[FileCheck] - Fix possible buffer out of bounds access when parsing --check-prefix.

[grimar]

Summary

FileCheck tool crashes when trying to parse check prefix if there is no any lines after it.

For example following test would crash if there is no symbols (no EOL mark) after boom:

# REQUIRES: x86
# RUN: <skipped few lines>
# RUN: llvm-readobj -t %t | FileCheck %s --check-prefix=boom

Patch fixes the issue.

[grimar]

• Added patch context.

[arsenm]

test/FileCheck/check-empty2.txt

1–3 ↗

(On Diff #129825)

Isn't this a non-empty file from the comments?

[grimar]

• Updated comment.

test/FileCheck/check-empty2.txt

1–3 ↗

(On Diff #129825)

Clarified the comment.

[arsenm]

LGTM

llvm/trunk/test/FileCheck/check-empty2.txt

+; Check that tool does not crash when there is no any data
+; in file after -check-prefix=PREFIX option.
+
+; RUN: not FileCheck -input-file %s %s -check-prefix=A
 No newline at end of file

llvm/trunk/utils/FileCheck/FileCheck.cpp

static size_t CheckTypeSize(Check::CheckType Ty) {
   case Check::CheckEOF:
     llvm_unreachable("Should not be using EOF size");
   }
 
   llvm_unreachable("Bad check type");
 }
 
 static Check::CheckType FindCheckType(StringRef Buffer, StringRef Prefix) {
+  if (Buffer.size() <= Prefix.size())
+    return Check::CheckNone;
+
   char NextChar = Buffer[Prefix.size()];
 
   // Verify that the : is present after the prefix.
   if (NextChar == ':')
     return Check::CheckPlain;
 
   if (NextChar != '-')
     return Check::CheckNone;