On Thu, Jul 21, 2022 at 12:59:09PM +0100, Lee Jones wrote:
> On Thu, 21 Jul 2022, Jiri Olsa wrote:
>
> > On Thu, Jul 21, 2022 at 12:14:30PM +0100, Lee Jones wrote:
> > > The documentation for find_pid() clearly states:
typo find_vpid
> > >
> > > "Must be called with the tasklist_lock or rcu_read_lock() held."
> > >
> > > Presently we do neither.
just curious, did you see crash related to this or you just spot that
> > >
> > > In an ideal world we would wrap the in-lined call to find_vpid() along
> > > with get_pid_task() in the suggested rcu_read_lock() and have done.
> > > However, looking at get_pid_task()'s internals, it already does that
> > > independently, so this would lead to deadlock.
> >
> > hm, we can have nested rcu_read_lock calls, right?
>
> I assumed not, but that might be an oversight on my part.
>
> Would that be your preference?
seems simpler than calling get/put for ppid
jirka
>
> > > Instead, we'll 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@xxxxxxxxxx>
> > > Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> > > Cc: John Fastabend <john.fastabend@xxxxxxxxx>
> > > Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > > Cc: Martin KaFai Lau <martin.lau@xxxxxxxxx>
> > > Cc: Song Liu <song@xxxxxxxxxx>
> > > Cc: Yonghong Song <yhs@xxxxxx>
> > > Cc: KP Singh <kpsingh@xxxxxxxxxx>
> > > Cc: Stanislav Fomichev <sdf@xxxxxxxxxx>
> > > Cc: Hao Luo <haoluo@xxxxxxxxxx>
> > > Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> > > Cc: bpf@xxxxxxxxxxxxxxx
> > > Fixes: 41bdc4b40ed6f ("bpf: introduce bpf subcommand BPF_TASK_FD_QUERY")
> > > Signed-off-by: Lee Jones <lee@xxxxxxxxxx>
> > > ---
> > > 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);
> > > if (!task)
> > > return -ENOENT;
> > >
>
> --
> Lee Jones [李琼斯]
