Re: [PATCH] tools: libxenstat: fix format string overflow

[Wei Liu <wei.liu2@citrix.com>]

On Fri, Feb 16, 2018 at 05:44:05PM +0000, Wei Liu wrote:
> On Fri, Feb 16, 2018 at 06:36:51PM +0100, Dario Faggioli wrote:
> > With gcc 7.3.0, the build fails like this:
> > 
> > src/xenstat_linux.c: In function ‘getBridge’
> > src/xenstat_linux.c:78:34: warning: ‘%s’ directive writing up to 255 bytes into a region of size 241 [-Wformat-overflow=]
> >      sprintf(tmp, "/sys/class/net/%s/bridge", de->d_name);
> >                                   ^~
> > src/xenstat_linux.c:78:5: note: ‘sprintf’ output between 23 and 278 bytes into a destination of size 256
> >      sprintf(tmp, "/sys/class/net/%s/bridge", de->d_name);
> >      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > 
> > Fix using asprintf().
> > 
> > Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
> > ---
> > Cc: Ian Jackson <ian.jackson@eu.citrix.com>
> > Cc: Wei Liu <wei.liu2@citrix.com>
> > ---
> > In case no one else have noticed and fixed this (I have checked xen-devel and
> > found nothing)
> > ---
> >  tools/xenstat/libxenstat/src/xenstat_linux.c |    8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> > 
> > diff --git a/tools/xenstat/libxenstat/src/xenstat_linux.c b/tools/xenstat/libxenstat/src/xenstat_linux.c
> > index 907d65fa63..396357511b 100644
> > --- a/tools/xenstat/libxenstat/src/xenstat_linux.c
> > +++ b/tools/xenstat/libxenstat/src/xenstat_linux.c
> > @@ -69,18 +69,20 @@ void getBridge(char *excludeName, char *result, size_t resultLen)
> >  	struct dirent *de;
> >  	DIR *d;
> >  
> > -	char tmp[256] = { 0 };
> > -
> >  	d = opendir("/sys/class/net");
> >  	while ((de = readdir(d)) != NULL) {
> >  		if ((strlen(de->d_name) > 0) && (de->d_name[0] != '.')
> >  			&& (strstr(de->d_name, excludeName) == NULL)) {
> > -				sprintf(tmp, "/sys/class/net/%s/bridge", de->d_name);
> > +				char *tmp;
> > +
> > +				asprintf(&tmp, "/sys/class/net/%s/bridge", de->d_name);
> 
> Need to check the return value of asprintf.
> 
> Preferably you also need to define _GNU_SOURCE at the beginning of this
> file. This file is Linux only makes it less of a problem.

To be precise: not necessarily at the beginning of this file, just
before the actual inclusion of stdio.h.

Wei.

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