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From:   Andy Lutomirski <luto@kernel.org>
Date:   Fri, 30 Nov 2018 08:35:45 -0800
Message-ID: <CALCETrW2aphWwEY4=hUwo_JBCvkQyMjxzxGd9FCW017kMLaMOQ@mail.gmail.com>
Subject: Re: [PATCH v2] signal: add procfd_signal() syscall
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Christian Brauner <christian@brauner.io>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Andrew Lutomirski <luto@kernel.org>,
        Florian Weimer <fweimer@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "Serge E. Hallyn" <serge@hallyn.com>, Jann Horn <jannh@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Daniel Colascione <dancol@google.com>,
        Tim Murray <timmurray@google.com>,
        linux-man <linux-man@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>
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On Fri, Nov 30, 2018 at 3:41 AM Arnd Bergmann <arnd@arndb.de> wrote:
> siginfo_t as it is now still has a number of other downsides, and Andy in
> particular didn't like the idea of having three new variants on x86
> (depending on how you count). His alternative suggestion of having
> a single syscall entry point that takes a 'signfo_t __user *' but interprets
> it as compat_siginfo depending on in_compat_syscall()/in_x32_syscall()
> should work correctly, but feels wrong to me, or at least inconsistent
> with how we do this elsewhere.

If everyone else is okay with it, I can get on board with three
variants on x86.  What I can't get on board with is *five* variants on
x86, which would be:

procfd_signal via int80 / the 32-bit vDSO: the ia32 structure

syscall64 with nr == 335 (presumably): 64-bit

syscall64 with nr == 548 | 0x40000000: x32

syscall64 with nr == 548: 64-bit entry but in_compat_syscall() ==
true, behavior is arbitrary

syscall64 with nr == 335 | 0x40000000: x32 entry, but
in_compat_syscall() == false, behavior is arbitrary

This mess isn't really Christian's fault -- it's been there for a
while, but it's awful and I don't think we want to perpetuate it.

Obviously, I'd prefer a variant where the structure that's passed in
is always the same.

BTW, do we consider siginfo_t to be extensible?  If so, and if we pass
in a pointer, presumably we should pass a length as well.