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From:   Keno Fischer <keno@juliacomputing.com>
Date:   Fri, 15 May 2020 07:15:35 -0400
Message-ID: <CABV8kRyHrDMK4o=UZZZWJMuQNjPA8Xuoj-JFF-Lsx26fBTR0WA@mail.gmail.com>
Subject: PTRACE_SYSEMU behavior difference on arm64
To:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Cc:     Oleg Nesterov <oleg@redhat.com>, Will Deacon <will.deacon@arm.com>,
        Sudeep Holla <sudeep.holla@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        linux-arm-kernel@lists.infradead.org
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The behavior of PTRACE_SYSEMU on arm64
appears to differ substantially from that of x86 and powerpc
(the other two architectures on which this feature is implemented).
In particular, after PTRACE_SYSEMU the syscall will always
be skipped on x86 and powerpc, but executed on arm64 unless
the syscall-entry stop was again continued using PTRACE_SYSEMU.
The skipping behavior is also documented in the manpage,
so I suspect this may just be a bug (the skipping behavior
makes sense to me and is what I would expect).
The reason this happens is that `syscall_trace_enter`
re-checks TIF_SYSCALL_EMU after the ptrace stop, but at that
point it may have already been superseded by a new ptrace
request. x86 and power save the original value of the flag,
rather than acting on the new value. I can submit a patch to
fix this, but wanted to check first whether this was intentional.
If it is, I can fix the man page instead.

Keno