linux-kernel - PTRACE_SYSEMU behavior difference on arm64

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Date:   Fri, 15 May 2020 07:15:35 -0400
From:   Keno Fischer <keno@...iacomputing.com>
To:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:     Oleg Nesterov <oleg@...hat.com>, Will Deacon <will.deacon@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org
Subject: PTRACE_SYSEMU behavior difference on arm64

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point it may have already been superseded by a new ptrace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