threads / patch / 61273

patchusage: report vsnprintf(3) failure

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

## tl;dr

2 messages between Apr 5, 2024 and Apr 5, 2024. Diffs are folded; open one to read it.

replies: 1people: 2as markdown or json

René Scharfe· Apr 5, 2024, 18:59 UTC · lore

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(-)

Show changes to usage.c +4 −2

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· Apr 5, 2024, 19:20 UTC · re: René Scharfe · lore

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

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

Show 24 quoted lines

> 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:

Show 14 quoted lines

> 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