X86: Fix use-after-realloc in X86AsmParser::ParseIntelExpression

[dexonsmith]

Summary

X86AsmParser::ParseIntelExpression has a while loop. In the body,
calls to MCAsmLexer::UnLex can force a reallocation in the MCAsmLexer's
CurToken SmallVector, invalidating saved references to
MCAsmLexer::getTok().

const MCAsmToken &Tok is such a saved reference, and this moves it
from outside the while loop to inside the body, fixing a
use-after-realloc.

Tok will still be reused across calls to Lex(), each of which
effectively destroys and constructs the pointed-to token. I'm a bit
skeptical of this usage pattern, but it seems broadly used in the
X86AsmParser (and others) so I'm leaving it alone (for now).

Somehow this bug was exposed by https://reviews.llvm.org/D94739,
resulting in test failures in dot-operator related tests in
llvm/test/tools/llvm-ml. I suspect the exposure path is related to
optimizer changes from splitting up the grow operation, but I haven't
dug all the way in. Regardless, there are already tests in tree that
cover this; they might fail consistently if we added ASan
instrumentation to SmallVector.

[dexonsmith]

(I had intended to reference https://reviews.llvm.org/D94739... not sure why it took me three tries to get the right link.)

@dblaikie, I've added you here since you reviewed the SmallVector patch that triggered the problem, but let me know if you'd rather I found another reviewer. In case it's helpful, the Lex and UnLex functions are defined at https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/MC/MCParser/MCAsmLexer.h#L77.

[dexonsmith]

Remove an unrelated whitespace change.

[dblaikie]

Sounds plausible enough to me.

Would be nice to have the asm annotations on SmallVector to back that up/make the codebase more robust.

llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp

if (Name.equals_lower("eq")) {
     return false;
   }
   End = consumeToken();
   return true;
 }
 
 bool X86AsmParser::ParseIntelExpression(IntelExprStateMachine &SM, SMLoc &End) {
   MCAsmParser &Parser = getParser();
-  const AsmToken &Tok = Parser.getTok();
   StringRef ErrMsg;
 
   AsmToken::TokenKind PrevTK = AsmToken::Error;
   bool Done = false;
   while (!Done) {
+    // Get a fresh reference on each loop iteration in case the previous
+    // iteration moved the token storage during UnLex().
+    const AsmToken &Tok = Parser.getTok();
+
     bool UpdateLocLex = true;
     AsmToken::TokenKind TK = getLexer().getKind();
 
     switch (TK) {
     default:
       if ((Done = SM.isValidEndState()))
         break;
       return Error(Tok.getLoc(), "unknown token in expression");