| Message ID | 20220505182645.497868-10-ebiederm@xmission.com (mailing list archive) |
|---|---|
| State | Handled Elsewhere, archived |
| Headers | show |
| Series | ptrace: cleaning up ptrace_stop | expand |
On 05/05, Eric W. Biederman wrote: > > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -103,7 +103,7 @@ struct task_group; > /* Convenience macros for the sake of set_current_state: */ > #define TASK_KILLABLE (TASK_WAKEKILL | TASK_UNINTERRUPTIBLE) > #define TASK_STOPPED (TASK_WAKEKILL | __TASK_STOPPED) > -#define TASK_TRACED (TASK_WAKEKILL | __TASK_TRACED) > +#define TASK_TRACED __TASK_TRACED however I personally still dislike this change. But let me read the code with this series applied, perhaps I will change my mind. If not, I will argue ;) Oleg.
Oleg Nesterov <oleg@redhat.com> writes: > On 05/05, Eric W. Biederman wrote: >> >> --- a/include/linux/sched.h >> +++ b/include/linux/sched.h >> @@ -103,7 +103,7 @@ struct task_group; >> /* Convenience macros for the sake of set_current_state: */ >> #define TASK_KILLABLE (TASK_WAKEKILL | TASK_UNINTERRUPTIBLE) >> #define TASK_STOPPED (TASK_WAKEKILL | __TASK_STOPPED) >> -#define TASK_TRACED (TASK_WAKEKILL | __TASK_TRACED) >> +#define TASK_TRACED __TASK_TRACED > > however I personally still dislike this change. But let me read the > code with this series applied, perhaps I will change my mind. If not, > I will argue ;) That is fair. I kind of grew on my after I implemented it and wrapped my head around what was going on, as it is simple and there are no implicit cases. Eric
On 05/05, Eric W. Biederman wrote: > > static void ptrace_unfreeze_traced(struct task_struct *task) > { > - if (READ_ONCE(task->__state) != __TASK_TRACED) > - return; > - > - WARN_ON(!task->ptrace || task->parent != current); > + unsigned long flags; > > /* > - * PTRACE_LISTEN can allow ptrace_trap_notify to wake us up remotely. > - * Recheck state under the lock to close this race. > + * The child may be awake and may have cleared > + * JOBCTL_PTRACE_FROZEN (see ptrace_resume). The child will > + * not set JOBCTL_PTRACE_FROZEN or enter __TASK_TRACED anew. > */ > - spin_lock_irq(&task->sighand->siglock); > - if (READ_ONCE(task->__state) == __TASK_TRACED) { > + if (lock_task_sighand(task, &flags)) { But I still think that a lockless if (!(task->jobctl & JOBCTL_PTRACE_FROZEN)) return; check at the start of ptrace_unfreeze_traced() makes sense to avoid lock_task_sighand() if possible. And ptrace_resume() can probably clear JOBCTL_PTRACE_FROZEN along with JOBCTL_TRACED to make this optimization work better. The same for ptrace_signal_wake_up(). Oleg.
Oleg Nesterov <oleg@redhat.com> writes: > On 05/05, Eric W. Biederman wrote: >> >> static void ptrace_unfreeze_traced(struct task_struct *task) >> { >> - if (READ_ONCE(task->__state) != __TASK_TRACED) >> - return; >> - >> - WARN_ON(!task->ptrace || task->parent != current); >> + unsigned long flags; >> >> /* >> - * PTRACE_LISTEN can allow ptrace_trap_notify to wake us up remotely. >> - * Recheck state under the lock to close this race. >> + * The child may be awake and may have cleared >> + * JOBCTL_PTRACE_FROZEN (see ptrace_resume). The child will >> + * not set JOBCTL_PTRACE_FROZEN or enter __TASK_TRACED anew. >> */ >> - spin_lock_irq(&task->sighand->siglock); >> - if (READ_ONCE(task->__state) == __TASK_TRACED) { >> + if (lock_task_sighand(task, &flags)) { > > But I still think that a lockless > > if (!(task->jobctl & JOBCTL_PTRACE_FROZEN)) > return; > > check at the start of ptrace_unfreeze_traced() makes sense to avoid > lock_task_sighand() if possible. > > And ptrace_resume() can probably clear JOBCTL_PTRACE_FROZEN along with > JOBCTL_TRACED to make this optimization work better. The same for > ptrace_signal_wake_up(). What do you have that suggests that taking siglock there is a problem? What you propose will definitely work as an incremental change, and in an incremental change we can explain why doing the stupid simple thing is not good enough. I am not really opposed on any grounds except that simplicity is good, and hard to get wrong. Eric
On 05/10, Eric W. Biederman wrote: > > > But I still think that a lockless > > > > if (!(task->jobctl & JOBCTL_PTRACE_FROZEN)) > > return; > > > > check at the start of ptrace_unfreeze_traced() makes sense to avoid > > lock_task_sighand() if possible. > > > > And ptrace_resume() can probably clear JOBCTL_PTRACE_FROZEN along with > > JOBCTL_TRACED to make this optimization work better. The same for > > ptrace_signal_wake_up(). > > What do you have that suggests that taking siglock there is a problem? Not necessarily a problem, but this optimization is free. If the tracee was resumed, it can compete for siglock with debugger. > What you propose will definitely work as an incremental change, and > in an incremental change we can explain why doing the stupid simple > thing is not good enough. OK. Oleg.
diff --git a/include/linux/sched.h b/include/linux/sched.h index d5e3c00b74e1..610f2fdb1e2c 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -103,7 +103,7 @@ struct task_group; /* Convenience macros for the sake of set_current_state: */ #define TASK_KILLABLE (TASK_WAKEKILL | TASK_UNINTERRUPTIBLE) #define TASK_STOPPED (TASK_WAKEKILL | __TASK_STOPPED) -#define TASK_TRACED (TASK_WAKEKILL | __TASK_TRACED) +#define TASK_TRACED __TASK_TRACED #define TASK_IDLE (TASK_UNINTERRUPTIBLE | TASK_NOLOAD) diff --git a/include/linux/sched/jobctl.h b/include/linux/sched/jobctl.h index fa067de9f1a9..d556c3425963 100644 --- a/include/linux/sched/jobctl.h +++ b/include/linux/sched/jobctl.h @@ -19,6 +19,7 @@ struct task_struct; #define JOBCTL_TRAPPING_BIT 21 /* switching to TRACED */ #define JOBCTL_LISTENING_BIT 22 /* ptracer is listening for events */ #define JOBCTL_TRAP_FREEZE_BIT 23 /* trap for cgroup freezer */ +#define JOBCTL_PTRACE_FROZEN_BIT 24 /* frozen for ptrace */ #define JOBCTL_STOP_DEQUEUED (1UL << JOBCTL_STOP_DEQUEUED_BIT) #define JOBCTL_STOP_PENDING (1UL << JOBCTL_STOP_PENDING_BIT) @@ -28,6 +29,7 @@ struct task_struct; #define JOBCTL_TRAPPING (1UL << JOBCTL_TRAPPING_BIT) #define JOBCTL_LISTENING (1UL << JOBCTL_LISTENING_BIT) #define JOBCTL_TRAP_FREEZE (1UL << JOBCTL_TRAP_FREEZE_BIT) +#define JOBCTL_PTRACE_FROZEN (1UL << JOBCTL_PTRACE_FROZEN_BIT) #define JOBCTL_TRAP_MASK (JOBCTL_TRAP_STOP | JOBCTL_TRAP_NOTIFY) #define JOBCTL_PENDING_MASK (JOBCTL_STOP_PENDING | JOBCTL_TRAP_MASK) diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h index 3c8b34876744..e66948abbee4 100644 --- a/include/linux/sched/signal.h +++ b/include/linux/sched/signal.h @@ -435,9 +435,10 @@ extern void calculate_sigpending(void); extern void signal_wake_up_state(struct task_struct *t, unsigned int state); -static inline void signal_wake_up(struct task_struct *t, bool resume) +static inline void signal_wake_up(struct task_struct *t, bool fatal) { - signal_wake_up_state(t, resume ? TASK_WAKEKILL : 0); + fatal = fatal && !(t->jobctl & JOBCTL_PTRACE_FROZEN); + signal_wake_up_state(t, fatal ? TASK_WAKEKILL | __TASK_TRACED : 0); } static inline void ptrace_signal_wake_up(struct task_struct *t, bool resume) { diff --git a/kernel/ptrace.c b/kernel/ptrace.c index 05953ac9f7bd..83ed28262708 100644 --- a/kernel/ptrace.c +++ b/kernel/ptrace.c @@ -197,7 +197,7 @@ static bool ptrace_freeze_traced(struct task_struct *task) spin_lock_irq(&task->sighand->siglock); if (task_is_traced(task) && !looks_like_a_spurious_pid(task) && !__fatal_signal_pending(task)) { - WRITE_ONCE(task->__state, __TASK_TRACED); + task->jobctl |= JOBCTL_PTRACE_FROZEN; ret = true; } spin_unlock_irq(&task->sighand->siglock); @@ -207,23 +207,19 @@ static bool ptrace_freeze_traced(struct task_struct *task) static void ptrace_unfreeze_traced(struct task_struct *task) { - if (READ_ONCE(task->__state) != __TASK_TRACED) - return; - - WARN_ON(!task->ptrace || task->parent != current); + unsigned long flags; /* - * PTRACE_LISTEN can allow ptrace_trap_notify to wake us up remotely. - * Recheck state under the lock to close this race. + * The child may be awake and may have cleared + * JOBCTL_PTRACE_FROZEN (see ptrace_resume). The child will + * not set JOBCTL_PTRACE_FROZEN or enter __TASK_TRACED anew. */ - spin_lock_irq(&task->sighand->siglock); - if (READ_ONCE(task->__state) == __TASK_TRACED) { + if (lock_task_sighand(task, &flags)) { + task->jobctl &= ~JOBCTL_PTRACE_FROZEN; if (__fatal_signal_pending(task)) wake_up_state(task, __TASK_TRACED); - else - WRITE_ONCE(task->__state, TASK_TRACED); + unlock_task_sighand(task, &flags); } - spin_unlock_irq(&task->sighand->siglock); } /** @@ -256,7 +252,6 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state) */ read_lock(&tasklist_lock); if (child->ptrace && child->parent == current) { - WARN_ON(READ_ONCE(child->__state) == __TASK_TRACED); /* * child->sighand can't be NULL, release_task() * does ptrace_unlink() before __exit_signal(). diff --git a/kernel/sched/core.c b/kernel/sched/core.c index d575b4914925..3c351707e830 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -6304,10 +6304,7 @@ static void __sched notrace __schedule(unsigned int sched_mode) /* * We must load prev->state once (task_struct::state is volatile), such - * that: - * - * - we form a control dependency vs deactivate_task() below. - * - ptrace_{,un}freeze_traced() can change ->state underneath us. + * that we form a control dependency vs deactivate_task() below. */ prev_state = READ_ONCE(prev->__state); if (!(sched_mode & SM_MASK_PREEMPT) && prev_state) { diff --git a/kernel/signal.c b/kernel/signal.c index d2d0c753156c..a58b68a2d3c6 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -2209,14 +2209,12 @@ static int ptrace_stop(int exit_code, int why, unsigned long message, } /* - * schedule() will not sleep if there is a pending signal that - * can awaken the task. - * - * After this point ptrace_signal_wake_up will clear TASK_TRACED - * if ptrace_unlink happens. Handle previous ptrace_unlinks - * here to prevent ptrace_stop sleeping in schedule. + * After this point ptrace_signal_wake_up or signal_wake_up + * will clear TASK_TRACED if ptrace_unlink happens or a fatal + * signal comes in. Handle previous ptrace_unlinks and fatal + * signals here to prevent ptrace_stop sleeping in schedule. */ - if (!current->ptrace) + if (!current->ptrace || __fatal_signal_pending(current)) return exit_code; set_special_state(TASK_TRACED); @@ -2305,7 +2303,7 @@ static int ptrace_stop(int exit_code, int why, unsigned long message, current->exit_code = 0; /* LISTENING can be set only during STOP traps, clear it */ - current->jobctl &= ~JOBCTL_LISTENING; + current->jobctl &= ~(JOBCTL_LISTENING | JOBCTL_PTRACE_FROZEN); /* * Queued signals ignored us while we were stopped for tracing.
Stop playing with tsk->__state to remove TASK_WAKEKILL while a ptrace command is executing. Instead remove TASK_WAKEKILL from the definition of TASK_TRACED, and implement a new jobctl flag TASK_PTRACE_FROZEN. This new flag is set in jobctl_freeze_task and cleared when ptrace_stop is awoken or in jobctl_unfreeze_task (when ptrace_stop remains asleep). In signal_wake_up add __TASK_TRACED to state along with TASK_WAKEKILL when the wake up is for a fatal signal. Skip adding __TASK_TRACED when TASK_PTRACE_FROZEN is not set. This has the same effect as changing TASK_TRACED to __TASK_TRACED as all of the wake_ups that use TASK_KILLABLE go through signal_wake_up. Handle a ptrace_stop being called with a pending fatal signal. Previously it would have been handled by schedule simply failing to sleep. As TASK_WAKEKILL is no longer part of TASK_TRACED schedule will sleep with a fatal_signal_pending. The code in signal_wake_up guarantees that the code will be awaked by any fatal signal that codes after TASK_TRACED is set. Previously the __state value of __TASK_TRACED was changed to TASK_RUNNING when woken up or back to TASK_TRACED when the code was left in ptrace_stop. Now when woken up ptrace_stop now clears JOBCTL_PTRACE_FROZEN and when left sleeping ptrace_unfreezed_traced clears JOBCTL_PTRACE_FROZEN. Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> --- include/linux/sched.h | 2 +- include/linux/sched/jobctl.h | 2 ++ include/linux/sched/signal.h | 5 +++-- kernel/ptrace.c | 21 ++++++++------------- kernel/sched/core.c | 5 +---- kernel/signal.c | 14 ++++++-------- 6 files changed, 21 insertions(+), 28 deletions(-)