Re: [PATCH] selftests/resctrl: Fix null pointer dereference on open failed

[Colin King (gmail)]

On 25/04/2022 16:51, Shuah Khan wrote:
> On 4/24/22 3:15 PM, Colin Ian King wrote:
> > Currently if opening /dev/null fails to open then file pointer fp
> > is null and further access to fp via fprintf will cause a null
> > pointer dereference. Fix this by returning a negative error value
> > when a null fp is detected.
>
> How did you find this problem and how can it be reproduced? Is there
> a case where test fails to open "/dev/null"?

Found with static analysis, cppcheck. Open on /dev/null is unlikely to 
fail, but it's good to fail reliably rather than have a SIGSEGV :-)

Colin

> > Fixes: a2561b12fe39 ("selftests/resctrl: Add built in benchmark")
> > Signed-off-by: Colin Ian King <colin.i.king@xxxxxxxxx>
> > ---
> >   tools/testing/selftests/resctrl/fill_buf.c | 4 +++-
> >   1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/testing/selftests/resctrl/fill_buf.c 
> > b/tools/testing/selftests/resctrl/fill_buf.c
> > index 51e5cf22632f..56ccbeae0638 100644
> > --- a/tools/testing/selftests/resctrl/fill_buf.c
> > +++ b/tools/testing/selftests/resctrl/fill_buf.c
> > @@ -121,8 +121,10 @@ static int fill_cache_read(unsigned char 
> > *start_ptr, unsigned char *end_ptr,
> >       /* Consume read result so that reading memory is not optimized 
> > out. */
> >       fp = fopen("/dev/null", "w");
> > -    if (!fp)
> > +    if (!fp) {
> >           perror("Unable to write to /dev/null");
> > +        return -1;
> > +    }
> >       fprintf(fp, "Sum: %d ", ret);
> >       fclose(fp);
>
> thanks,
> -- Shuah