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. Hongyan
