netdev - Re: [PATCH v6 08/12] tools/bpf/bpftool/skeleton: make it adopt to task comm size change

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Date:   Tue, 26 Oct 2021 10:18:51 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Valentin Schneider <valentin.schneider@....com>,
        Qiang Zhang <qiang.zhang@...driver.com>,
        robdclark <robdclark@...omium.org>,
        christian <christian@...uner.io>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        dennis.dalessandro@...nelisnetworks.com,
        mike.marciniszyn@...nelisnetworks.com, dledford@...hat.com,
        jgg@...pe.ca, linux-rdma@...r.kernel.org,
        netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        "linux-perf-use." <linux-perf-users@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel test robot <oliver.sang@...el.com>,
        kbuild test robot <lkp@...el.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Subject: Re: [PATCH v6 08/12] tools/bpf/bpftool/skeleton: make it adopt to
 task comm size change

On Tue, Oct 26, 2021 at 5:24 AM Kees Cook <keescook@...omium.org> wrote:
>
> On Mon, Oct 25, 2021 at 08:33:11AM +0000, Yafang Shao wrote:
> > bpf_probe_read_kernel_str() will add a nul terminator to the dst, then
> > we don't care about if the dst size is big enough.
> >
> > Signed-off-by: Yafang Shao <laoar.shao@...il.com>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > Cc: Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
> > Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Cc: Al Viro <viro@...iv.linux.org.uk>
> > Cc: Kees Cook <keescook@...omium.org>
> > Cc: Petr Mladek <pmladek@...e.com>
>
> So, if we're ever going to copying these buffers out of the kernel (I
> don't know what the object lifetime here in bpf is for "e", etc), we
> should be zero-padding (as get_task_comm() does).
>
> Should this, instead, be using a bounce buffer?

The comment in bpf_probe_read_kernel_str_common() says

  :      /*
  :       * The strncpy_from_kernel_nofault() call will likely not fill the
  :       * entire buffer, but that's okay in this circumstance as we're probing
  :       * arbitrary memory anyway similar to bpf_probe_read_*() and might
  :       * as well probe the stack. Thus, memory is explicitly cleared
  :       * only in error case, so that improper users ignoring return
  :       * code altogether don't copy garbage; otherwise length of string
  :       * is returned that can be used for bpf_perf_event_output() et al.
  :       */

It seems that it doesn't matter if the buffer is filled as that is
probing arbitrary memory.

>
> get_task_comm(comm, task->group_leader);

This helper can't be used by the BPF programs, as it is not exported to BPF.

> bpf_probe_read_kernel_str(&e.comm, sizeof(e.comm), comm);
>
> -Kees
>
> > ---
> >  tools/bpf/bpftool/skeleton/pid_iter.bpf.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/bpf/bpftool/skeleton/pid_iter.bpf.c b/tools/bpf/bpftool/skeleton/pid_iter.bpf.c
> > index d9b420972934..f70702fcb224 100644
> > --- a/tools/bpf/bpftool/skeleton/pid_iter.bpf.c
> > +++ b/tools/bpf/bpftool/skeleton/pid_iter.bpf.c
> > @@ -71,8 +71,8 @@ int iter(struct bpf_iter__task_file *ctx)
> >
> >       e.pid = task->tgid;
> >       e.id = get_obj_id(file->private_data, obj_type);
> > -     bpf_probe_read_kernel(&e.comm, sizeof(e.comm),
> > -                           task->group_leader->comm);
> > +     bpf_probe_read_kernel_str(&e.comm, sizeof(e.comm),
> > +                               task->group_leader->comm);
> >       bpf_seq_write(ctx->meta->seq, &e, sizeof(e));
> >
> >       return 0;
> > --
> > 2.17.1
> >
>
> --
> Kees Cook



-- 
Thanks
Yafang