On 7/17/2026 7:01 PM, Will Deacon wrote: > On Fri, Jul 17, 2026 at 01:51:12AM +0000, Hongyan Xia wrote: >> On 7/16/2026 11:20 PM, Will Deacon wrote: >>> On Thu, Jul 16, 2026 at 02:38:58PM +0000, Pu Hu wrote: >>>> On 7/16/2026 9:24 PM, Will Deacon wrote: >>>>> On Fri, Jul 10, 2026 at 06:32:55AM +0000, Pu Hu wrote: >>>>>> From: Pu Hu <[email protected]> >>>>>> >>>>>> A kprobe can be hit while another kprobe is in KPROBE_HIT_SS state. This >>>>>> can happen when tracing or perf code runs from the debug exception path >>>>>> while the first kprobe is preparing or executing its out-of-line >>>>>> single-step instruction. >>>>> >>>>> I don't understand this part. The single-step runs with debug exceptions >>>>> disabled (kprobes_save_local_irqflag() sets PSTATE.D) so how do we end >>>>> up taking one? >>>> >>>> You are right that the single-step runs with debug exceptions disabled. >>>> However, the case I was referring to is not a hardware breakpoint or a >>>> software-step exception, but another Breakpoint Instruction exception >>>> generated by executing a BRK instruction. A BRK instruction exception is >>>> not masked by PSTATE.D, so it can still be taken while handling a kprobe. >>>> >>>> As far as I understand the architecture, there are two different cases >>>> here: >>>> >>>> - Breakpoint Instruction exceptions, generated by executing a BRK >>>> instruction. >>>> - Breakpoint exceptions, generated by the debug logic, for example by >>>> programmed breakpoint registers. >>>> >>>> PSTATE.D masks debug exceptions such as hardware breakpoints, >>>> watchpoints and software-step exceptions, but it does not mask >>>> Breakpoint Instruction exceptions generated by BRK. This also seems >>>> consistent with the pseudocode for BRK, >>>> Arch64.SoftwareBreakpoint(imm16), which does not appear to check >>>> PSTATE.D before taking the exception. >>>> >>>> Therefore, even if kprobes_save_local_irqflag() sets PSTATE.D while >>>> handling the first kprobe, if the code executed from that path reaches >>>> another instruction patched with BRK, it can still take a Breakpoint >>>> Instruction exception. In other words, the nested case I mentioned is >>>> another kprobe BRK being hit, not a hardware debug exception or a >>>> software-step exception. >>> >>> Yes, that's correct, but if we're doing the out-of-line step, how do we >>> end up executing a BRK? Or are you saying that it's the kprobes >>> BRK64_OPCODE_KPROBES_SS instruction that we use to implement the >>> single-step that is the problem? If so, how does taking that exception >>> result in us executing tracing or perf code? >>> >>> Sorry for all the questions, I just haven't understood what's going on >>> here from the commit message. >> >> The key is that, when you use 'perf --call-graph dwarf' to sample >> certain events, kernel perf code will sample a piece of user stack each >> time those events are hit, and copy_to/from_user() triggers page faults. >> Say you are profiling preempt_enable events: >> >> 1st BRK -> preempt_disable() -> debug_exception() -> set SS state -> >> preempt_enable() -> triggers perf -> perf_sample() -> sample user stack >> using copy_to/from_user() -> page fault or 2nd BRK on the page fault path. >> >> The key is perf sampling the user stack while the 1st BRK is still >> running. When a page fault is hit, a can of worms is released, including >> a possible 2nd BRK. > > Thanks. So perf is run synchronously from the debug exception entry path,
Yes, exactly. > rather than because of a second exception taking place. Got it. But then > it sounds like we should really make the debug exception handling path (at > least, the part that runs for handling the kprobe step) noinstr to avoid > getting into this state to begin with. Is that practical? Not sure about making the whole path noinstr (@Masami might have a better opinion on this than me). Personally I don't mind either disallowing it or making it correct. But it might be a good idea not to diverge too much between ISAs. This patch is pretty much mirroring what the x86 side handles this situation. > > Will
