compiler-rt: avoid segfaults when re-exec'ing with ASLR

After 930a7c2 ("compiler-rt: re-exec with ASLR disabled when
necessary") and 96fe7c8 ("compiler-rt: support ReExec() on
FreeBSD"), binaries linked against the sanitizer libraries may segfault
due to procctl(2) being intercepted. Instead, the non-intercepted
internal_procctl() should be called.

Similarly, the ReExec() function that re-executes the binary after
turning off ASLR should not call elf_aux_info(3) and realpath(3), since
these will also be intercepted. Instead, loop directly over the elf aux
info vector to find the executable path, and avoid calling realpath(3)
since it is actually unwanted for this use case.

Fixes:		930a7c2, 96fe7c8
MFC after:	3 days

Author: DimitryAndric

contrib/llvm-project/compiler-rt/lib/sanitizer_common/sanitizer_linux.cpp

@@ -2323,7 +2323,7 @@ void CheckASLR() {
                "and binaries compiled with PIE\n"
                "ASLR will be disabled and the program re-executed.\n");
     int aslr_ctl = PROC_ASLR_FORCE_DISABLE;
-    CHECK_NE(procctl(P_PID, 0, PROC_ASLR_CTL, &aslr_ctl), -1);
+    CHECK_NE(internal_procctl(P_PID, 0, PROC_ASLR_CTL, &aslr_ctl), -1);
     ReExec();
   }
 #  elif SANITIZER_PPC64V2

contrib/llvm-project/compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cpp

@@ -56,6 +56,7 @@
 // that, it was never implemented. So just define it to zero.
 #undef MAP_NORESERVE
 #define MAP_NORESERVE 0
+extern const Elf_Auxinfo *__elf_aux_vector;
 #endif
 
 #if SANITIZER_NETBSD
@@ -947,11 +948,11 @@ void ReExec() {
   const char *pathname = "/proc/self/exe";
 
 #if SANITIZER_FREEBSD
-  char exe_path[PATH_MAX];
-  if (elf_aux_info(AT_EXECPATH, exe_path, sizeof(exe_path)) == 0) {
-    char link_path[PATH_MAX];
-    if (realpath(exe_path, link_path))
-      pathname = link_path;
+  for (const auto *aux = __elf_aux_vector; aux->a_type != AT_NULL; aux++) {
+    if (aux->a_type == AT_EXECPATH) {
+      pathname = static_cast<const char *>(aux->a_un.a_ptr);
+      break;
+    }
   }
 #elif SANITIZER_NETBSD
   static const int name[] = {