Messages in this thread | | | From | "Eric W. Biederman" <> | Date | Mon, 28 Mar 2022 09:41:37 -0500 | Subject | Re: [PATCH] signal/x86: Delay calling signals in atomic |
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"Eric W. Biederman" <ebiederm@xmission.com> writes:
> Sebastian Andrzej Siewior <bigeasy@linutronix.de> writes: > >> From: Oleg Nesterov <oleg@redhat.com> >> Date: Tue, 14 Jul 2015 14:26:34 +0200 >> >> On x86_64 we must disable preemption before we enable interrupts >> for stack faults, int3 and debugging, because the current task is using >> a per CPU debug stack defined by the IST. If we schedule out, another task >> can come in and use the same stack and cause the stack to be corrupted >> and crash the kernel on return. >> >> When CONFIG_PREEMPT_RT is enabled, spinlock_t locks become sleeping, and >> one of these is the spin lock used in signal handling. >> >> Some of the debug code (int3) causes do_trap() to send a signal. >> This function calls a spinlock_t lock that has been converted to a >> sleeping lock. If this happens, the above issues with the corrupted >> stack is possible. >> >> Instead of calling the signal right away, for PREEMPT_RT and x86, >> the signal information is stored on the stacks task_struct and >> TIF_NOTIFY_RESUME is set. Then on exit of the trap, the signal resume >> code will send the signal when preemption is enabled. > > Folks I really would have appreciated being copied on a signal handling > patch like this. > > It is too late to nack, but I think this buggy patch deserved one. Can > we please fix PREEMPT_RT instead? > > As far as I can tell this violates all of rules from > implementing/maintaining the RT kernel. Instead of coming up with new > abstractions that makes sense and can use by everyone this introduces > a hack only for PREEMPT_RT and a pretty horrible one at that. > > This talks about int3, but the code looks for in_atomic(). Which means > that essentially every call of force_sig will take this path as they > almost all come from exception handlers. It is the nature of signals > that report on faults. An exception is raised and the kernel reports it > to userspace with a fault signal (aka force_sig_xxx). > > Further this code is buggy. TIF_NOTIFY_RESUME is not the correct > flag to set to enter into exit_to_usermode_loop. TIF_NOTIFY_RESUME is > about that happens after signal handling. This very much needs to be > TIF_SIGPENDING with recalc_sigpending and friends updated to know about > "task->force_info". > > Does someone own this problem? Can that person please fix this > properly? > > I really don't think it is going to be maintainable for PREEMPT_RT to > maintain a separate signal delivery path for faults from the rest of > linux.
I want to say the patch below looks like it was a perfectly fine debug patch to see if what someone thinks is the issue is the issue. It is not a good final solution for the reasons I have already mentioned.
May I ask where the rest of the conversation was? I can only find the single posting of this patch on linux-kernel without any conversation, and the description indicates this change has seen several rounds of development.
Eric
>> [ rostedt: Switched from #ifdef CONFIG_PREEMPT_RT to >> ARCH_RT_DELAYS_SIGNAL_SEND and added comments to the code. ] >> [bigeasy: Add on 32bit as per Yang Shi, minor rewording. ] >> >> Signed-off-by: Oleg Nesterov <oleg@redhat.com> >> Signed-off-by: Steven Rostedt <rostedt@goodmis.org> >> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> >> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> >> --- >> arch/x86/include/asm/signal.h | 13 +++++++++++++ >> include/linux/sched.h | 3 +++ >> kernel/entry/common.c | 9 +++++++++ >> kernel/signal.c | 28 ++++++++++++++++++++++++++++ >> 4 files changed, 53 insertions(+) >> >> diff --git a/arch/x86/include/asm/signal.h b/arch/x86/include/asm/signal.h >> index 2dfb5fea13aff..fc03f4f7ed84c 100644 >> --- a/arch/x86/include/asm/signal.h >> +++ b/arch/x86/include/asm/signal.h >> @@ -28,6 +28,19 @@ typedef struct { >> #define SA_IA32_ABI 0x02000000u >> #define SA_X32_ABI 0x01000000u >> >> +/* >> + * Because some traps use the IST stack, we must keep preemption >> + * disabled while calling do_trap(), but do_trap() may call >> + * force_sig_info() which will grab the signal spin_locks for the >> + * task, which in PREEMPT_RT are mutexes. By defining >> + * ARCH_RT_DELAYS_SIGNAL_SEND the force_sig_info() will set >> + * TIF_NOTIFY_RESUME and set up the signal to be sent on exit of the >> + * trap. >> + */ >> +#if defined(CONFIG_PREEMPT_RT) >> +#define ARCH_RT_DELAYS_SIGNAL_SEND >> +#endif >> + >> #ifndef CONFIG_COMPAT >> #define compat_sigset_t compat_sigset_t >> typedef sigset_t compat_sigset_t; >> diff --git a/include/linux/sched.h b/include/linux/sched.h >> index 75ba8aa60248b..0514237cee3fc 100644 >> --- a/include/linux/sched.h >> +++ b/include/linux/sched.h >> @@ -1087,6 +1087,9 @@ struct task_struct { >> /* Restored if set_restore_sigmask() was used: */ >> sigset_t saved_sigmask; >> struct sigpending pending; >> +#ifdef CONFIG_PREEMPT_RT >> + struct kernel_siginfo forced_info; >> +#endif >> unsigned long sas_ss_sp; >> size_t sas_ss_size; >> unsigned int sas_ss_flags; >> diff --git a/kernel/entry/common.c b/kernel/entry/common.c >> index bad713684c2e3..216dbf46e05f5 100644 >> --- a/kernel/entry/common.c >> +++ b/kernel/entry/common.c >> @@ -162,6 +162,15 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs, >> if (ti_work & _TIF_NEED_RESCHED) >> schedule(); >> >> +#ifdef ARCH_RT_DELAYS_SIGNAL_SEND >> + if (unlikely(current->forced_info.si_signo)) { >> + struct task_struct *t = current; >> + >> + force_sig_info(&t->forced_info); >> + t->forced_info.si_signo = 0; >> + } >> +#endif >> + >> if (ti_work & _TIF_UPROBE) >> uprobe_notify_resume(regs); >> >> diff --git a/kernel/signal.c b/kernel/signal.c >> index 9b04631acde8f..cb2b28c17c0a5 100644 >> --- a/kernel/signal.c >> +++ b/kernel/signal.c >> @@ -1327,6 +1327,34 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t, >> struct k_sigaction *action; >> int sig = info->si_signo; >> >> + /* >> + * On some archs, PREEMPT_RT has to delay sending a signal from a trap >> + * since it can not enable preemption, and the signal code's spin_locks >> + * turn into mutexes. Instead, it must set TIF_NOTIFY_RESUME which will >> + * send the signal on exit of the trap. >> + */ >> +#ifdef ARCH_RT_DELAYS_SIGNAL_SEND >> + if (in_atomic()) { >> + struct task_struct *t = current; >> + >> + if (WARN_ON_ONCE(t->forced_info.si_signo)) >> + return 0; >> + >> + if (is_si_special(info)) { >> + WARN_ON_ONCE(info != SEND_SIG_PRIV); >> + t->forced_info.si_signo = info->si_signo; >> + t->forced_info.si_errno = 0; >> + t->forced_info.si_code = SI_KERNEL; >> + t->forced_info.si_pid = 0; >> + t->forced_info.si_uid = 0; >> + } else { >> + t->forced_info = *info; >> + } >> + >> + set_tsk_thread_flag(t, TIF_NOTIFY_RESUME); >> + return 0; >> + } >> +#endif >> spin_lock_irqsave(&t->sighand->siglock, flags); >> action = &t->sighand->action[sig-1]; >> ignored = action->sa.sa_handler == SIG_IGN;
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