Re: [PATCH v4 bpf-next 2/3] bpf: Support "%c" in bpf_bprintf_prepare().

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, Aug 11, 2021 at 7:15 PM Kuniyuki Iwashima <kuniyu@amazon.co.jp> wrote:
>
> From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
> Date:   Wed, 11 Aug 2021 14:15:50 -0700
> > On Tue, Aug 10, 2021 at 2:29 AM Kuniyuki Iwashima <kuniyu@amazon.co.jp> wrote:
> > >
> > > /proc/net/unix uses "%c" to print a single-byte character to escape '\0' in
> > > the name of the abstract UNIX domain socket.  The following selftest uses
> > > it, so this patch adds support for "%c".  Note that it does not support
> > > wide character ("%lc" and "%llc") for simplicity.
> > >
> > > Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
> > > ---
> > >  kernel/bpf/helpers.c | 14 ++++++++++++++
> > >  1 file changed, 14 insertions(+)
> > >
> > > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > > index 15746f779fe1..6d3aaf94e9ac 100644
> > > --- a/kernel/bpf/helpers.c
> > > +++ b/kernel/bpf/helpers.c
> > > @@ -907,6 +907,20 @@ int bpf_bprintf_prepare(char *fmt, u32 fmt_size, const u64 *raw_args,
> > >                         tmp_buf += err;
> > >                         num_spec++;
> > >
> > > +                       continue;
> > > +               } else if (fmt[i] == 'c') {
> >
> > you are adding new features to printk-like helpers, please add
> > corresponding tests as well. I'm particularly curious how something
> > like "% 9c" (which is now allowed, along with a few other unusual
> > combinations) will work.
>
> I see. I'll add a test.
> I'm now thinking of test like:
>   1. pin the bpf prog that outputs "% 9c" and other format strings.
>   2. read and validate it

Simpler. Use bpf_snprintf() to test all this logic.
bpf_trace_printk(), bpf_snprintf() and bpf_seq_printf() share the same
"backend" in kernel. No need to use bpf_iter program for testing this.
Look for other snprintf() tests and just extend them.

>
> Is there any related test ?
> and is there other complicated fomat strings to test ?
>
> Also, "% 9c" worked as is :)
>
> ---8<---
> $ sudo ./tools/bpftool/bpftool iter pin ./bpf_iter_unix.o /sys/fs/bpf/unix
> $ sudo cat /sys/fs/bpf/unix | head -n 1
>         a
> $ git diff
> diff --git a/tools/testing/selftests/bpf/progs/bpf_iter_unix.c b/tools/testing/selftests/bpf/progs/bpf_iter_unix.c
> index ad397e2962cf..8a7d5aa4c054 100644
> --- a/tools/testing/selftests/bpf/progs/bpf_iter_unix.c
> +++ b/tools/testing/selftests/bpf/progs/bpf_iter_unix.c
> @@ -34,8 +34,10 @@ int dump_unix(struct bpf_iter__unix *ctx)
>
>         seq = ctx->meta->seq;
>         seq_num = ctx->meta->seq_num;
> -       if (seq_num == 0)
> +       if (seq_num == 0) {
> +               BPF_SEQ_PRINTF(seq, "% 9c\n", 'a');
>                 BPF_SEQ_PRINTF(seq, "Num               RefCount Protocol Flags    Type St Inode    Path\n");
> +       }
>
>         BPF_SEQ_PRINTF(seq, "%pK: %08X %08X %08X %04X %02X %8lu",
>                        unix_sk,
> ---8<---
>
>
>
> >
> > > +                       if (!tmp_buf)
> > > +                               goto nocopy_fmt;
> > > +
> > > +                       if (tmp_buf_end == tmp_buf) {
> > > +                               err = -ENOSPC;
> > > +                               goto out;
> > > +                       }
> > > +
> > > +                       *tmp_buf = raw_args[num_spec];
> > > +                       tmp_buf++;
> > > +                       num_spec++;
> > > +
> > >                         continue;
> > >                 }
> > >
> > > --
> > > 2.30.2