Re: Breakage (?) in configure and git_vsnprintf()

[Jeff King <peff@peff.net>]

On Mon, Dec 12, 2011 at 10:30:27AM +0100, Andreas Schwab wrote:

> Jeff King <peff@peff.net> writes:
> 
> >  	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);
> 
> You also need to call va_end on the copy.

Silly me. Thanks for catching.

Junio, here's a corrected version.

-- >8 --
Subject: [PATCH] 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>
---
 compat/snprintf.c |    9 +++++++--
 1 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/compat/snprintf.c b/compat/snprintf.c
index e1e0e75..42ea1ac 100644
--- a/compat/snprintf.c
+++ b/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;
 	}
-- 
1.7.8.13.g74677