From mboxrd@z Thu Jan 1 00:00:00 1970 From: ji.zhang@mediatek.com (Ji.Zhang) Date: Wed, 11 Apr 2018 14:30:28 +0800 Subject: [PATCH] arm64: avoid race condition issue in dump_backtrace In-Reply-To: <20180409112559.uh76jpiytznymw6w@lakrids.cambridge.arm.com> References: <1521687960-3744-1-git-send-email-ji.zhang@mediatek.com> <20180322055929.z25brvwlmdighz66@salmiak> <1521711329.26617.31.camel@mtksdccf07> <20180326113932.2i6qp3776jtmcqk4@lakrids.cambridge.arm.com> <1522229612.26617.47.camel@mtksdccf07> <20180328101240.moo44g5qd3qjuxgn@lakrids.cambridge.arm.com> <1522397292.26617.63.camel@mtksdccf07> <20180404090431.rqwtaqovipxa5gta@lakrids.cambridge.arm.com> <1523174328.26617.91.camel@mtksdccf07> <20180409112559.uh76jpiytznymw6w@lakrids.cambridge.arm.com> Message-ID: <1523428228.26617.100.camel@mtksdccf07> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Mon, 2018-04-09 at 12:26 +0100, Mark Rutland wrote: > On Sun, Apr 08, 2018 at 03:58:48PM +0800, Ji.Zhang wrote: > > Yes, I see where the loop is, I have missed that the loop may cross > > different stacks. > > Define a nesting order and check against is a good idea, and it can > > resolve the issue exactly, but as you mentioned before, we have no idea > > how to handle with overflow and sdei stack, and the nesting order is > > strongly related with the scenario of the stack, which means if someday > > we add another stack, we should consider the relationship of the new > > stack with other stacks. From the perspective of your experts, is that > > suitable for doing this in unwind? > > > > Or could we just find some way easier but not so accurate, eg. > > Proposal 1: > > When we do unwind and detect that the stack spans, record the last fp of > > previous stack and next time if we get into the same stack, compare it > > with that last fp, the new fp should still smaller than last fp, or > > there should be potential loop. > > For example, when we unwind from irq to task, we record the last fp in > > irq stack such as last_irq_fp, and if it unwind task stack back to irq > > stack, no matter if it is the same irq stack with previous, just let it > > go and compare the new irq fp with last_irq_fp, although the process may > > be wrong since from task stack it could not unwind to irq stack, but the > > whole process will eventually stop. > > I agree that saving the last fp per-stack could work. > > > Proposal 2: > > So far we have four types of stack: task, irq, overflow and sdei, could > > we just assume that the MAX number of stack spanning is just 3 > > times?(task->irq->overflow->sdei or task->irq->sdei->overflow), if yes, > > we can just check the number of stack spanning when we detect the stack > > spans. > > I also agree that counting the number of stack transitions will prevent > an inifinite loop, even if less accurately than proposal 1. > > I don't have a strong preference either way. Thank you for your comment. Compared with proposal 1 and 2, I decide to use proposal2 since proposal1 seems a little complicated and it is not as easy as proposal2 when new stack is added. The sample is as below: diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h index 902f9ed..72d1f34 100644 --- a/arch/arm64/include/asm/stacktrace.h +++ b/arch/arm64/include/asm/stacktrace.h @@ -92,4 +92,22 @@ static inline bool on_accessible_stack(struct task_struct *tsk, unsigned long sp return false; } +#define MAX_STACK_SPAN 3 +DECLARE_PER_CPU(int, num_stack_span); + +static inline bool on_same_stack(struct task_struct *tsk, + unsigned long sp1, unsigned long sp2) +{ + if (on_task_stack(tsk, sp1) && on_task_stack(tsk, sp2)) + return true; + if (on_irq_stack(sp1) && on_irq_stack(sp2)) + return true; + if (on_overflow_stack(sp1) && on_overflow_stack(sp2)) + return true; + if (on_sdei_stack(sp1) && on_sdei_stack(sp2)) + return true; + + return false; +} + #endif /* __ASM_STACKTRACE_H */ diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c index d5718a0..db905e8 100644 --- a/arch/arm64/kernel/stacktrace.c +++ b/arch/arm64/kernel/stacktrace.c @@ -27,6 +27,8 @@ #include #include +DEFINE_PER_CPU(int, num_stack_span); + /* * AArch64 PCS assigns the frame pointer to x29. * @@ -56,6 +58,20 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame) frame->fp = READ_ONCE_NOCHECK(*(unsigned long *)(fp)); frame->pc = READ_ONCE_NOCHECK(*(unsigned long *)(fp + 8)); + if (!on_same_stack(tsk, fp, frame->fp)) { + int num = (int)__this_cpu_read(num_stack_span); + + if (num >= MAX_STACK_SPAN) + return -EINVAL; + num++; + __this_cpu_write(num_stack_span, num); + fp = frame->fp + 0x8; + } + if (fp <= frame->fp) { + pr_notice("fp invalid, stop unwind\n"); + return -EINVAL; + } + #ifdef CONFIG_FUNCTION_GRAPH_TRACER if (tsk->ret_stack && (frame->pc == (unsigned long)return_to_handler)) { diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c index eb2d151..e6b5181 100644 --- a/arch/arm64/kernel/traps.c +++ b/arch/arm64/kernel/traps.c @@ -102,6 +102,8 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk) struct stackframe frame; int skip; + __this_cpu_write(num_stack_span, 0); + pr_debug("%s(regs = %p tsk = %p)\n", __func__, regs, tsk); if (!tsk) @@ -144,6 +146,7 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk) } while (!unwind_frame(tsk, &frame)); put_task_stack(tsk); + __this_cpu_write(num_stack_span, 0); } void show_stack(struct task_struct *tsk, unsigned long *sp) -- 1.9.1 If that is ok then I will submit a new patch. Best Regards, Ji > > Thanks, > Mark.