On FreeBSD use AT_EXECPATH from ELF auxiliary vectors for getExecutablePath

[emaste]

Summary

/proc/curproc/file and the KERN_PROC_PATHNAME sysctl may not return the desired path if there are multiple hardlinks to the file.

Unfortunately we do not yet have a standard interface to obtain this in FreeBSD, but one will be added shortly. I will post a followup change to move to that based on FreeBSD version leaving this as a fallback, and eventually we can leave only that one.

[emaste]

See https://bugs.freebsd.org/241932

[emaste]

Use elf_aux_info() if we can

llvm/lib/Support/Unix/Path.inc

 
 #include <dirent.h>
 #include <pwd.h>
 
 #ifdef __APPLE__
 #include <mach-o/dyld.h>
 #include <sys/attr.h>
 #include <copyfile.h>
+#elif defined(__FreeBSD__)
+#include <osreldate.h>
+#if __FreeBSD_version >= 1300057
+#include <sys/auxv.h>
+#else
+#include <machine/elf.h>
+extern char **environ;
+#endif
 #elif defined(__DragonFly__)
 #include <sys/mount.h>
 #endif
 
 // Both stdio.h and cstdio are included via different paths and
 // stdcxx's cstdio doesn't include stdio.h, so it doesn't #undef the macros
 // either.
 #undef ferror
#if defined(__APPLE__)
   // binaries with symbols.
   char exe_path[MAXPATHLEN];
   uint32_t size = sizeof(exe_path);
   if (_NSGetExecutablePath(exe_path, &size) == 0) {
     char link_path[MAXPATHLEN];
     if (realpath(exe_path, link_path))
       return link_path;
   }
-#elif defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__) ||   \
-    defined(__minix) || defined(__DragonFly__) ||                              \
-    defined(__FreeBSD_kernel__) || defined(_AIX)
+#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
+  char exe_path[PATH_MAX];
+  if (elf_aux_info(AT_EXECPATH, exe_path, sizeof(exe_path)) == 0)
+    return exe_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 (;;) {
+    switch (*(uintptr_t *)p++) {
+    case AT_EXECPATH:
+      return *p;
+    case AT_NULL:
+      break;
+    }
+    p++;
+  }
+#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";
   char exe_path[PATH_MAX];
-  // /proc is not mounted by default under FreeBSD, but gives more accurate
-  // information than argv[0] when it is.
   if (sys::fs::exists(curproc)) {
     ssize_t len = readlink(curproc, exe_path, sizeof(exe_path));
     if (len > 0) {
       // Null terminate the string for realpath. readlink never null
       // terminates its output.
       len = std::min(len, ssize_t(sizeof(exe_path) - 1));
       exe_path[len] = '\0';
       return exe_path;