linux-kernel - Re: [PATCH v2 1/1] bpf: Drop unprotected find_vpid() in favour of find_get

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Date:   Tue, 9 Aug 2022 07:50:02 +0100
From:   Lee Jones <lee@...nel.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH v2 1/1] bpf: Drop unprotected find_vpid() in favour of
 find_get_pid()

On Thu, 04 Aug 2022, Alexei Starovoitov wrote:

> On Wed, Aug 3, 2022 at 6:48 AM Lee Jones <lee@...nel.org> wrote:
> >
> > The documentation for find_pid() clearly states:
> >
> >   "Must be called with the tasklist_lock or rcu_read_lock() held."
> >
> > Presently we do neither.
> >
> > Let's use find_get_pid() which searches for the vpid, then takes a
> > reference to it preventing early free, all within the safety of
> > rcu_read_lock().  Once we have our reference we can safely make use of
> > it up until the point it is put.
> >
> > Cc: Alexei Starovoitov <ast@...nel.org>
> > Cc: Daniel Borkmann <daniel@...earbox.net>
> > Cc: John Fastabend <john.fastabend@...il.com>
> > Cc: Andrii Nakryiko <andrii@...nel.org>
> > Cc: Martin KaFai Lau <martin.lau@...ux.dev>
> > Cc: Song Liu <song@...nel.org>
> > Cc: Yonghong Song <yhs@...com>
> > Cc: KP Singh <kpsingh@...nel.org>
> > Cc: Stanislav Fomichev <sdf@...gle.com>
> > Cc: Hao Luo <haoluo@...gle.com>
> > Cc: Jiri Olsa <jolsa@...nel.org>
> > Cc: bpf@...r.kernel.org
> > Fixes: 41bdc4b40ed6f ("bpf: introduce bpf subcommand BPF_TASK_FD_QUERY")
> > Signed-off-by: Lee Jones <lee@...nel.org>
> > ---
> >
> > v1 => v2:
> >   * Commit log update - no code differences
> >
> >  kernel/bpf/syscall.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > index 83c7136c5788d..c20cff30581c4 100644
> > --- a/kernel/bpf/syscall.c
> > +++ b/kernel/bpf/syscall.c
> > @@ -4385,6 +4385,7 @@ static int bpf_task_fd_query(const union bpf_attr *attr,
> >         const struct perf_event *event;
> >         struct task_struct *task;
> >         struct file *file;
> > +       struct pid *ppid;
> >         int err;
> >
> >         if (CHECK_ATTR(BPF_TASK_FD_QUERY))
> > @@ -4396,7 +4397,9 @@ static int bpf_task_fd_query(const union bpf_attr *attr,
> >         if (attr->task_fd_query.flags != 0)
> >                 return -EINVAL;
> >
> > -       task = get_pid_task(find_vpid(pid), PIDTYPE_PID);
> > +       ppid = find_get_pid(pid);
> > +       task = get_pid_task(ppid, PIDTYPE_PID);
> > +       put_pid(ppid);
> 
> rcu_read_lock/unlock around this line
> would be a cheaper and faster alternative than pid's
> refcount inc/dec.

This was already discussed here:

https://lore.kernel.org/all/YtsFT1yFtb7UW2Xu@krava/

-- 
Lee Jones [李琼斯]