git: 2171b2cbe084 - main - diff: avoid applying offsets to null pointer

[Piotr Pawel Stefaniak]

The branch main has been updated by pstef:

URL: https://cgit.FreeBSD.org/src/commit/?id=2171b2cbe084118e0e8f7de658f0302d0feb8827

commit 2171b2cbe084118e0e8f7de658f0302d0feb8827
Author:     Piotr Pawel Stefaniak <pstef at FreeBSD.org>
AuthorDate: 2021-09-05 16:59:08 +0000
Commit:     Piotr Pawel Stefaniak <pstef at FreeBSD.org>
CommitDate: 2021-09-15 23:46:44 +0000

    diff: avoid applying offsets to null pointer
    
    This was the only instance of undefined behavior I could find so far.
---
 usr.bin/diff/diffreg.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/usr.bin/diff/diffreg.c b/usr.bin/diff/diffreg.c
index c743c862f2d6..4a00aff9243b 100644
--- a/usr.bin/diff/diffreg.c
+++ b/usr.bin/diff/diffreg.c
@@ -255,7 +255,6 @@ diffreg(char *file1, char *file2, int flags, int capsicum)
 	anychange = 0;
 	lastline = 0;
 	lastmatchline = 0;
-	context_vec_ptr = context_vec_start - 1;
 
 	 /*
 	  * hw excludes padding and make sure when -t is not used,
@@ -1050,8 +1049,12 @@ proceed:
 		/*
 		 * Allocate change records as needed.
 		 */
-		if (context_vec_ptr == context_vec_end - 1) {
-			ptrdiff_t offset = context_vec_ptr - context_vec_start;
+		if (context_vec_start == NULL ||
+		    context_vec_ptr == context_vec_end - 1) {
+			ptrdiff_t offset = -1;
+
+			if (context_vec_start != NULL)
+				offset = context_vec_ptr - context_vec_start;
 			max_context <<= 1;
 			context_vec_start = xreallocarray(context_vec_start,
 			    max_context, sizeof(*context_vec_start));