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. Thanks, Mauro
