linux-kernel - Re: [PATCH] Documentation: syscalls: add a note about ABI-agnostic types

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Date:   Wed, 14 Apr 2021 15:38:47 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Yury Norov <yury.norov@...il.com>, linux-api@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Alexander A. Klimov" <grandmaster@...klimov.de>,
        André Almeida <andrealmeid@...labora.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>, David Sterba <dsterba@...e.com>,
        Joe Perches <joe@...ches.com>,
        Jonathan Corbet <corbet@....net>,
        Aleksa Sarai <cyphar@...har.com>
Subject: Re: [PATCH] Documentation: syscalls: add a note about  ABI-agnostic
 types

On Wed, Apr 14, 2021 at 12:46:01PM +0300, Mike Rapoport wrote:
> On Wed, Apr 14, 2021 at 10:46:05AM +0200, Christian Brauner wrote:
> > On Wed, Apr 14, 2021 at 08:14:22AM +0200, Mauro Carvalho Chehab wrote:
> > > Em Tue, 13 Apr 2021 21:40:20 -0700
> > > Yury Norov <yury.norov@...il.com> escreveu:
> > > 
> > > > Ping?
> > > > 
> > > > On Fri, Apr 09, 2021 at 01:43:04PM -0700, Yury Norov wrote:
> > > > > Recently added memfd_secret() syscall had a flags parameter passed
> > > > > as unsigned long, which requires creation of compat entry for it.
> > > > > It was possible to change the type of flags to unsigned int and so
> > > > > avoid bothering with compat layer.
> > > > > 
> > > > > https://www.spinics.net/lists/linux-mm/msg251550.html
> > > > > 
> > > > > Documentation/process/adding-syscalls.rst doesn't point clearly about
> > > > > preference of ABI-agnostic types. This patch adds such notification.
> > > > > 
> > > > > Signed-off-by: Yury Norov <yury.norov@...il.com>
> > > > > ---
> > > > >  Documentation/process/adding-syscalls.rst | 7 +++++++
> > > > >  1 file changed, 7 insertions(+)
> > > > > 
> > > > > diff --git a/Documentation/process/adding-syscalls.rst b/Documentation/process/adding-syscalls.rst
> > > > > index 9af35f4ec728..46add16edf14 100644
> > > > > --- a/Documentation/process/adding-syscalls.rst
> > > > > +++ b/Documentation/process/adding-syscalls.rst
> > > > > @@ -172,6 +172,13 @@ arguments (i.e. parameter 1, 3, 5), to allow use of contiguous pairs of 32-bit
> > > > >  registers.  (This concern does not apply if the arguments are part of a
> > > > >  structure that's passed in by pointer.)
> > > > >  
> > > > > +Whenever possible, try to use ABI-agnostic types for passing parameters to
> > > > > +a syscall in order to avoid creating compat entry for it. Linux supports two
> > > > > +ABI models - ILP32 and LP64. 
> > > 
> > > > > + The types like ``void *``, ``long``, ``size_t``,
> > > > > +``off_t`` have different size in those ABIs;
> > > 
> > > In the case of pointers, the best is to use __u64. The pointer can then
> > > be read on Kernelspace with something like this:
> > > 
> > > 	static inline void __user *media_get_uptr(__u64 arg)
> > > 	{
> > > 		return (void __user *)(uintptr_t)arg;
> > > 	}
> > > 
> > > 
> > > > > types like ``char`` and  ``int``
> > > > > +have the same size and don't require a compat layer support. For flags, it's
> > > > > +always better to use ``unsigned int``.
> > > > > +
> > > 
> > > I don't think this is true for all compilers on userspace, as the C
> > > standard doesn't define how many bits an int/unsigned int has. 
> > > So, even if this is today's reality, things may change in the future.
> > > 
> > > For instance, I remember we had to replace "int" and "enum" by "__u32" 
> > > and "long" by "__u64" at the media uAPI in the past, when we start
> > > seeing x86_64 Kernels with 32-bits userspace and when cameras started 
> > > being supported on arm32.
> > > 
> > > We did have some real bugs with "enum", as, on that time, some
> > > compilers (gcc, I guess) were optimizing them to have less than
> > > 32 bits on certain architectures, when it fits.
> > 
> > Fwiw, Aleksa and I have written extended syscall documentation
> > documenting the agreement that we came to in a dedicated session with a
> > wide range of kernel folks during Linux Plumbers last year. We simply
> > never had time to actually send this series but fwiw here it is. It also
> > mentions the use of correct types. If people feel it's worth it I can
> > send as a proper series:
> 
> Yes, please.

Ok, I'll try to fix the commit messages and send it out.

Christian