linux-kernel - Re: [PATCH 0/5] Enable pt_regs based syscalls for x86-32 native

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Message-ID: <CAMzpN2iQuaNdTdL6G1rGbUFo+r16iRFo1zbiD_VMrrjtGf0acw@mail.gmail.com>
Date:   Fri, 21 Feb 2020 08:15:19 -0500
From:   Brian Gerst <brgerst@...il.com>
To:     Dominik Brodowski <linux@...inikbrodowski.net>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH 0/5] Enable pt_regs based syscalls for x86-32 native

On Fri, Feb 21, 2020 at 2:07 AM Dominik Brodowski
<linux@...inikbrodowski.net> wrote:
>
> Brian,
>
> On Fri, Feb 21, 2020 at 12:09:29AM -0500, Brian Gerst wrote:
> > This patch series cleans up the x86 syscall wrapper code and converts
> > the 32-bit native kernel over to pt_regs based syscalls.
>
> thanks for your patchset. Could you explain a bit more what the rationale
> is. Due to asmlinkage, it doesn't leak "random user-provided register
> content down the call chain" (as was the case for x86-64). But it may be
> cleaner, and you mention in patch 5/5 that the new way is "a bit more
> efficient" -- do you have numbers?

The main rationale for this patch set is to make the 32-bit native
kernel consistent with the 64-bit kernel.  It's also slightly more
efficient because the old code pushed all 6 arguments onto the stack
whereas the new code only reads the args the syscall needs, with the
pt_regs pointer passed in through a register.  By efficient I mean
that it uses fewer instructions and stack accesses, not that it will
actually have a significant difference on a benchmark.

--
Brian Gerst