[llvm] Fix undefined-behaviour in regex engine. (PR #73071)

[via llvm-commits]

https://github.com/tanmaysachan created https://github.com/llvm/llvm-project/pull/73071

Running the `mlir-text-parser-fuzzer` discovers a path that causes application of offset to a null pointer (UB) in the regex engine.

This patch adds a check.

>From e24049717c1bacfc7d62bd94513fb0bd207aef3e Mon Sep 17 00:00:00 2001
From: tanmaysachan <tnmysachan at gmail.com>
Date: Wed, 22 Nov 2023 08:09:08 +0530
Subject: [PATCH] Fix undefined-behaviour in regex engine.

- Running the regex engine on an empty string causes "Applying non-zero offset to null pointer" UB.
- This patch puts a check in the matcher.
- Bug discovered through "mlir-text-parser-fuzzer" module.
---
 llvm/lib/Support/regengine.inc | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Support/regengine.inc b/llvm/lib/Support/regengine.inc
index f23993abc6e7e71..54dd96ab9cfada5 100644
--- a/llvm/lib/Support/regengine.inc
+++ b/llvm/lib/Support/regengine.inc
@@ -146,7 +146,9 @@ matcher(struct re_guts *g, const char *string, size_t nmatch,
 	const char *stop;
 
 	/* simplify the situation where possible */
-	if (g->cflags&REG_NOSUB)
+        if (!string)
+		return(REG_INVARG);
+        if (g->cflags&REG_NOSUB)
 		nmatch = 0;
 	if (eflags&REG_STARTEND) {
 		start = string + pmatch[0].rm_so;