linux-kernel - Re: [PATCH v3 1/4] x86/syscall: sign-extend system calls on entry to int

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Message-ID: <4127D74A-A110-4EAB-9745-46997B1B031C@zytor.com>
Date:   Mon, 17 May 2021 13:23:59 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Ingo Molnar <mingo@...nel.org>
CC:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/4] x86/syscall: sign-extend system calls on entry to int

No, this is what the Linux kernel did for a very long time.

On May 16, 2021 12:48:19 AM PDT, Ingo Molnar <mingo@...nel.org> wrote:
>
>* H. Peter Anvin <hpa@...or.com> wrote:
>
>> This is an ABI change, but is in fact a revert to the original x86-64
>> ABI. The original assembly entry code would zero-extend the system
>> call number; this patch uses sign extend to be explicit that this is
>> treated as a signed number (although in practice it makes no
>> difference, of course) and to avoid people getting the idea of
>> "optimizing" it, as has happened on at least two(!) separate
>> occasions.
>
>The original x86-64 ABI as documented by AMD, as we (probably) never
>had 
>this in Linux, right?
>
>Sounds sensible to do this, assuming nothing relies on the weirdness.
>
>Thanks,
>
>	Ingo

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