Re: [PATCH] perf cpumap: Use scnprintf instead of snprintf

[Christophe JAILLET]

Le 23/03/2020 Ã 12:03, Dan Carpenter a ÃcritÂ:
> On Sun, Mar 22, 2020 at 06:25:23PM +0100, Christophe JAILLET wrote:
> > 'scnprintf' returns the number of characters written in the output buffer
> > excluding the trailing '\0', instead of the number of characters which
> > would be generated for the given input.
> >
> > Both function return a number of characters, excluding the trailing '\0'.
> > So comparaison to check if it overflows, should be done against max_size-1.
> > Comparaison against max_size can never match.
> >
> > Fixes: 7780c25bae59f ("perf tools: Allow ability to map cpus to nodes easily")
> > Fixes: a24020e6b7cf6 ("perf tools: Change cpu_map__fprintf output")
> > Fixes: 92a7e1278005b ("perf cpumap: Add cpu__max_present_cpu()")
> > Signed-off-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>
> > ---
> >   tools/perf/util/cpumap.c | 39 ++++++++++++++++++++-------------------
> >   1 file changed, 20 insertions(+), 19 deletions(-)
> >
> > diff --git a/tools/perf/util/cpumap.c b/tools/perf/util/cpumap.c
> > index 983b7388f22b..b87e7ef4d130 100644
> > --- a/tools/perf/util/cpumap.c
> > +++ b/tools/perf/util/cpumap.c
> > @@ -316,8 +316,8 @@ static void set_max_cpu_num(void)
> >   		goto out;
> >   
> >   	/* get the highest possible cpu number for a sparse allocation */
> > -	ret = snprintf(path, PATH_MAX, "%s/devices/system/cpu/possible", mnt);
> > -	if (ret == PATH_MAX) {
> > +	ret = scnprintf(path, PATH_MAX, "%s/devices/system/cpu/possible", mnt);
> > +	if (ret == PATH_MAX-1) {
> This should be a static analysis warning.
>
> But isn't this stuff userspace?  I can't figure out how to compile it on
> Debian so I'm not sure.  There is no scnprintf() in user space.
>
> regards,
> dan carpenter

I compiled it with:

ÂÂÂ make tools/perf

the cpumap.o is generated and if I introduce an error, 
'scn<SPACE>printf' for example, gcc triggers a built error.

I though it was enough to validate the patch, before sending it.


Anyway, keeping 'snprintf' could be better to check for the overflow, 
but 'if (ret == PATH_MAX)' should be turned in 'if (ret >= PATH_MAX)'.
If agreed, I can send a V2.

CJ