[TySan] False positive with global structs

[TheLastRar]

This was previously reported in the discourse, since TySan has now been merged I'm reposting it on Github

This is the reduced version from gbMattN, the original report used a std::array<float,2>

Tysan reports a type violation when arr is in the global scope https://godbolt.org/z/TxeaaY5cc

#include <stdio.h>

struct array_type{
    int inner[1];
};

struct array_type arr;

int main() {
  arr.inner[0] = 5;
  return 0;
}

TySan currently reports

==1==ERROR: TypeSanitizer: type-aliasing-violation on address 0x5d14c0846c5c (pc 0x5d14bfeefb4b bp 0x7ffd3e06b7a0 sp 0x7ffd3e06b730 tid 1)
WRITE of size 4 at 0x5d14c0846c5c with type int accesses an existing object of type array_type
    #0 0x5d14bfeefb4a  (/app/output.s+0x2ab4a)

[gbMattN]

Ah! Thanks for pointing this out again. This feels like something #120406 should fix but it doesn't.... hmm

TySan sets the shadow memory for globals at the start. &arr and &arr.inner[0] is the same, so the TD is array_type as it was set at the start. The mistake here is that TySan doesn't dig down into the array_type TD to see that the first member is an int.

Will certainly give it a look

[gbMattN]

Aha. The inner array type is given the incorrect TD. Sounds like a TBAA issue. I think the draft solution to #119615 should be extendable to fix this too