On Thu, Feb 04, 2021 at 10:54:42AM +0100, Dmitry Vyukov wrote: > On Thu, Feb 4, 2021 at 10:39 AM Peter Zijlstra <pet...@infradead.org> wrote:
> > OTOH, we're using ptrace permission checks, and ptrace() can inject > > signals just fine. But it's a fairly big departure from what perf set > > out to be. > > Oh, I see, I did not think about this. > > FWIW it's doable today by attaching a BPF program. Sorta. For one, I can't operate BPF to save my life. Secondly, BPF has some very dodgy recursion rules and it's trivial to loose BPF invocations because another BPF is already running. > Will it help if this mode is restricted to monitoring the current > process? Sending signals indeed usually requires cooperation, so doing > it for the current process looks like a reasonable restriction. > This may be not a fundamental restriction, but rather "we don't have > any use cases and are not sure about implications, so this is a > precaution measure, may be relaxed in future". Yeah, limiting it might help. I can trivially add attr::thread_only, that requires attr::inherit and will limit it to CLONE_THREAD (find below). What do we do then? The advantage of IOC_REFRESH is that it disables the event until it gets explicitly re-armed, avoiding recursion issues etc. Do you want those semantics? If so, we'd need to have IOC_REFRESH find the actual event for the current task, which should be doable I suppose. And I need to dig into that fcntl() crud again, see if that's capable of doing a SIGTRAP and if it's possible to target that to the task raising it, instead of doing a process wide signal delivery. Lemme rummage about a bit. --- --- a/include/linux/perf_event.h +++ b/include/linux/perf_event.h @@ -955,7 +955,7 @@ extern void __perf_event_task_sched_in(s struct task_struct *task); extern void __perf_event_task_sched_out(struct task_struct *prev, struct task_struct *next); -extern int perf_event_init_task(struct task_struct *child); +extern int perf_event_init_task(struct task_struct *child, unsigned long clone_flags); extern void perf_event_exit_task(struct task_struct *child); extern void perf_event_free_task(struct task_struct *task); extern void perf_event_delayed_put(struct task_struct *task); @@ -1446,7 +1446,8 @@ perf_event_task_sched_in(struct task_str static inline void perf_event_task_sched_out(struct task_struct *prev, struct task_struct *next) { } -static inline int perf_event_init_task(struct task_struct *child) { return 0; } +static inline int perf_event_init_task(struct task_struct *child, + unsigned long clone_flags) { return 0; } static inline void perf_event_exit_task(struct task_struct *child) { } static inline void perf_event_free_task(struct task_struct *task) { } static inline void perf_event_delayed_put(struct task_struct *task) { } --- a/include/uapi/linux/perf_event.h +++ b/include/uapi/linux/perf_event.h @@ -388,7 +388,8 @@ struct perf_event_attr { aux_output : 1, /* generate AUX records instead of events */ cgroup : 1, /* include cgroup events */ text_poke : 1, /* include text poke events */ - __reserved_1 : 30; + thread_only : 1, /* only inherit on threads */ + __reserved_1 : 29; union { __u32 wakeup_events; /* wakeup every n events */ --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -12776,12 +12776,13 @@ static int inherit_task_group(struct perf_event *event, struct task_struct *parent, struct perf_event_context *parent_ctx, struct task_struct *child, int ctxn, - int *inherited_all) + unsigned long clone_flags, int *inherited_all) { int ret; struct perf_event_context *child_ctx; - if (!event->attr.inherit) { + if (!event->attr.inherit || + (event->attr.thread_only && !(clone_flags & CLONE_THREAD))) { *inherited_all = 0; return 0; } @@ -12813,7 +12814,7 @@ inherit_task_group(struct perf_event *ev /* * Initialize the perf_event context in task_struct */ -static int perf_event_init_context(struct task_struct *child, int ctxn) +static int perf_event_init_context(struct task_struct *child, int ctxn, unsigned long clone_flags) { struct perf_event_context *child_ctx, *parent_ctx; struct perf_event_context *cloned_ctx; @@ -12853,7 +12854,8 @@ static int perf_event_init_context(struc */ perf_event_groups_for_each(event, &parent_ctx->pinned_groups) { ret = inherit_task_group(event, parent, parent_ctx, - child, ctxn, &inherited_all); + child, ctxn, clone_flags, + &inherited_all); if (ret) goto out_unlock; } @@ -12869,7 +12871,8 @@ static int perf_event_init_context(struc perf_event_groups_for_each(event, &parent_ctx->flexible_groups) { ret = inherit_task_group(event, parent, parent_ctx, - child, ctxn, &inherited_all); + child, ctxn, clone_flags, + &inherited_all); if (ret) goto out_unlock; } @@ -12911,7 +12914,7 @@ static int perf_event_init_context(struc /* * Initialize the perf_event context in task_struct */ -int perf_event_init_task(struct task_struct *child) +int perf_event_init_task(struct task_struct *child, unsigned long clone_flags) { int ctxn, ret; @@ -12920,7 +12923,7 @@ int perf_event_init_task(struct task_str INIT_LIST_HEAD(&child->perf_event_list); for_each_task_context_nr(ctxn) { - ret = perf_event_init_context(child, ctxn); + ret = perf_event_init_context(child, ctxn, clone_flags); if (ret) { perf_event_free_task(child); return ret; --- a/kernel/fork.c +++ b/kernel/fork.c @@ -2068,7 +2068,7 @@ static __latent_entropy struct task_stru if (retval) goto bad_fork_cleanup_policy; - retval = perf_event_init_task(p); + retval = perf_event_init_task(p, clone_flags); if (retval) goto bad_fork_cleanup_policy; retval = audit_alloc(p);