[compiler-rt] __asan_register_elf_globals: properly check the "no instrumented global variable" case (PR #96529)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay created https://github.com/llvm/llvm-project/pull/96529

On ELF platforms, the instrumentation registers global variables using
`__asan_register_elf_globals` for the default `UseGlobalsGC` case. If
all instrumented global variables in a module are discarded by linker
GC, we will have `start == stop`.

Normally `start == 0`, but `start != 0` is possible with a linker script
retaining `asan_globals`. The called `__asan_register_globals` would
access out-of-bounds `globals[n-1]`, though there is likely no runtime
failure.


>From 0c579172a067dd5004a0fdf2f1e0d39cdc8be640 Mon Sep 17 00:00:00 2001
From: Fangrui Song <i at maskray.me>
Date: Mon, 24 Jun 2024 11:10:17 -0700
Subject: [PATCH] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20initia?=
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Created using spr 1.3.5-bogner
---
 compiler-rt/lib/asan/asan_globals.cpp | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/compiler-rt/lib/asan/asan_globals.cpp b/compiler-rt/lib/asan/asan_globals.cpp
index 6ac64c4b776bb..d413b1ebc9fc0 100644
--- a/compiler-rt/lib/asan/asan_globals.cpp
+++ b/compiler-rt/lib/asan/asan_globals.cpp
@@ -344,8 +344,8 @@ void __asan_unregister_image_globals(uptr *flag) {
 }
 
 void __asan_register_elf_globals(uptr *flag, void *start, void *stop) {
-  if (*flag) return;
-  if (!start) return;
+  if (*flag || start == stop)
+    return;
   CHECK_EQ(0, ((uptr)stop - (uptr)start) % sizeof(__asan_global));
   __asan_global *globals_start = (__asan_global*)start;
   __asan_global *globals_stop = (__asan_global*)stop;
@@ -354,8 +354,8 @@ void __asan_register_elf_globals(uptr *flag, void *start, void *stop) {
 }
 
 void __asan_unregister_elf_globals(uptr *flag, void *start, void *stop) {
-  if (!*flag) return;
-  if (!start) return;
+  if (!*flag || start == stop)
+    return;
   CHECK_EQ(0, ((uptr)stop - (uptr)start) % sizeof(__asan_global));
   __asan_global *globals_start = (__asan_global*)start;
   __asan_global *globals_stop = (__asan_global*)stop;