linux-kernel - Re: [PATCH] epoll: fix compat syscall wire up of epoll

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Message-ID: <CAK8P3a2fUpnhxmOPbzD=z10_uuWQsHyEUv+YQ19wNWrxiUVwvQ@mail.gmail.com>
Date:   Mon, 21 Dec 2020 00:18:32 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Peter Anvin <hpa@...or.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        Matthew Wilcox <willy@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        "David S. Miller" <davem@...emloft.net>,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [PATCH] epoll: fix compat syscall wire up of epoll_pwait2

On Sun, Dec 20, 2020 at 10:06 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Sun, Dec 20, 2020 at 12:14 PM Arnd Bergmann <arnd@...nel.org> wrote:
> >
> > The sigset_t argument is actually compatible between x86-32 and x86-64
>
> Well, random high bits in size_t or the pointer value aren't. So it
> still looks a bit iffy to me.
>
> But it might end up working almost by accident.

The direct syscall arguments all get the correct zero-extension with
asm/syscall_wrapper.h, just like they do with any other syscall that
is used in both native and compat mode, like epoll_wait().

It probably makes sense to change it just for consistency with the other
architectures, but I would assume that it was intentional when this was
added originally, as the compat handling for epoll_pwait() went through
several iterations before it first worked correctly.

        Arnd