linux-kernel - Re: [PATCH] riscv: entry: Save a0 prior syscall_enter_from_user

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Message-ID: <CABgGipVHV9C3QwsCkaU+ecTPqhdztn7EwWOr1Hc1ETqEWhhdxQ@mail.gmail.com>
Date:   Mon, 10 Apr 2023 17:41:57 +0800
From:   Andy Chiu <andy.chiu@...ive.com>
To:     Björn Töpel <bjorn@...nel.org>
Cc:     Albert Ou <aou@...s.berkeley.edu>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        linux-riscv@...ts.infradead.org,
        Guo Ren <guoren@...ux.alibaba.com>,
        Björn Töpel <bjorn@...osinc.com>,
        Heiko Stübner <heiko@...ech.de>,
        Conor Dooley <conor@...nel.org>,
        Yipeng Zou <zouyipeng@...wei.com>,
        linux-kernel@...r.kernel.org, Palmer Dabbelt <palmer@...osinc.com>,
        Heiko Stuebner <heiko.stuebner@...ll.eu>
Subject: Re: [PATCH] riscv: entry: Save a0 prior syscall_enter_from_user_mode()

On Mon, Apr 3, 2023 at 2:54 PM Björn Töpel <bjorn@...nel.org> wrote:
>
> From: Björn Töpel <bjorn@...osinc.com>
>
> The RISC-V calling convention passes the first argument, and the
> return value in the a0 register. For this reason, the a0 register
> needs some extra care; When handling syscalls, the a0 register is
> saved into regs->orig_a0, so a0 can be properly restored for,
> e.g. interrupted syscalls.
>
> This functionality was broken with the introduction of the generic
> entry patches. Here, a0 was saved into orig_a0 after calling
> syscall_enter_from_user_mode(), which can change regs->a0 for some
> paths, incorrectly restoring a0.
>
> This is resolved, by saving a0 prior doing the
> syscall_enter_from_user_mode() call.
>
> Fixes: f0bddf50586d ("riscv: entry: Convert to generic entry")
> Reviewed-by: Heiko Stuebner <heiko.stuebner@...ll.eu>
> Tested-by: Heiko Stuebner <heiko.stuebner@...ll.eu>
> Signed-off-by: Björn Töpel <bjorn@...osinc.com>

This fix works for me, thanks!

Tested-by: Andy Chiu <andy.chiu@...ive.com>