Messages in this thread |  | | Date | Mon, 18 Jan 2016 19:39:06 +0800 | From | "Wangnan (F)" <> | Subject | Re: [PATCH v2] arm64: Store breakpoint single step state into pstate |
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On 2016/1/13 1:06, Will Deacon wrote: > On Tue, Jan 05, 2016 at 01:06:15PM +0800, Wangnan (F) wrote: >> On 2016/1/5 0:55, Will Deacon wrote: >>> The problem seems to be that we take the debug exception before the >>> breakpointed instruction has been executed and call perf_bp_event at >>> that moment, so when we single-step the faulting instruction we actually >>> step into the SIGIO handler and end up getting stuck. >>> >>> Your fix doesn't really address this afaict, in that you don't (can't?) >>> handle: >>> >>> * A longjmp out of a signal handler >>> * A watchpoint and a breakpoint that fire on the same instruction >>> * User-controlled single-step from a signal handler that enables a >>> breakpoint explicitly >>> * Nested signals >> Please have a look at [1], which I improve test__bp_signal() to >> check bullet 2 and 4 you mentioned above. Seems my fix is correct. >> >> [1] http://lkml.kernel.org/g/1451969880-14877-1-git-send-email-wangnan0@huawei.com > I'm still really uneasy about this change. Pairing up the signal delivery > with the sigreturn to keep track of the debug state is extremely fragile > and I'm not keen on adding this logic there. I also think we need to > track the address that the breakpoint is originally taken on so that we > can only perform the extra sigreturn work if we're returning to the same > instruction. Furthermore, I wouldn't want to do this for signals other > than those generated directly by a breakpoint. > > An alternative would be to postpone the signal delivery until after the > stepping has been taken care of, but that's a change in ABI and I worry > we'll break somebody relying on the current behaviour. > > What exactly does x86 do? I couldn't figure it out from the code.
Actually x86 does similar thing as what this patch does.
RF bit in x86_64's eflags prohibit debug exception raises. It is set by x86_64's debug handler to avoid recursion. x86_64 need setting this bit in breakpoint handler because it needs to jump back to original instruction and single-step on it, similar to ARM64.
The RF bit in eflags records a state that the process shouldn't generate debug exception. It is part of the state of a process, and should be saved and cleared if transfers to signal handler.
This patch does the same thing: create two bits in pstate to indicate the states that 'a process should not raises watchpoint/breakpoint exceptions', maintains them in kernel, cleans them for signal handler and save them in signal frame.
Thank you.
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