linux-kernel - Re: [GIT PULL] thread fixes v5.7-rc5

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Date:   Thu, 14 May 2020 11:35:29 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] thread fixes v5.7-rc5

On Thu, May 14, 2020 at 11:22 AM Christian Brauner
<christian.brauner@...ntu.com> wrote:
>
> Seemed weird to me to change something that's been exposed to userspace for that long.

Well, the internal declarations aren't actually "exposed" to user
space - it's not like it's the declaration of the system call, that's
separate.

And we have done that before: we have had a lot of history of using
"unsigned long" to basically mean "register", and then ended up
cleaning up types afterwards.

In fact, if you look at the macros that do SYSCALL_DEFINE() (hint -
don't actually do it, you'll go mad) you'll see that magical
__SC_LONG() thing, which actually declares _all_ arguments as either
"unsigned long" or "unsigned long long".

That's the "native" representation of the low-level system call (it's
also marked "asmlinkage" etc).

We then end up casting them to the internal representation with that
__SC_CAST() macro.

So the actual types that we get are intentionally "cleaned up"
versions of the raw registers that were passed in.

But you really don't want to understand the __SYSCALL_DEFINEx() macro.
It's clever, but it really is the Cthulhu of macros. Just looking at
it might drive you mad.

                Linus