netdev - Re: [PATCH iproute2-next] bpf: fix snprintf truncation warning

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Date:   Fri, 6 Sep 2019 12:19:05 +0200
From:   Andrea Claudi <aclaudi@...hat.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     David Ahern <dsahern@...il.com>,
        linux-netdev <netdev@...r.kernel.org>,
        David Ahern <dsahern@...nel.org>
Subject: Re: [PATCH iproute2-next] bpf: fix snprintf truncation warning

On Thu, Sep 5, 2019 at 5:51 PM Stephen Hemminger
<stephen@...workplumber.org> wrote:
>
> On Thu, 5 Sep 2019 13:44:55 +0200
> Andrea Claudi <aclaudi@...hat.com> wrote:
>
> > On Thu, Sep 5, 2019 at 12:15 AM David Ahern <dsahern@...il.com> wrote:
> > >
> > > On 9/4/19 9:50 AM, Andrea Claudi wrote:
> > > > gcc v9.2.1 produces the following warning compiling iproute2:
> > > >
> > > > bpf.c: In function ‘bpf_get_work_dir’:
> > > > bpf.c:784:49: warning: ‘snprintf’ output may be truncated before the last format character [-Wformat-truncation=]
> > > >   784 |  snprintf(bpf_wrk_dir, sizeof(bpf_wrk_dir), "%s/", mnt);
> > > >       |                                                 ^
> > > > bpf.c:784:2: note: ‘snprintf’ output between 2 and 4097 bytes into a destination of size 4096
> > > >   784 |  snprintf(bpf_wrk_dir, sizeof(bpf_wrk_dir), "%s/", mnt);
> > > >       |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > >
> > > > Fix it extending bpf_wrk_dir size by 1 byte for the extra "/" char.
> > > >
> > > > Signed-off-by: Andrea Claudi <aclaudi@...hat.com>
> > > > ---
> > > >  lib/bpf.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/lib/bpf.c b/lib/bpf.c
> > > > index 7d2a322ffbaec..95de7894a93ce 100644
> > > > --- a/lib/bpf.c
> > > > +++ b/lib/bpf.c
> > > > @@ -742,7 +742,7 @@ static int bpf_gen_hierarchy(const char *base)
> > > >  static const char *bpf_get_work_dir(enum bpf_prog_type type)
> > > >  {
> > > >       static char bpf_tmp[PATH_MAX] = BPF_DIR_MNT;
> > > > -     static char bpf_wrk_dir[PATH_MAX];
> > > > +     static char bpf_wrk_dir[PATH_MAX + 1];
> > > >       static const char *mnt;
> > > >       static bool bpf_mnt_cached;
> > > >       const char *mnt_env = getenv(BPF_ENV_MNT);
> > > >
> > >
> > > PATH_MAX is meant to be the max length for a filesystem path including
> > > the null terminator, so I think it would be better to change the
> > > snprintf to 'sizeof(bpf_wrk_dir) - 1'.
> >
> > With 'sizeof(bpf_wrk_dir) - 1' snprintf simply truncates at byte 4095
> > instead of byte 4096.
> > This means that bpf_wrk_dir can again be truncated before the final
> > "/", as it is by now.
> > Am I missing something?
> >
> > Trying your suggestion I have this slightly different warning message:
> >
> > bpf.c: In function ‘bpf_get_work_dir’:
> > bpf.c:784:52: warning: ‘/’ directive output may be truncated writing 1
> > byte into a region of size between 0 and 4095 [-Wformat-truncation=]
> >   784 |  snprintf(bpf_wrk_dir, sizeof(bpf_wrk_dir) - 1, "%s/", mnt);
> >       |                                                    ^
> > bpf.c:784:2: note: ‘snprintf’ output between 2 and 4097 bytes into a
> > destination of size 4095
> >   784 |  snprintf(bpf_wrk_dir, sizeof(bpf_wrk_dir) - 1, "%s/", mnt);
> >       |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> Why not rework this to use asprintf and avoid having huge buffers on stack?

Thanks for the suggestion. There are a lot of similar usages in
lib/bpf.c, I'll send a v2 to rework them all.