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From:   Lai Jiangshan <jiangshanlai@gmail.com>
Date:   Fri, 20 May 2022 09:11:55 +0800
Message-ID: <CAJhGHyC0sztc2_jTT+U1M2+OSNxfNgmaKJyZgySwYqKM06q7hg@mail.gmail.com>
Subject: Re: [PATCH 3/6] x86/entry: Use PUSH_AND_CLEAR_REGS for compat
To:     Josh Poimboeuf <jpoimboe@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Guenter Roeck <linux@roeck-us.net>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Brian Gerst <brgerst@gmail.com>,
        Andrew Cooper <Andrew.Cooper3@citrix.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Borislav Petkov <bp@suse.de>
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On Fri, May 20, 2022 at 1:35 AM Josh Poimboeuf <jpoimboe@kernel.org> wrote:
>
> diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
> index ed2be3615b50..f76e674d22c4 100644
> --- a/arch/x86/entry/entry_64_compat.S
> +++ b/arch/x86/entry/entry_64_compat.S
> @@ -200,7 +200,7 @@ SYM_INNER_LABEL(entry_SYSCALL_compat_safe_stack, SYM_L_GLOBAL)
>  SYM_INNER_LABEL(entry_SYSCALL_compat_after_hwframe, SYM_L_GLOBAL)
>         movl    %eax, %eax              /* discard orig_ax high bits */
>         pushq   %rax                    /* pt_regs->orig_ax */
> -       PUSH_AND_CLEAR_REGS rax=$-ENOSYS
> +       PUSH_AND_CLEAR_REGS rcx=%rbp rax=$-ENOSYS

Some comments need to be here to explain why %rcx is stashed in %rbp.

The code doing the stash in userspace may be in
arch/x86/entry/vdso/vdso32/system_call.S (see SYSCALL_SEQUENCE)

Thanks
Lai