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

[Mauro Carvalho Chehab <mchehab+huawei@kernel.org>]

Em Tue, 13 Apr 2021 21:40:20 -0700
Yury Norov <yury.norov@gmail.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@gmail.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.

Thanks,
Mauro