Shaohua Li wrote: > On Tue, 2007-11-13 at 12:07 +0100, Petr Tesarik wrote: >> On Mon, 2007-11-12 at 16:30 -0800, Roland McGrath wrote: >>> [...] >>> If you do the artificial test using a long sleep in arch_ptrace_stop, >>> then you can probably produce this by hand with gdb. Have the process >>> doing raise(SIGCHLD) or some other harmless signal. The traced >>> process will stop to report the signal to gdb, and then gdb will sit >>> at the prompt before resuming it (given "handle SIGFOO stop" if not default). >>> If your sleep is long enough, it won't be hard to get your SIGKILL in there. >>> Then when gdb is sitting, the traced process may still be sitting too. >>> But it should have gone away instantly from SIGKILL. >> I found it extremely difficult to trigger the race condition without the >> articifial test - arch_ptrace_stop() only sleeps if the user page is not >> present, but in the common case the register stack backing store will >> have been quite recently accessed by the process. >> >> It should be possible to create a large file, flush the page cache, put >> the RSE into lazy mode, flush it and map the register stack from that >> file, so that no memory accesses to the backing store are done before >> ptrace_stop(), but for the time being I placed an msleep(100) after >> arch_ptrace_stop(). >> >> Anyway, I produced a test case which succeeds when the call to >> sigkill_pending() is in and fails when it's commented out. I'm attaching >> it here (the kernel patch to follow). > So without the sigkill_pending() check, your test case will print an > error, and otherwise, nothing. This is expected output, right? Looks I > can reproduce it here. Yes - you can also check the exit code - it is TEST_OK (=0) if the test passes, TEST_FAILED (=1) if it fails in a known way and TEST_ABORTED (=2) if something went wrong unexpectedly. I was able to manage my test case to run even without the msleep() - I'll send the instruction in the following mail. Anyway, I was finally running with the following patch: diff -ru linux-2.6.orig/arch/ia64/kernel/ptrace.c linux-2.6/arch/ia64/kernel/ptrace.c --- linux-2.6.orig/arch/ia64/kernel/ptrace.c 2007-10-09 22:31:38.000000000 +0200 +++ linux-2.6/arch/ia64/kernel/ptrace.c 2007-11-13 11:34:03.664000000 +0100 @@ -547,6 +547,64 @@ return 0; } +static long +ia64_sync_kernel_rbs (struct task_struct *child, struct switch_stack *sw, + unsigned long user_rbs_start, unsigned long user_rbs_end) +{ + unsigned long addr, val; + long ret; + + /* now copy word for word from user rbs to kernel rbs: */ + for (addr = user_rbs_start; addr < user_rbs_end; addr += 8) { + if (access_process_vm(child, addr, &val, sizeof(val), 0) + != sizeof(val)) + return -EIO; + + ret = ia64_poke(child, sw, user_rbs_end, addr, val); + if (ret < 0) + return ret; + } + return 0; +} + +#define RBS_SYNC_TO_USER 0 +#define RBS_SYNC_TO_KERNEL 1 +static void do_sync_rbs(struct unw_frame_info *info, void *arg) +{ + struct pt_regs *pt; + unsigned long urbs_end; + int to_user = (unsigned long)arg; + + if (unw_unwind_to_user(info) < 0) + return; + pt = task_pt_regs(info->task); + urbs_end = ia64_get_user_rbs_end(info->task, pt, NULL); + + if (to_user == RBS_SYNC_TO_USER) + ia64_sync_user_rbs(info->task, info->sw, pt->ar_bspstore, urbs_end); + else + ia64_sync_kernel_rbs(info->task, info->sw, pt->ar_bspstore, urbs_end); +} + +/* + * when a thread is stopped (ptraced), debugger might change thread's user + * stack (change memory directly), and we must avoid the RSE stored in kernel + * to override user stack (user space's RSE is newer than kernel's in the + * case). To workaround the issue, we copy kernel RSE to user RSE before the + * task is stopped, so user RSE has updated data. we then copy user RSE to + * kernel after the task is resummed from traced stop and kernel will use the + * newer RSE to return to user. + */ +void ia64_ptrace_stop(void) +{ + unw_init_running(do_sync_rbs, (void *)RBS_SYNC_TO_USER); +} + +void ia64_ptrace_resume(void) +{ + unw_init_running(do_sync_rbs, (void *)RBS_SYNC_TO_KERNEL); +} + static inline int thread_matches (struct task_struct *thread, unsigned long addr) { diff -ru linux-2.6.orig/include/asm-ia64/ptrace.h linux-2.6/include/asm-ia64/ptrace.h --- linux-2.6.orig/include/asm-ia64/ptrace.h 2007-10-09 22:31:38.000000000 +0200 +++ linux-2.6/include/asm-ia64/ptrace.h 2007-11-13 11:35:17.848000000 +0100 @@ -303,6 +303,12 @@ extern void ia64_increment_ip (struct pt_regs *pt); extern void ia64_decrement_ip (struct pt_regs *pt); + extern void ia64_ptrace_stop(void); + extern void ia64_ptrace_resume(void); + + #define arch_ptrace_stop ia64_ptrace_stop + #define arch_ptrace_resume ia64_ptrace_resume + #define arch_ptrace_stop_needed() 1 #endif /* !__KERNEL__ */ /* pt_all_user_regs is used for PTRACE_GETREGS PTRACE_SETREGS */ diff -ru linux-2.6.orig/include/linux/ptrace.h linux-2.6/include/linux/ptrace.h --- linux-2.6.orig/include/linux/ptrace.h 2007-10-09 22:31:38.000000000 +0200 +++ linux-2.6/include/linux/ptrace.h 2007-11-13 11:32:01.568000000 +0100 @@ -128,6 +128,18 @@ #define force_successful_syscall_return() do { } while (0) #endif +#ifndef arch_ptrace_stop +#define arch_ptrace_stop() do { } while (0) +#endif + +#ifndef arch_ptrace_resume +#define arch_ptrace_resume() do { } while (0) +#endif + +#ifndef arch_ptrace_stop_needed +#define arch_ptrace_stop_needed() 0 +#endif + #endif #endif diff -ru linux-2.6.orig/kernel/signal.c linux-2.6/kernel/signal.c --- linux-2.6.orig/kernel/signal.c 2007-11-13 14:19:35.267851100 +0100 +++ linux-2.6/kernel/signal.c 2007-11-13 11:37:49.236000000 +0100 @@ -1569,6 +1570,17 @@ } /* + * Return non-zero 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. @@ -1581,6 +1593,26 @@ */ static void ptrace_stop(int exit_code, int nostop_code, siginfo_t *info) { + int killed = 0; + + if (arch_ptrace_stop_needed()) { + /* + * 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(); + spin_lock_irq(¤t->sighand->siglock); + killed = sigkill_pending(current); + } + /* * If there is a group stop in progress, * we must participate in the bookkeeping. @@ -1596,10 +1630,11 @@ spin_unlock_irq(¤t->sighand->siglock); try_to_freeze(); read_lock(&tasklist_lock); - if (may_ptrace_stop()) { + if (likely(!killed) && may_ptrace_stop()) { do_notify_parent_cldstop(current, CLD_TRAPPED); read_unlock(&tasklist_lock); schedule(); + arch_ptrace_resume(); } else { /* * By the time we got the lock, our tracer went away. - 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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