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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Tue, 4 Apr 2023 17:06:35 +0200
Message-ID: <CAMuHMdV+sqGB5V4LZAcHw_4n6=ZVPs_v0uB9V-sPTUL1EpL6eg@mail.gmail.com>
Subject: Re: [PATCH] riscv: entry: Save a0 prior syscall_enter_from_user_mode()
To:     =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>
Cc:     Albert Ou <aou@eecs.berkeley.edu>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        linux-riscv@lists.infradead.org,
        Guo Ren <guoren@linux.alibaba.com>,
        =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@rivosinc.com>,
        =?UTF-8?Q?Heiko_St=C3=BCbner?= <heiko@sntech.de>,
        Conor Dooley <conor@kernel.org>,
        Yipeng Zou <zouyipeng@huawei.com>,
        linux-kernel@vger.kernel.org, Palmer Dabbelt <palmer@rivosinc.com>,
        Heiko Stuebner <heiko.stuebner@vrull.eu>
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On Mon, Apr 3, 2023 at 8:54 AM Björn Töpel <bjorn@kernel.org> wrote:
> From: Björn Töpel <bjorn@rivosinc.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@vrull.eu>
> Tested-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
> Signed-off-by: Björn Töpel <bjorn@rivosinc.com>

Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds