compat/win32/syslog: fix use-after-realloc

Git for Windows' SDK recently upgraded to GCC v12.x which points out
that the `pos` variable might be used even after the corresponding
memory was `realloc()`ed and therefore potentially no longer valid.

Since a subset of this SDK is used in Git's CI/PR builds, we need to fix
this to continue to be able to benefit from the CI/PR runs.

Note: This bug has been with us since 2a6b149c64f6 (mingw: avoid using
strbuf in syslog, 2011-10-06), and while it looks tempting to replace
the hand-rolled string manipulation with a `strbuf`-based one, that
commit's message explains why we cannot do that: The `syslog()` function
is called as part of the function in `daemon.c` which is set as the
`die()` routine, and since `strbuf_grow()` can call that function if it
runs out of memory, this would cause a nasty infinite loop that we do
not want to re-introduce.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/compat/win32/syslog.c b/compat/win32/syslog.c
index 161978d720aed9db5a00c77d1c6bd9a073544f15..1f8d8934cc90dc2a5988265c555271c8afd18084 100644 (file)
--- a/compat/win32/syslog.c
+++ b/compat/win32/syslog.c
@@ -43,6 +43,7 @@ void syslog(int priority, const char *fmt, ...)
        va_end(ap);
 
        while ((pos = strstr(str, "%1")) != NULL) {
+               size_t offset = pos - str;
                char *oldstr = str;
                str = realloc(str, st_add(++str_len, 1));
                if (!str) {
@@ -50,6 +51,7 @@ void syslog(int priority, const char *fmt, ...)
                        warning_errno("realloc failed");
                        return;
                }
+               pos = str + offset;
                memmove(pos + 2, pos + 1, strlen(pos));
                pos[1] = ' ';
        }