Re: [PATCH 1/1] bpf: Drop unprotected find_vpid() in favour of find_get_pid()

[Yonghong Song <yhs@fb.com>]

On 7/21/22 5:14 AM, Jiri Olsa wrote:
> 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.

 From kernel documentation, nested rcu_read_lock is allowed.
https://www.kernel.org/doc/Documentation/RCU/Design/Requirements/Requirements.html

RCU's grace-period guarantee allows updaters to wait for the completion 
of all pre-existing RCU read-side critical sections. An RCU read-side 
critical section begins with the marker rcu_read_lock() and ends with 
the marker rcu_read_unlock(). These markers may be nested, and RCU 
treats a nested set as one big RCU read-side critical section. 
Production-quality implementations of rcu_read_lock() and 
rcu_read_unlock() are extremely lightweight, and in fact have exactly 
zero overhead in Linux kernels built for production use with 
CONFIG_PREEMPT=n.

>>
>> Would that be your preference?
> 
> seems simpler than calling get/put for ppid

The current implementation seems okay since we can hide
rcu_read_lock() inside find_get_pid(). We can also avoid
nested rcu_read_lock(), which is although allowed but
not pretty.

> 
> 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@kernel.org>
>>>> Cc: Daniel Borkmann <daniel@iogearbox.net>
>>>> Cc: John Fastabend <john.fastabend@gmail.com>
>>>> Cc: Andrii Nakryiko <andrii@kernel.org>
>>>> Cc: Martin KaFai Lau <martin.lau@linux.dev>
>>>> Cc: Song Liu <song@kernel.org>
>>>> Cc: Yonghong Song <yhs@fb.com>
>>>> Cc: KP Singh <kpsingh@kernel.org>
>>>> Cc: Stanislav Fomichev <sdf@google.com>
>>>> Cc: Hao Luo <haoluo@google.com>
>>>> Cc: Jiri Olsa <jolsa@kernel.org>
>>>> Cc: bpf@vger.kernel.org
>>>> Fixes: 41bdc4b40ed6f ("bpf: introduce bpf subcommand BPF_TASK_FD_QUERY")
>>>> Signed-off-by: Lee Jones <lee@kernel.org>
>>>> ---
>>>>   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]