| Message ID | 20220518225355.784371-7-ebiederm@xmission.com (mailing list archive) |
|---|---|
| State | Handled Elsewhere, archived |
| Headers | show |
| Series | ptrace: cleanups and calling do_cldstop with only siglock | expand |
I fail to understand this patch... On 05/18, Eric W. Biederman wrote: > > Today if a process is ptraced only the ptracer will ever be woken up in > wait and why is this wrong? > Fixes: 75b95953a569 ("job control: Add @for_ptrace to do_notify_parent_cldstop()") how does this change fix 75b95953a569? > static int child_wait_callback(wait_queue_entry_t *wait, unsigned mode, > int sync, void *key) > { > struct wait_opts *wo = container_of(wait, struct wait_opts, > child_wait); > - struct task_struct *p = key; > + struct child_wait_info *info = key; > > - if (!eligible_pid(wo, p)) > + if (!eligible_pid(wo, info->p)) > return 0; > > - if ((wo->wo_flags & __WNOTHREAD) && wait->private != p->parent) > - return 0; > + if ((wo->wo_flags & __WNOTHREAD) && (wait->private != info->parent)) > + return 0; So. wait->private is the task T which sleeping on wait_chldexit. Before the patch the logic is clear. T called do_wait(__WNOTHREAD) and we do not need to wake it up if it is not the "actual" parent of p. After the patch we check it T is actual to the "parent" arg passed to __wake_up_parent(). Why??? This arg is only used to find the ->signal->wait_chldexit wait_queue_head, and this is fine. As I said, I don't understand this patch. But at least this change is wrong in case when __wake_up_parent() is calles by __ptrace_detach(). (you removed it in 5/16 but this looks wrong too). Sure, we can change ptrace_detach() to use __wake_up_parent(p, p->parent), but for what? I must have missed something. Oleg.
On 05/24, Oleg Nesterov wrote: > > I fail to understand this patch... > > On 05/18, Eric W. Biederman wrote: > > > > Today if a process is ptraced only the ptracer will ever be woken up in > > wait > > and why is this wrong? > > > Fixes: 75b95953a569 ("job control: Add @for_ptrace to do_notify_parent_cldstop()") > > how does this change fix 75b95953a569? OK, I guess you mean the 2nd do_notify_parent_cldstop() in ptrace_stop(), the problematic case is current->ptrace == T. Right? I dislike this patch anyway, but let me think more about it. Oleg.
On 05/24, Oleg Nesterov wrote: > > On 05/24, Oleg Nesterov wrote: > > > > I fail to understand this patch... > > > > On 05/18, Eric W. Biederman wrote: > > > > > > Today if a process is ptraced only the ptracer will ever be woken up in > > > wait > > > > and why is this wrong? > > > > > Fixes: 75b95953a569 ("job control: Add @for_ptrace to do_notify_parent_cldstop()") > > > > how does this change fix 75b95953a569? > > OK, I guess you mean the 2nd do_notify_parent_cldstop() in ptrace_stop(), > the problematic case is current->ptrace == T. Right? > > I dislike this patch anyway, but let me think more about it. OK, now that I understand the problem, the patch doesn't look bad to me, although I'd ask to make the changelog more clear. After this change __wake_up_parent() can't accept any "parent" from p->parent thread group, but all callers look fine except ptrace_detach(). Oleg.
Oleg Nesterov <oleg@redhat.com> writes: > On 05/24, Oleg Nesterov wrote: >> >> On 05/24, Oleg Nesterov wrote: >> > >> > I fail to understand this patch... >> > >> > On 05/18, Eric W. Biederman wrote: >> > > >> > > Today if a process is ptraced only the ptracer will ever be woken up in >> > > wait >> > >> > and why is this wrong? >> > >> > > Fixes: 75b95953a569 ("job control: Add @for_ptrace to do_notify_parent_cldstop()") >> > >> > how does this change fix 75b95953a569? >> >> OK, I guess you mean the 2nd do_notify_parent_cldstop() in ptrace_stop(), >> the problematic case is current->ptrace == T. Right? >> >> I dislike this patch anyway, but let me think more about it. > > OK, now that I understand the problem, the patch doesn't look bad to me, > although I'd ask to make the changelog more clear. I will see what I can do. > After this change __wake_up_parent() can't accept any "parent" from > p->parent thread group, but all callers look fine except > ptrace_detach(). Having looked at it a little more I think the change was too restrictive. For the !ptrace_reparented case there are possibly two threads of the parent process that wait_consider_task will allow to wait even with __WNOTHREAD specified. It is desirable to wake them both up. Which if I have had enough sleep reduces this patch to just: diff --git a/kernel/exit.c b/kernel/exit.c index f072959fcab7..c8156366b722 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -1431,8 +1431,10 @@ static int child_wait_callback(wait_queue_entry_t *wait, unsigned mode, if (!eligible_pid(wo, p)) return 0; - if ((wo->wo_flags & __WNOTHREAD) && wait->private != p->parent) - return 0; + if ((wo->wo_flags & __WNOTHREAD) && + (wait->private != p->parent) && + (wait->private != p->real_parent)) + return 0; return default_wake_function(wait, mode, sync, key); } I think that solves the issue without missing wake-ups without adding any more. For the same set of reasons it looks like the __wake_up_parent in __ptrace_detach is just simply dead code. I don't think there is a case where when !ptrace_reparented the thread that is the real_parent can sleep in do_wait when the thread that was calling ptrace could not. That needs a very close look to confirm. Eric
On 06/06, Eric W. Biederman wrote: > > Which if I have had enough sleep reduces this patch to just: > > diff --git a/kernel/exit.c b/kernel/exit.c > index f072959fcab7..c8156366b722 100644 > --- a/kernel/exit.c > +++ b/kernel/exit.c > @@ -1431,8 +1431,10 @@ static int child_wait_callback(wait_queue_entry_t *wait, unsigned mode, > if (!eligible_pid(wo, p)) > return 0; > > - if ((wo->wo_flags & __WNOTHREAD) && wait->private != p->parent) > - return 0; > + if ((wo->wo_flags & __WNOTHREAD) && > + (wait->private != p->parent) && > + (wait->private != p->real_parent)) > + return 0; > > return default_wake_function(wait, mode, sync, key); > } > > > I think that solves the issue without missing wake-ups without adding > any more. Agreed, and looks much simpler. > For the same set of reasons it looks like the __wake_up_parent in > __ptrace_detach is just simply dead code. I don't think there is a case > where when !ptrace_reparented the thread that is the real_parent can > sleep in do_wait when the thread that was calling ptrace could not. Yes... this doesn't really differ from the case when one thread reaps a natural child and another thread sleep in do_wait(). > That needs a very close look to confirm. Yes. Oleg.
diff --git a/kernel/exit.c b/kernel/exit.c index f072959fcab7..0e26f73c49ac 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -1421,26 +1421,35 @@ static int ptrace_do_wait(struct wait_opts *wo, struct task_struct *tsk) return 0; } +struct child_wait_info { + struct task_struct *p; + struct task_struct *parent; +}; + static int child_wait_callback(wait_queue_entry_t *wait, unsigned mode, int sync, void *key) { struct wait_opts *wo = container_of(wait, struct wait_opts, child_wait); - struct task_struct *p = key; + struct child_wait_info *info = key; - if (!eligible_pid(wo, p)) + if (!eligible_pid(wo, info->p)) return 0; - if ((wo->wo_flags & __WNOTHREAD) && wait->private != p->parent) - return 0; + if ((wo->wo_flags & __WNOTHREAD) && (wait->private != info->parent)) + return 0; return default_wake_function(wait, mode, sync, key); } void __wake_up_parent(struct task_struct *p, struct task_struct *parent) { + struct child_wait_info info = { + .p = p, + .parent = parent, + }; __wake_up_sync_key(&parent->signal->wait_chldexit, - TASK_INTERRUPTIBLE, p); + TASK_INTERRUPTIBLE, &info); } static bool is_effectively_child(struct wait_opts *wo, bool ptrace,
Today if a process is ptraced only the ptracer will ever be woken up in wait, if the parent is waiting with __WNOTHREAD. Update the code so that the real_parent can also be woken up with __WNOTHREAD even when the code is ptraced. Fixes: 75b95953a569 ("job control: Add @for_ptrace to do_notify_parent_cldstop()") Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> --- kernel/exit.c | 19 ++++++++++++++----- 1 file changed, 14 insertions(+), 5 deletions(-)