linux-kernel - RE: [GIT PULL] pidfd updates

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Date:   Fri, 28 Apr 2023 08:40:31 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        Christian Brauner <brauner@...nel.org>
CC:     Al Viro <viro@...iv.linux.org.uk>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [GIT PULL] pidfd updates

From: Linus Torvalds
> Sent: 25 April 2023 17:29
> 
> On Tue, Apr 25, 2023 at 5:34 AM Christian Brauner <brauner@...nel.org> wrote:
> >
> > Hell, you could even extend that proposal below to wrap the
> > put_user()...
> >
> > struct fd_file {
> >         struct file *file;
> >         int fd;
> >         int __user *fd_user;
> > };
> 
> So I don't like this extended version, but your proposal patch below
> looks good to me.
> 
> Why? Simply because the "two-word struct" is actually a good way to
> return two values. But a three-word one would be passed on the stack.
> 
> Both gcc and clang return small structs (where "small" is literally
> just two words) in registers, and it's part of most (all?) ABIs and
> we've relied on that before.

It is definitely architecture dependant.
x86-64 and arm-64 will return two 64bit values in registers.
x86-32 and arm-32 return two 32bit values on stack.

Pretty much everything passes short structures directly by value.
(I'm not sure about sparc-32 though, I'm sure it passed all
structures by reference back in the 1980s)

	David

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