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

[Andrea Claudi <aclaudi@redhat.com>]

On Thu, Sep 5, 2019 at 12:15 AM David Ahern <dsahern@gmail.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@redhat.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);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~