Subject: Re: [PATCH v3 02/14] mingw: implement syslog

From: Eric Sunshine <[email-masked]>
Date: Sun, 10 Oct 2010 19:20:35 -0400

On 10/10/2010 6:16 PM, Erik Faye-Lund wrote:
> On Sun, Oct 10, 2010 at 11:28 PM, Eric Sunshine<[email-masked]>  wrote:
>> On 10/10/2010 4:37 PM, Erik Faye-Lund wrote:
>>> This is the result of the feed-back in v1, where we tried to implement
>>> all format strings. But that turned out to be very complex (due to the
>>> lack of a portable va_copy()) and since we control all call-sites for
>>> syslog and already only use "%s" as the format, it should be OK.
>>
>> Do you mean vsnprintf() rather than va_copy()?
>
> The problem was lack of portable va_copy, because I tried to add a
> non-variadic version of strbuf_addf(), namely strbuf_vaddf() to do the
> work.
>
> I guess it could be implemented pretty easily with vsnprintf(),
> though. I was afraid of doing that originally because I know there's
> portability issues with the return value of snprintf. Luckily it seems
> that we have a fix for that in compat/sprintf.c, and we rely on the
> return value being correct in strbuf_addf() so it would probably be
> safe.
>
> Something like this (on top)
>
> diff --git a/compat/mingw.c b/compat/mingw.c
> index bbe45d0..e3f3f92 100644
> --- a/compat/mingw.c
> +++ b/compat/mingw.c
> @@ -1435,17 +1435,24 @@ void openlog(const char *ident, int logopt,
> int facility)
>   		warning("RegisterEventSource() failed: %lu", GetLastError());
>   }
>
> -void syslog(int priority, const char *fmt, const char *arg)
> +void syslog(int priority, const char *fmt, ...)
>   {
>   	WORD logtype;
> +	char *str;
> +	int str_len;
> +	va_list ap;
>
>   	if (!ms_eventlog)
>   		return;
>
> -	if (strcmp(fmt, "%s")) {
> -		warning("format string of syslog() not implemented");
> -		return;
> -	}
> +	va_start(ap, fmt);
> +	str_len = vsnprintf(NULL, 0, fmt, ap);
> +	va_end(ap);

vsnprintf() can return -1 on error (even the compat/snprintf.c version 
can do so), so perhaps check for this condition before the subsequent 
malloc(str_len+1)?

> +
> +	str = malloc(str_len + 1);
> +	va_start(ap, fmt);
> +	vsnprintf(str, str_len, fmt, ap);
> +	va_end(ap);
>
>   	switch (priority) {
>   	case LOG_EMERG:
> @@ -1478,8 +1485,9 @@ void syslog(int priority, const char *fmt, const
> char *arg)
>   	    NULL,
>   	    1,
>   	    0,
> -	    (const char **)&arg,
> +	    (const char **)&str,
>   	    NULL);
> +	free(str);
>   }

Other than the note about -1 return value, this revision looks fine.

>> (On the other hand, for the '%s' check above, the code does report a warning
>> and then exits, so it is not inconceivable that a '%n' could also emit a
>> warning.)
>
> I guess I could add something like this:
>
> if (strstr(arg, "%1"))
> 	warning("arg contains %1, message might be corrupted");
>
> I don't want to return in that case, because I think some output is
> better than no output, and it seems to work on Vista.

Rather than emitting a warning, it might be reasonable to perform a 
simple transformation on the string if it contains a %1 (or %n 
generally) in order to avoid ReportEvent()'s shortcoming. Even something 
as simple as inserting a space between '%' and '1' might be sufficiently 
defensive.

-- ES