linux-kernel - Re: [PATCH 1/3] arm64: armv8_deprecated: Fix swp

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Date:   Thu, 22 Apr 2021 13:59:04 +0100
From:   Will Deacon <will@...nel.org>
To:     Liam Howlett <liam.howlett@...cle.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Julien Grall <julien.grall@....com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
Subject: Re: [PATCH 1/3] arm64: armv8_deprecated: Fix swp_handler() signal
 generation

On Tue, Apr 20, 2021 at 04:50:12PM +0000, Liam Howlett wrote:
> arm64_notify_segfault() was written to decide on the si_code from the
> assembly emulation of the swp_handler(), but was also used for the
> signal generation from failed access_ok() and unaligned instructions.
> 
> When access_ok() fails, there is no need to search for the offending
> address in the VMA space.  Instead, simply set the error to SIGSEGV with
> si_code SEGV_ACCERR.
> 
> Change the return code from emulate_swpX() when there is an unaligned
> pointer so the caller can differentiate from the EFAULT.  It is
> unnecessary to search the VMAs in the case of an unaligned pointer.
> This change uses SIGSEGV and SEGV_ACCERR instead of SIGBUS to keep with
> what was returned before.
> 
> Fixes: bd35a4adc413 (arm64: Port SWP/SWPB emulation support from arm)
> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> ---
>  arch/arm64/kernel/armv8_deprecated.c | 20 +++++++++++++-------

Can you give an example of something that is fixed by this, please? At first
glance, it doesn't look like it changes the user-visible behaviour.

We should also be compatible with arch/arm/ here (see set_segfault()).

Will