NULL; p++) {
-    if (*(uintptr_t *)p++ == AT_EXECPATH)
-      return *p;
+  for (Elf_Auxinfo *aux = (Elf_Auxinfo *)p; aux->a_type != AT_NULL; aux++) {
+    if (aux->a_type == AT_EXECPATH) {
+      char link_path[PATH_MAX];
+      if (realpath((char *)aux->a_un.a_ptr, link_path))
+        return link_path;
+    }
   }
 #endif
   // Fall back to argv[0] if auxiliary vectors are not available.
   if (getprogpath(exe_path, argv0) != NULL)
     return exe_path;
 #elif defined(__NetBSD__) || defined(__OpenBSD__) || defined(__minix) ||       \
     defined(__DragonFly__) || defined(__FreeBSD_kernel__) || defined(_AIX)
   const char *curproc = "/proc/curproc/file";