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@xxxxxxxxx> 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@xxxxxxxxx> > > > > --- > > > > 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. > From 9035258aaa23c5e1bb5bc2242f97221a3e5b9a87 Mon Sep 17 00:00:00 2001 > From: Christian Brauner <christian.brauner@xxxxxxxxxx> > Date: Fri, 4 Sep 2020 14:27:47 +0200 > Subject: [PATCH 1/6] docs: split extensibility section into two subsections ... -- Sincerely yours, Mike.
