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:
> >
> > "Must be called with the tasklist_lock or rcu_read_lock() held."
> >
> > Presently we do neither.
> >
> > 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?
> > 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]
