Messages in this thread | | | Date | Tue, 27 Oct 2020 09:15:59 +0100 | From | Peter Zijlstra <> | Subject | Re: [REGRESSION] x86/debug: After PTRACE_SINGLESTEP DR_STEP is no longer reported in dr6 |
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On Mon, Oct 26, 2020 at 04:45:18PM -0700, Andy Lutomirski wrote:
> PeterZ, this new scheme of having handlers clear bits in dr6 to > consume them and set bits in virtual_dr6 to send signals is > incomprehensible -- there is no possible way to read traps.c and tell > what the code does :(
IIRC, it was you who suggested it. Also the old code was equally incomprehensible. Now it has actual semantics.
> I attached a test case. I'll make a real patch out of this in a bit. > This passes on 5.8, and I haven't tested it yet on 5.10-rc1. The real > patch will also test ICEBP, and I'm sure we'll be quite unhappy with > the result of that.
> diff --git a/tools/testing/selftests/x86/single_step_syscall.c b/tools/testing/selftests/x86/single_step_syscall.c > index 5a4c6e06872e..f6abefd4066e 100644 > --- a/tools/testing/selftests/x86/single_step_syscall.c > +++ b/tools/testing/selftests/x86/single_step_syscall.c > @@ -72,7 +72,6 @@ static unsigned char altstack_data[SIGSTKSZ]; > static void sigtrap(int sig, siginfo_t *info, void *ctx_void) > { > ucontext_t *ctx = (ucontext_t*)ctx_void; > - unsigned long dr6 = info->si_code; > > if (get_eflags() & X86_EFLAGS_TF) { > set_eflags(get_eflags() & ~X86_EFLAGS_TF); > @@ -89,7 +88,10 @@ static void sigtrap(int sig, siginfo_t *info, void *ctx_void) > (unsigned long)ctx->uc_mcontext.gregs[REG_IP]); > } > > - printf("DR6 = 0x%lx\n", dr6); > + if (info->si_code != TRAP_TRACE) { > + printf("[FAIL]\tsi_code was 0x%lx; should have been TRAP_TRACE (0x%lx)\n", (unsigned long)info->si_code, (unsigned long)TRAP_TRACE); > + _exit(1); > + } > }
That actually works (tested). Nothing here cares about the virtual_dr6 value.
Think of virtual_dr6 as the value used by ptrace_{get,set}_debugreg(6). It (should) only contain bits that results from other ptrace() actions.
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