From: Linus Torvalds
> Sent: 25 April 2023 17:29
>
> On Tue, Apr 25, 2023 at 5:34 AM Christian Brauner <brauner@xxxxxxxxxx> 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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