On Sun, 2007-11-11 at 18:14 -0800, Roland McGrath wrote: > Sorry for the late reply on this. I've been juggling too many other > things, and also I was out for most of a couple of weeks in October. > > I did indeed promise to do the ptrace/signals magic part of the equation > once you ia64 folks ironed out the ia64-specific code for this plan. > I'm very glad to do it and see this wart finally get thoroughly cleaned up! > > To integrate this with the ia64 code, I imagine in asm-ia64/ptrace.h adding: > > #define arch_ptrace_stop_needed(code, info) \ > (!test_thread_flag(TIF_RESTORE_RSE)) > #define arch_ptrace_stop(code, info) ia64_rse_writeback() > > I have not tested this patch except to verify that it compiles away to no > change when the two new macros are not defined. It may be difficult to > really test for the case it's particularly intended for, since it's a race. > You need something like repeated runs of strace /bin/true while > asynchronously trying kill -9 on the traced /bin/true process (but might > need to avoid killing the whole pgrp including strace), and see if it ever > leaves off with the traced process stuck in TASK_TRACED (T) with a pending > SIGKILL, rather than being a dead zombie (and cleaned up by strace). If > you try to produce a test scenario for the problem, you can test it with a > version that never calls sigkill_pending after calling arch_ptrace_stop > with the siglock dropped. Once you produce the bug with that kernel and > believe you can make it hit with a known test scenario, try the proper > patch as below to verify it can no longer hit. Hi Roland, the trouble is I used the current ia64 patch and even inserted an msleep(10) into ptrace_stop() to make sure it does sleep but I don't see any problems. I added the following code between arch_ptrace_stop(1) and set_current_state(TASK_TRACED): msleep(10); if (unlikely(sigismember(¤t->pending.signal, SIGKILL))) printk(KERN_INFO "%d (%s): Got SIGKILL in ptrace_stop\n", current->pid, current->comm); I ran strace on a simple program (calling gettimeofday() in an endless loop) and killed it with SIGKILL. The program exited correctly and I got the message in syslog. I'm puzzled. :/ Is this not the correct place where the race condition should happen? Kind regards, Petr Tesarik > If you are happy with this patch and the results of integrating it with > your ia64 changes, then either you or I can post this machine-independent > patch upstream for inclusion in the mainline kernel. I'm eager to see all > the old cruft in ia64 ptrace get cleaned out after we have this in. > > > Thanks, > Roland > > --- > [PATCH] arch_ptrace_stop > > This adds support to allow asm/ptrace.h to define two new macros, > arch_ptrace_stop_needed and arch_ptrace_stop. These control special > machine-specific actions to be done before a ptrace stop. The new > code compiles away to nothing when the new macros are not defined. > This is the case on all machines to begin with. > > On ia64, these macros will be defined to solve the long-standing > issue of ptrace vs register backing store. > > Signed-off-by: Roland McGrath <roland@xxxxxxxxxx> > --- > kernel/signal.c | 48 +++++++++++++++++++++++++++++++++++++++++++++++- > 1 files changed, 47 insertions(+), 1 deletions(-) > > diff --git a/kernel/signal.c b/kernel/signal.c > index 909a0cc..0ac614a 100644 > --- a/kernel/signal.c > +++ b/kernel/signal.c > @@ -1594,6 +1594,32 @@ static inline int may_ptrace_stop(void) > } > > /* > + * The arch can have machine-specific work to be done before ptrace stops. > + * On ia64, register backing store gets written back to user memory here. > + * Since this can be costly (requires dropping the siglock), we only do it > + * when the arch requires it for this particular stop. For example, if > + * the thread has not been back to user mode since the last stop, the > + * thread state might indicate that nothing needs to be done. > + */ > +#ifndef arch_ptrace_stop_needed > +#define arch_ptrace_stop_needed(code, info) 0 > +#endif > +#ifndef arch_ptrace_stop > +#define arch_ptrace_stop(code, info) do { } while (0) > +#endif > + > +/* > + * Return nonzero if there is a SIGKILL that should be waking us up. > + * Called with the siglock held. > + */ > +static int sigkill_pending(struct task_struct *tsk) > +{ > + return ((sigismember(&tsk->pending.signal, SIGKILL) || > + sigismember(&tsk->signal->shared_pending.signal, SIGKILL)) && > + !unlikely(sigismember(&tsk->blocked, SIGKILL))); > +} > + > +/* > * This must be called with current->sighand->siglock held. > * > * This should be the path for all ptrace stops. > @@ -1606,6 +1632,26 @@ static inline int may_ptrace_stop(void) > */ > static void ptrace_stop(int exit_code, int nostop_code, siginfo_t *info) > { > + int killed = 0; > + > + if (arch_ptrace_stop_needed(exit_code, info)) { > + /* > + * The arch code has something special to do before a > + * ptrace stop. This is allowed to block, e.g. for faults > + * on user stack pages. We can't keep the siglock while > + * calling arch_ptrace_stop, so we must release it now. > + * To preserve proper semantics, we must do this before > + * any signal bookkeeping like checking group_stop_count. > + * Meanwhile, a SIGKILL could come in before we retake the > + * siglock. That must prevent us from sleeping in TASK_TRACED. > + * So after regaining the lock, we must check for SIGKILL. > + */ > + spin_unlock_irq(¤t->sighand->siglock); > + arch_ptrace_stop(exit_code, info); > + spin_lock_irq(¤t->sighand->siglock); > + killed = sigkill_pending(current); > + } > + > /* > * If there is a group stop in progress, > * we must participate in the bookkeeping. > @@ -1621,7 +1667,7 @@ static void ptrace_stop(int exit_code, int nostop_code, siginfo_t *info) > spin_unlock_irq(¤t->sighand->siglock); > try_to_freeze(); > read_lock(&tasklist_lock); > - if (may_ptrace_stop()) { > + if (!unlikely(killed) && may_ptrace_stop()) { > do_notify_parent_cldstop(current, CLD_TRAPPED); > read_unlock(&tasklist_lock); > schedule(); - To unsubscribe from this list: send the line "unsubscribe linux-ia64" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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