[PATCH] compat/win32: correct for incorrect compiler warning

From: Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>
Date: Jul 19, 2022, 18:45 UTC
Message-ID: <pull.1294.git.1658256354725.gitgitgadget@gmail.com>

From: Derrick Stolee <derrickstolee@github.com>

The 'win build' job of our CI build is failing with the following error:

compat/win32/syslog.c: In function 'syslog':
compat/win32/syslog.c:53:17: error: pointer 'pos' may be used after \
				    'realloc' [-Werror=use-after-free]
   53 |                 memmove(pos + 2, pos + 1, strlen(pos));
    CC compat/poll/poll.o
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compat/win32/syslog.c:47:23: note: call to 'realloc' here
   47 |                 str = realloc(str, st_add(++str_len, 1));
      |                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

However, between this realloc() and the use we have a line that resets the value of 'pos'. Thus, this error is incorrect. It is likely due to a new version of the compiler on the CI machines.

Instead of waiting for a new compiler, create a new variable to avoid this error.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
---
    Fix error in response to compiler bug
    
    See [1] for an example of this error in the wild.
    
    [1]
    https://github.com/gitgitgadget/git/runs/7413762368?check_suite_focus=true#step:4:297
    
    Thanks, -Stolee

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1294%2Fderrickstolee%2Frealloc-error-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1294/derrickstolee/realloc-error-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1294

 compat/win32/syslog.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/compat/win32/syslog.c b/compat/win32/syslog.c
index 1f8d8934cc9..0af18d88825 100644
--- a/compat/win32/syslog.c
+++ b/compat/win32/syslog.c
@@ -44,6 +44,7 @@ void syslog(int priority, const char *fmt, ...)
 
 	while ((pos = strstr(str, "%1")) != NULL) {
 		size_t offset = pos - str;
+		char *new_pos;
 		char *oldstr = str;
 		str = realloc(str, st_add(++str_len, 1));
 		if (!str) {
@@ -51,9 +52,9 @@ void syslog(int priority, const char *fmt, ...)
 			warning_errno("realloc failed");
 			return;
 		}
-		pos = str + offset;
-		memmove(pos + 2, pos + 1, strlen(pos));
-		pos[1] = ' ';
+		new_pos = str + offset;
+		memmove(new_pos + 2, new_pos + 1, strlen(new_pos));
+		new_pos[1] = ' ';
 	}
 
 	switch (priority) {

base-commit: 71a8fab31b70c417e8f5b5f716581f89955a7082

-- 
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