linux-kernel - Re: [PATCH] bpf: btf: Fix vsnprintf return value check

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Message-ID: <CAEf4BzYGjNaqL4h8=4Jw7O_xxMfy=TbUg94VO6RZT5wOtV+_wQ@mail.gmail.com>
Date:   Wed, 13 Jul 2022 11:40:01 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jiri Olsa <olsajiri@...il.com>,
        Alan Maguire <alan.maguire@...cle.com>
Cc:     Fedor Tokarev <ftokarev@...il.com>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bpf: btf: Fix vsnprintf return value check

On Mon, Jul 11, 2022 at 2:45 PM Jiri Olsa <olsajiri@...il.com> wrote:
>
> On Mon, Jul 11, 2022 at 11:13:17PM +0200, Fedor Tokarev wrote:
> > vsnprintf returns the number of characters which would have been written if
> > enough space had been available, excluding the terminating null byte. Thus,
> > the return value of 'len_left' means that the last character has been
> > dropped.
>
> should we have test for this in progs/test_snprintf.c ?

It might be too annoying to set up such test, and given the fix is
pretty trivial IMO it's ok without extra test. But cc Alan for ack.
Alan, please take a look as well.

>
> jirka
>
> >
> > Signed-off-by: Fedor Tokarev <ftokarev@...il.com>
> > ---
> >  kernel/bpf/btf.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index eb12d4f705cc..a9c1c98017d4 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -6519,7 +6519,7 @@ static void btf_snprintf_show(struct btf_show *show, const char *fmt,
> >       if (len < 0) {
> >               ssnprintf->len_left = 0;
> >               ssnprintf->len = len;
> > -     } else if (len > ssnprintf->len_left) {
> > +     } else if (len >= ssnprintf->len_left) {
> >               /* no space, drive on to get length we would have written */
> >               ssnprintf->len_left = 0;
> >               ssnprintf->len += len;
> > --
> > 2.25.1
> >