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compat/snprintf: don't look at va_list twice

If you define SNPRINTF_RETURNS_BOGUS, we use a special
git_vsnprintf wrapper assumes that vsnprintf returns "-1"
instead of the number of characters that you would need to
store the result.

To do this, it invokes vsnprintf multiple times, growing a
heap buffer until we have enough space to hold the result.
However, this means we evaluate the va_list parameter
multiple times, which is generally a bad thing (it may be
modified by calls to vsnprintf, yielding undefined
behavior).

Instead, we must va_copy it and hand the copy to vsnprintf,
so we always have a pristine va_list.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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commit a9bfbc5b698103b1553e1023c4f77001cc861e79 1 parent 26db0f2
@peff peff authored gitster committed
Showing with 7 additions and 2 deletions.
  1. +7 −2 compat/snprintf.c
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9 compat/snprintf.c
@@ -19,11 +19,14 @@
#undef vsnprintf
int git_vsnprintf(char *str, size_t maxsize, const char *format, va_list ap)
{
+ va_list cp;
char *s;
int ret = -1;
if (maxsize > 0) {
- ret = vsnprintf(str, maxsize-SNPRINTF_SIZE_CORR, format, ap);
+ va_copy(cp, ap);
+ ret = vsnprintf(str, maxsize-SNPRINTF_SIZE_CORR, format, cp);
+ va_end(cp);
if (ret == maxsize-1)
ret = -1;
/* Windows does not NUL-terminate if result fills buffer */
@@ -42,7 +45,9 @@ int git_vsnprintf(char *str, size_t maxsize, const char *format, va_list ap)
if (! str)
break;
s = str;
- ret = vsnprintf(str, maxsize-SNPRINTF_SIZE_CORR, format, ap);
+ va_copy(cp, ap);
+ ret = vsnprintf(str, maxsize-SNPRINTF_SIZE_CORR, format, cp);
+ va_end(cp);
if (ret == maxsize-1)
ret = -1;
}
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