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Date:   Mon, 31 Jan 2022 18:55:26 +0100
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Subject: Test 73 Sig_trap fails on arm64
From:   Dmitry Vyukov <dvyukov@google.com>
To:     john.garry@huawei.com, will@kernel.org
Cc:     acme@kernel.org, elver@google.com, gor@linux.ibm.com,
        hca@linux.ibm.com, leo.yan@linaro.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-perf-users@vger.kernel.org, mark.rutland@arm.com,
        sumanthk@linux.ibm.com, svens@linux.ibm.com, tmricht@linux.ibm.com
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> On 18/01/2022 12:43, Leo Yan wrote:
>
> Hi Will,
>
> Can you kindly check below the question from Leo on this issue?
>
> You were cc'ed earlier in this thread so should be able to find more
> context, if needed.

Hi Will, John,

I wonder if PSTATE.D flag can be used to resolve this
(similar to x86's use of EFLAGS.RF)?
I naively tried to do:

void OnSigtrap(int sig, siginfo_t* info, void* uctx) {
  auto& mctx =3D static_cast<ucontext_t*>(uctx)->uc_mcontext;
  mctx.pstate |=3D PSR_D_BIT;
}

But then I got a SIGSEGV from kernel.
But I wasn't able to track yet what part of the kernel did
not like setting of D bit.


> Cheers,
> John
>
> > On Tue, Jan 18, 2022 at 12:40:04PM +0100, Marco Elver wrote:
> >
> > [...]
> >
> >>> Both Arm and Arm64 platforms cannot support signal handler with
> >>> breakpoint, please see the details in [1]. =C2=A0So I think we need
> >>> something like below:
> >>>
> >>> static int test__sigtrap(struct test_suite *test __maybe_unused, int =
subtest __maybe_unused)
> >>> {
> >>> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0...
> >>>
> >>> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0if (!BP_SIGNAL_IS_SUPPORTED) {
> >>> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0pr_debu=
g("Test not supported on this architecture");
> >>> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0return =
TEST_SKIP;
> >>> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0}
> >>>
> >>> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0...
> >>> }
> >>>
> >>> Since we have defined BP_SIGNAL_IS_SUPPORTED, I think we can reuse it=
 at
> >>> here.
> >>>
> >>> [1]https://lore.kernel.org/lkml/157169993406.29376.124737710291797557=
67.tip-bot2@tip-bot2/
> >> Does this limitation also exist for address watchpoints? The sigtrap
> >> test does not make use of instruction breakpoints, but instead just
> >> sets up a watchpoint on access to a data address.
> > Yes, after reading the code, the flow for either instrution breakpoint
> > or watchpoint both use the single step [1], thus the signal handler wil=
l
> > take the single step execution and lead to the infinite loop.
> >
> > I am not the best person to answer this question; @Will, could you
> > confirm for this? =C2=A0Thanks!
> >
> > Leo
> >
> > [1]https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/t=
ree/arch/arm64/kernel/hw_breakpoint.c