[scudo] Always exclude Tag 0

[vitalybuka]

Summary

prepareTaggedChunk uses Tag 0 for header.

Android already PR_MTE_TAG_MASK to 0xfffe,
but with the patch we will not need to deppend
on the system configuration.

[pcc]

There are probably going to be more callers of selectRandomTag and I don't think we want to have to deal with tag 0 in any of them. Could we just or 1 into the mask in selectRandomTag?

[vitalybuka]

Could we just or 1 into the mask in selectRandomTag?

done

[vitalybuka]

ping

[pcc]

LGTM

compiler-rt/lib/scudo/standalone/memtag.h

__asm__ __volatile__(
         msr tco, %0
         )"
         :
         : "r"(PrevTCO));
   }
 };
 
 inline uptr selectRandomTag(uptr Ptr, uptr ExcludeMask) {
+  ExcludeMask |= 1; // Always exclude Tag 0.
   uptr TaggedPtr;
   __asm__ __volatile__(
       R"(
       .arch_extension memtag
       irg %[TaggedPtr], %[Ptr], %[ExcludeMask]
       )"
       : [TaggedPtr] "=r"(TaggedPtr)
       : [Ptr] "r"(Ptr), [ExcludeMask] "r"(ExcludeMask));