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

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Date:   Sun, 20 Dec 2020 21:14:34 +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 7:51 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Sun, Dec 20, 2020 at 10:22 AM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > Slightly tangential, it's not immediately clear to me why in
> > arch/x86/entry/syscalls/syscall_32.tbl epoll_pwait does not need a
> > compat entry, unlike on other architectures and unlike signalfd.
>
> Hmm. Good question. That looks like a bug to me. Probably nobody
> noticed because it's so rarely used.
>
> Or maybe I'm missing something too.
>
> Adding x86 entry code people to the participants.

The sigset_t argument is actually compatible between x86-32 and x86-64
because

- The bits are in the same order on little-endian machines
- _NSIG is the same as _COMPAT_NSIG (unlike old sparc kernels)
- accessing a 64-bit with 32-bit alignment is always allowed on x86

All other architectures with compat mode support big-endian
code at least as an option, so they have to use the compat
version.

         Arnd