From: Keno Fischer <keno@juliacomputing.com>
To: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
Luis Machado <luis.machado@linaro.org>,
kernel-team@android.com, stable@vger.kernel.org,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 1/2] arm64: Override SPSR.SS when single-stepping is enabled
Date: Wed, 3 Jun 2020 11:42:46 -0400 [thread overview]
Message-ID: <CABV8kRwrnixNc074-jQhZzeucGHx9_e5FnQmBS=VuL=tFGjY-Q@mail.gmail.com> (raw)
In-Reply-To: <20200603151033.11512-2-will@kernel.org>
On Wed, Jun 3, 2020 at 11:10 AM Will Deacon <will@kernel.org> wrote:
>
> Luis reports that, when reverse debugging with GDB, single-step does not
> function as expected on arm64:
>
> | I've noticed, under very specific conditions, that a PTRACE_SINGLESTEP
> | request by GDB won't execute the underlying instruction. As a consequence,
> | the PC doesn't move, but we return a SIGTRAP just like we would for a
> | regular successful PTRACE_SINGLESTEP request.
>
> The underlying problem is that when the CPU register state is restored
> as part of a reverse step, the SPSR.SS bit is cleared and so the hardware
> single-step state can transition to the "active-pending" state, causing
> an unexpected step exception to be taken immediately if a step operation
> is attempted.
We saw this issue also and worked around it in user-space [1]. That said,
I think I'm ok with this change in the kernel, since I can't think of
a particularly useful usecase for this feature.
However, at the same time as changing this, we should probably make sure
to enable the syscall exit pseudo-singlestep trap (similar issue as the other
patch I had sent for the signal pseudo-singlestep trap), since otherwise
ptracers might get confused about the lack of singlestep trap during a
singlestep -> seccomp -> singlestep path (which would give one trap
less with this patch than before).
Keno
[1] https://github.com/mozilla/rr/blob/36aa5328a2240dc3d794c14926e0754f66ee28e0/src/Task.cc#L1352-L1362
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next prev parent reply other threads:[~2020-06-03 15:43 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-03 15:10 [PATCH 0/2] arm64: Fix single-stepping with reverse debugging Will Deacon
2020-06-03 15:10 ` [PATCH 1/2] arm64: Override SPSR.SS when single-stepping is enabled Will Deacon
2020-06-03 15:42 ` Keno Fischer [this message]
2020-06-03 15:53 ` Will Deacon
2020-06-03 16:56 ` Keno Fischer
2020-06-04 8:32 ` Will Deacon
2020-06-04 22:32 ` Keno Fischer
2020-06-05 4:50 ` Luis Machado
2020-06-05 20:12 ` Keno Fischer
2020-06-03 15:10 ` [PATCH 2/2] arm64: Use test_tsk_thread_flag() for checking TIF_SINGLESTEP Will Deacon
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