__asan_register_elf_globals: properly check the "no instrumented global variable" case

[MaskRay]

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.

[llvmbot]

@llvm/pr-subscribers-compiler-rt-sanitizer

Author: Fangrui Song (MaskRay)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/96529.diff

1 Files Affected:

• (modified) compiler-rt/lib/asan/asan_globals.cpp (+4-4)

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;

[hctim]

LGTM