2022-08-03 14:31:07

by Lee Jones

Subject: [PATCH v2 1/1] bpf: Drop unprotected find_vpid() in favour of find_get_pid()

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 <[email protected]>
Cc: Daniel Borkmann <[email protected]>
Cc: John Fastabend <[email protected]>
Cc: Andrii Nakryiko <[email protected]>
Cc: Martin KaFai Lau <[email protected]>
Cc: Song Liu <[email protected]>
Cc: Yonghong Song <[email protected]>
Cc: KP Singh <[email protected]>
Cc: Stanislav Fomichev <[email protected]>
Cc: Hao Luo <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: [email protected]
Fixes: 41bdc4b40ed6f ("bpf: introduce bpf subcommand BPF_TASK_FD_QUERY")
Signed-off-by: Lee Jones <[email protected]>
---

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);
if (!task)
return -ENOENT;

--
2.37.1.455.g008518b4e5-goog

2022-08-03 15:34:27

by Jiri Olsa

Subject: Re: [PATCH v2 1/1] bpf: Drop unprotected find_vpid() in favour of find_get_pid()

On Wed, Aug 03, 2022 at 02:48:21PM +0100, Lee Jones wrote:
> The documentation for find_pid() clearly states:

nit: typo find_vpid

>
> "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 <[email protected]>
> Cc: Daniel Borkmann <[email protected]>
> Cc: John Fastabend <[email protected]>
> Cc: Andrii Nakryiko <[email protected]>
> Cc: Martin KaFai Lau <[email protected]>
> Cc: Song Liu <[email protected]>
> Cc: Yonghong Song <[email protected]>
> Cc: KP Singh <[email protected]>
> Cc: Stanislav Fomichev <[email protected]>
> Cc: Hao Luo <[email protected]>
> Cc: Jiri Olsa <[email protected]>
> Cc: [email protected]
> Fixes: 41bdc4b40ed6f ("bpf: introduce bpf subcommand BPF_TASK_FD_QUERY")
> Signed-off-by: Lee Jones <[email protected]>

Acked-by: Jiri Olsa <[email protected]>

jirka

> ---
>
> 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);
> if (!task)
> return -ENOENT;
>
> --
> 2.37.1.455.g008518b4e5-goog
>

2022-08-04 18:07:06

by Alexei Starovoitov

Subject: Re: [PATCH v2 1/1] bpf: Drop unprotected find_vpid() in favour of find_get_pid()

On Wed, Aug 3, 2022 at 6:48 AM Lee Jones <[email protected]> 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 <[email protected]>
> Cc: Daniel Borkmann <[email protected]>
> Cc: John Fastabend <[email protected]>
> Cc: Andrii Nakryiko <[email protected]>
> Cc: Martin KaFai Lau <[email protected]>
> Cc: Song Liu <[email protected]>
> Cc: Yonghong Song <[email protected]>
> Cc: KP Singh <[email protected]>
> Cc: Stanislav Fomichev <[email protected]>
> Cc: Hao Luo <[email protected]>
> Cc: Jiri Olsa <[email protected]>
> Cc: [email protected]
> Fixes: 41bdc4b40ed6f ("bpf: introduce bpf subcommand BPF_TASK_FD_QUERY")
> Signed-off-by: Lee Jones <[email protected]>
> ---
>
> 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.

2022-08-09 06:55:16

by Lee Jones

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 <[email protected]> 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 <[email protected]>
> > Cc: Daniel Borkmann <[email protected]>
> > Cc: John Fastabend <[email protected]>
> > Cc: Andrii Nakryiko <[email protected]>
> > Cc: Martin KaFai Lau <[email protected]>
> > Cc: Song Liu <[email protected]>
> > Cc: Yonghong Song <[email protected]>
> > Cc: KP Singh <[email protected]>
> > Cc: Stanislav Fomichev <[email protected]>
> > Cc: Hao Luo <[email protected]>
> > Cc: Jiri Olsa <[email protected]>
> > Cc: [email protected]
> > Fixes: 41bdc4b40ed6f ("bpf: introduce bpf subcommand BPF_TASK_FD_QUERY")
> > Signed-off-by: Lee Jones <[email protected]>
> > ---
> >
> > 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]

2022-08-09 07:05:45

by Lee Jones

Subject: Re: [PATCH v2 1/1] bpf: Drop unprotected find_vpid() in favour of find_get_pid()

On Wed, 03 Aug 2022, Jiri Olsa wrote:

> On Wed, Aug 03, 2022 at 02:48:21PM +0100, Lee Jones wrote:
> > The documentation for find_pid() clearly states:
>
> nit: typo find_vpid

Sorry missed this.

Will fix.

> > "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 <[email protected]>
> > Cc: Daniel Borkmann <[email protected]>
> > Cc: John Fastabend <[email protected]>
> > Cc: Andrii Nakryiko <[email protected]>
> > Cc: Martin KaFai Lau <[email protected]>
> > Cc: Song Liu <[email protected]>
> > Cc: Yonghong Song <[email protected]>
> > Cc: KP Singh <[email protected]>
> > Cc: Stanislav Fomichev <[email protected]>
> > Cc: Hao Luo <[email protected]>
> > Cc: Jiri Olsa <[email protected]>
> > Cc: [email protected]
> > Fixes: 41bdc4b40ed6f ("bpf: introduce bpf subcommand BPF_TASK_FD_QUERY")
> > Signed-off-by: Lee Jones <[email protected]>
>
> Acked-by: Jiri Olsa <[email protected]>

Thanks.

--
Lee Jones [李琼斯]

2022-08-09 15:05:56

by Alexei Starovoitov

Subject: Re: [PATCH v2 1/1] bpf: Drop unprotected find_vpid() in favour of find_get_pid()

On Mon, Aug 8, 2022 at 11:50 PM Lee Jones <[email protected]> wrote:
>
> On Thu, 04 Aug 2022, Alexei Starovoitov wrote:
>
> > On Wed, Aug 3, 2022 at 6:48 AM Lee Jones <[email protected]> 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 <[email protected]>
> > > Cc: Daniel Borkmann <[email protected]>
> > > Cc: John Fastabend <[email protected]>
> > > Cc: Andrii Nakryiko <[email protected]>
> > > Cc: Martin KaFai Lau <[email protected]>
> > > Cc: Song Liu <[email protected]>
> > > Cc: Yonghong Song <[email protected]>
> > > Cc: KP Singh <[email protected]>
> > > Cc: Stanislav Fomichev <[email protected]>
> > > Cc: Hao Luo <[email protected]>
> > > Cc: Jiri Olsa <[email protected]>
> > > Cc: [email protected]
> > > Fixes: 41bdc4b40ed6f ("bpf: introduce bpf subcommand BPF_TASK_FD_QUERY")
> > > Signed-off-by: Lee Jones <[email protected]>
> > > ---
> > >
> > > 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/

Since several people thought about rcu_read_lock instead of your
approach it means that it's preferred.
Sooner or later somebody will send a patch to optimize
refcnt into rcu_read_lock.
So let's avoid the churn and do it now.

2022-08-10 11:12:50

by Lee Jones

Subject: Re: [PATCH v2 1/1] bpf: Drop unprotected find_vpid() in favour of find_get_pid()

On Tue, 09 Aug 2022, Alexei Starovoitov wrote:

> On Mon, Aug 8, 2022 at 11:50 PM Lee Jones <[email protected]> wrote:
> >
> > On Thu, 04 Aug 2022, Alexei Starovoitov wrote:
> >
> > > On Wed, Aug 3, 2022 at 6:48 AM Lee Jones <[email protected]> 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 <[email protected]>
> > > > Cc: Daniel Borkmann <[email protected]>
> > > > Cc: John Fastabend <[email protected]>
> > > > Cc: Andrii Nakryiko <[email protected]>
> > > > Cc: Martin KaFai Lau <[email protected]>
> > > > Cc: Song Liu <[email protected]>
> > > > Cc: Yonghong Song <[email protected]>
> > > > Cc: KP Singh <[email protected]>
> > > > Cc: Stanislav Fomichev <[email protected]>
> > > > Cc: Hao Luo <[email protected]>
> > > > Cc: Jiri Olsa <[email protected]>
> > > > Cc: [email protected]
> > > > Fixes: 41bdc4b40ed6f ("bpf: introduce bpf subcommand BPF_TASK_FD_QUERY")
> > > > Signed-off-by: Lee Jones <[email protected]>
> > > > ---
> > > >
> > > > 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/
>
> Since several people thought about rcu_read_lock instead of your
> approach it means that it's preferred.
> Sooner or later somebody will send a patch to optimize
> refcnt into rcu_read_lock.
> So let's avoid the churn and do it now.

I'm not wed to either approach. Please discuss it with Yonghong and
Jiri and I'll do whatever is agreed upon.

--
Lee Jones [李琼斯]

2022-08-10 12:15:56

by Jiri Olsa

Subject: Re: [PATCH v2 1/1] bpf: Drop unprotected find_vpid() in favour of find_get_pid()

On Wed, Aug 10, 2022 at 12:03:33PM +0100, Lee Jones wrote:
> On Tue, 09 Aug 2022, Alexei Starovoitov wrote:
>
> > On Mon, Aug 8, 2022 at 11:50 PM Lee Jones <[email protected]> wrote:
> > >
> > > On Thu, 04 Aug 2022, Alexei Starovoitov wrote:
> > >
> > > > On Wed, Aug 3, 2022 at 6:48 AM Lee Jones <[email protected]> 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 <[email protected]>
> > > > > Cc: Daniel Borkmann <[email protected]>
> > > > > Cc: John Fastabend <[email protected]>
> > > > > Cc: Andrii Nakryiko <[email protected]>
> > > > > Cc: Martin KaFai Lau <[email protected]>
> > > > > Cc: Song Liu <[email protected]>
> > > > > Cc: Yonghong Song <[email protected]>
> > > > > Cc: KP Singh <[email protected]>
> > > > > Cc: Stanislav Fomichev <[email protected]>
> > > > > Cc: Hao Luo <[email protected]>
> > > > > Cc: Jiri Olsa <[email protected]>
> > > > > Cc: [email protected]
> > > > > Fixes: 41bdc4b40ed6f ("bpf: introduce bpf subcommand BPF_TASK_FD_QUERY")
> > > > > Signed-off-by: Lee Jones <[email protected]>
> > > > > ---
> > > > >
> > > > > 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/
> >
> > Since several people thought about rcu_read_lock instead of your
> > approach it means that it's preferred.
> > Sooner or later somebody will send a patch to optimize
> > refcnt into rcu_read_lock.
> > So let's avoid the churn and do it now.
>
> I'm not wed to either approach. Please discuss it with Yonghong and
> Jiri and I'll do whatever is agreed upon.

yea, I thought using rcu_read_lock would be better, but I did not
have strong feelings against doing the pid's refcount inc/dec when
Yonghong supported that.. now with Alexei it's 2 against 1 in favour
of using rcu_read_lock ;-)

jirka

2022-08-10 15:38:41

by Yonghong Song

Subject: Re: [PATCH v2 1/1] bpf: Drop unprotected find_vpid() in favour of find_get_pid()

On 8/10/22 4:03 AM, Lee Jones wrote:
> On Tue, 09 Aug 2022, Alexei Starovoitov wrote:
>
>> On Mon, Aug 8, 2022 at 11:50 PM Lee Jones <[email protected]> wrote:
>>>
>>> On Thu, 04 Aug 2022, Alexei Starovoitov wrote:
>>>
>>>> On Wed, Aug 3, 2022 at 6:48 AM Lee Jones <[email protected]> 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 <[email protected]>
>>>>> Cc: Daniel Borkmann <[email protected]>
>>>>> Cc: John Fastabend <[email protected]>
>>>>> Cc: Andrii Nakryiko <[email protected]>
>>>>> Cc: Martin KaFai Lau <[email protected]>
>>>>> Cc: Song Liu <[email protected]>
>>>>> Cc: Yonghong Song <[email protected]>
>>>>> Cc: KP Singh <[email protected]>
>>>>> Cc: Stanislav Fomichev <[email protected]>
>>>>> Cc: Hao Luo <[email protected]>
>>>>> Cc: Jiri Olsa <[email protected]>
>>>>> Cc: [email protected]
>>>>> Fixes: 41bdc4b40ed6f ("bpf: introduce bpf subcommand BPF_TASK_FD_QUERY")
>>>>> Signed-off-by: Lee Jones <[email protected]>
>>>>> ---
>>>>>
>>>>> 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/
>>
>> Since several people thought about rcu_read_lock instead of your
>> approach it means that it's preferred.
>> Sooner or later somebody will send a patch to optimize
>> refcnt into rcu_read_lock.
>> So let's avoid the churn and do it now.
>
> I'm not wed to either approach. Please discuss it with Yonghong and
> Jiri and I'll do whatever is agreed upon.

Hi, Lee, Let us just do rcu_read_lock() approach then. I am okay with
that. Thanks!

>