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

[Yury Norov <yury.norov@gmail.com>]

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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; 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``.
> +
>  
>  Proposing the API
>  -----------------
> -- 
> 2.25.1