# [PATCH] usage: report vsnprintf(3) failure

2 messages from 2024-04-05 to 2024-04-05. Participants: René Scharfe, Jeff King.
Thread: https://gitlist.dev/t/61273

## René Scharfe, 2024-04-05 18:59

Subject: [PATCH] usage: report vsnprintf(3) failure
Message-ID: <3da13298-b6a6-4391-b8e8-5dae9a28b860@web.de>
URL: https://gitlist.dev/e/3da13298-b6a6-4391-b8e8-5dae9a28b860%40web.de

```
vreportf(), which is used e.g. by die() and warning() by default, calls
vsnprintf(3) to format the message to report.  If that call fails, it
only prints the prefix, e.g. "fatal: " or "warning: ".  This at least
informs users that they were supposed to get a message and reveals its
severity, but leaves them wondering what it may have been about.

Here's an example where vreportf() tries to print a message with a 2GB
string, which is too much for vsnprintf(3):

  $ perl -le 'print "create refs/heads/", "a"x2**31' | git update-ref --stdin
  fatal:

At least report the formatting error along with the offending message
(unformatted) to indicate why that message is empty.  Use fprintf(3)
instead of error() to get the message out directly and avoid recursing
back into vreportf().

With this patch we get:

  $ perl -le 'print "create refs/heads/", "a"x2**31' | git update-ref --stdin
  error: unable to format message: invalid ref format: %s
  fatal:

... which allows users to at least get an idea of what went wrong.

Suggested-by: Jeff King <peff@peff.net>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: René Scharfe <l.s.r@web.de>
---
 usage.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/usage.c b/usage.c
index 09f0ed509b..7a2f7805f5 100644
--- a/usage.c
+++ b/usage.c
@@ -19,8 +19,11 @@ static void vreportf(const char *prefix, const char *err, va_list params)
 	}
 	memcpy(msg, prefix, prefix_len);
 	p = msg + prefix_len;
-	if (vsnprintf(p, pend - p, err, params) < 0)
+	if (vsnprintf(p, pend - p, err, params) < 0) {
+		fprintf(stderr, _("error: unable to format message: %s\n"),
+			err);
 		*p = '\0'; /* vsnprintf() failed, clip at prefix */
+	}

 	for (; p != pend - 1 && *p; p++) {
 		if (iscntrl(*p) && *p != '\t' && *p != '\n')
--
2.44.0

```

## Jeff King, 2024-04-05 19:20

Subject: Re: [PATCH] usage: report vsnprintf(3) failure
Message-ID: <20240405192024.GB2561807@coredump.intra.peff.net>
URL: https://gitlist.dev/e/20240405192024.GB2561807%40coredump.intra.peff.net
In-Reply-To: <3da13298-b6a6-4391-b8e8-5dae9a28b860@web.de>

```
On Fri, Apr 05, 2024 at 08:59:52PM +0200, René Scharfe wrote:

> vreportf(), which is used e.g. by die() and warning() by default, calls
> vsnprintf(3) to format the message to report.  If that call fails, it
> only prints the prefix, e.g. "fatal: " or "warning: ".  This at least
> informs users that they were supposed to get a message and reveals its
> severity, but leaves them wondering what it may have been about.
> 
> Here's an example where vreportf() tries to print a message with a 2GB
> string, which is too much for vsnprintf(3):
> 
>   $ perl -le 'print "create refs/heads/", "a"x2**31' | git update-ref --stdin
>   fatal:
> 
> At least report the formatting error along with the offending message
> (unformatted) to indicate why that message is empty.  Use fprintf(3)
> instead of error() to get the message out directly and avoid recursing
> back into vreportf().
> 
> With this patch we get:
> 
>   $ perl -le 'print "create refs/heads/", "a"x2**31' | git update-ref --stdin
>   error: unable to format message: invalid ref format: %s
>   fatal:
> 
> ... which allows users to at least get an idea of what went wrong.

Thanks, I think this is a good change and you've nicely summarized the
situation above. And the patch itself:

> diff --git a/usage.c b/usage.c
> index 09f0ed509b..7a2f7805f5 100644
> --- a/usage.c
> +++ b/usage.c
> @@ -19,8 +19,11 @@ static void vreportf(const char *prefix, const char *err, va_list params)
>  	}
>  	memcpy(msg, prefix, prefix_len);
>  	p = msg + prefix_len;
> -	if (vsnprintf(p, pend - p, err, params) < 0)
> +	if (vsnprintf(p, pend - p, err, params) < 0) {
> +		fprintf(stderr, _("error: unable to format message: %s\n"),
> +			err);
>  		*p = '\0'; /* vsnprintf() failed, clip at prefix */
> +	}

is nice and simply, and shouldn't have any unexpected side effects.

-Peff

```