linux-kernel - Test 73 Sig_trap fails on arm64

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Message-Id: <20220131175526.1777801-1-dvyukov@google.com>
Date:   Mon, 31 Jan 2022 18:55:26 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     john.garry@...wei.com, will@...nel.org
Cc:     acme@...nel.org, elver@...gle.com, gor@...ux.ibm.com,
        hca@...ux.ibm.com, leo.yan@...aro.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org, mark.rutland@....com,
        sumanthk@...ux.ibm.com, svens@...ux.ibm.com, tmricht@...ux.ibm.com
Subject: Test 73 Sig_trap fails on arm64

> 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 = static_cast<ucontext_t*>(uctx)->uc_mcontext;
  mctx.pstate |= 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].  So I think we need
> >>> something like below:
> >>>
> >>> static int test__sigtrap(struct test_suite *test __maybe_unused, int subtest __maybe_unused)
> >>> {
> >>>          ...
> >>>
> >>>          if (!BP_SIGNAL_IS_SUPPORTED) {
> >>>                  pr_debug("Test not supported on this architecture");
> >>>                  return TEST_SKIP;
> >>>          }
> >>>
> >>>          ...
> >>> }
> >>>
> >>> Since we have defined BP_SIGNAL_IS_SUPPORTED, I think we can reuse it at
> >>> here.
> >>>
> >>> [1]https://lore.kernel.org/lkml/157169993406.29376.12473771029179755767.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 will
> > 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?  Thanks!
> >
> > Leo
> >
> > [1]https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/kernel/hw_breakpoint.c