linux-kernel - Re: [RFC][PATCH 1/5] exit: kill struct waitid

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Date:   Wed, 24 Jul 2019 10:37:34 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Christian Brauner <christian@...uner.io>
Cc:     Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tejun Heo <tj@...nel.org>, David Howells <dhowells@...hat.com>,
        Jann Horn <jannh@...gle.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [RFC][PATCH 1/5] exit: kill struct waitid_info

On Wed, Jul 24, 2019 at 7:47 AM Christian Brauner <christian@...uner.io> wrote:
>
> The code here uses a struct waitid_info to catch basic information about
> process exit including the pid, uid, status, and signal that caused the
> process to exit. This information is then stuffed into a struct siginfo
> for the waitid() syscall. That seems like an odd thing to do. We can
> just pass down a siginfo_t struct directly which let's us cleanup and
> simplify the whole code quite a bit.

Ack. Except I'd like the commit message to explain where this comes
from instead of that "That seems like an odd thing to do".

The _original_ reason for "struct waitid_info" was that "siginfo_t" is
huge because of all the insane padding that various architectures do.

So it was introduced by commit 67d7ddded322 ("waitid(2): leave copyout
of siginfo to syscall itself") very much to avoid stack usage issues.
And I quote:

    collect the information needed for siginfo into
    a small structure (waitid_info)

simply because "sigset_t" was big.

But that size came from the explicit "pad it out to 128 bytes for
future expansion that will never happen", and the kernel using the
same exact sigset_t that was exposed to user space.

Then in commit 4ce5f9c9e754 ("signal: Use a smaller struct siginfo in
the kernel") we got rid of the insane padding for in-kernel use,
exactly because it causes issues like this.

End result: that "struct waitid_info" no longer makes sense. It's not
appreciably smaller than kernel_siginfo_t is today, but it made sense
at the time.

                 Linus