-fsanitize=memtag-globals doesn't tag global variables in the presence of __attribute__ section

[tarcisiofischer]

The following program:

__attribute__((section("foo_section"))) int foo = 42;
int main() { return foo; }

compiled with -fsanitize=memtag-globals yields

$ llvm-readelf --memtag a.out
Memtag Dynamic Entries:                 
    AARCH64_MEMTAG_MODE: Synchronous (0)
    AARCH64_MEMTAG_HEAP: Disabled (0)   
    AARCH64_MEMTAG_STACK: Disabled (0)  
Memtag Android Note:                    
    Tagging Mode: SYNC                  
    Heap: Disabled                      
    Stack: Disabled                     

I'd expect the same result as:

int foo = 42;
int main() { return foo; }

$ llvm-readelf --memtag a.out
Memtag Dynamic Entries:
    AARCH64_MEMTAG_MODE: Synchronous (0)
    AARCH64_MEMTAG_HEAP: Disabled (0)
    AARCH64_MEMTAG_STACK: Disabled (0)
    AARCH64_MEMTAG_GLOBALS: 0x338
    AARCH64_MEMTAG_GLOBALSSZ: 3
Memtag Android Note:
    Tagging Mode: SYNC
    Heap: Disabled
    Stack: Disabled
Memtag Global Descriptors:
    0x89b0: 0x10

The reason seems to be the following code: https://github.com/llvm/llvm-project/blob/main/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp#L2608-L2627

I tried removing this code, and I can confirm the memory in the section is tagged, and my test executable runs just fine 👍 But I didn't run into any of the issues mentioned in the comments... So I'm creating this issue hoping to either remove that bit of code or to understand more about the issues mentioned in the comments. (Marking @fmayer since he appears in the blame in that code)

Thanks in advance.

[fmayer]

The comment doesn't mean that all tagged variables in sections will cause trouble. Yours trivially won't because nothing assumes its layout. But for some global variables in sections, the runtime will assume the layout, e.g. that they are contiguous.

That code definitely cannot be removed. If you want to tag variables in sections, you could e.g. add an explicit __attribute__ to opt them in even though they are in sections.

[DavidSpickett]

That code definitely cannot be removed. If you want to tag variables in sections, you could e.g. add an explicit attribute to opt them in even though they are in sections.

The test clang/test/CodeGen/memtag-globals.cpp only has no_sanitize.... I don't see one for opt-in, is there one or do you mean the OP could implement one for this purpose?

[fmayer]

do you mean the OP could implement one for this purpose?

Yes, sorry.