Hi Christian, Thanks a bunch for chiming in. On Thu, Nov 24, 2022 at 01:49:27PM +0100, Christian Brauner wrote: > Alternatively, you could also introduce a simple struct versioned by > size for this system call similar to mount_setatt() and clone3() and so > on. This way you don't need to worry about future extensibilty. Just a > thought. Briefly considered that, but it seemed a bit heavy for something like this. I'm not super heavily opposed, but just seemed like a bit much. > > > >> >> > +SYSCALL_DEFINE3(vgetrandom_alloc, unsigned long __user *, num, > > > >> >> > + unsigned long __user *, size_per_each, unsigned int, flags) > > > >> >> > > > >> >> I think you should make this __u64, so that you get a consistent > > > >> >> userspace interface on all architectures, without the need for compat > > > >> >> system calls. > > > >> > > > > >> > That would be quite unconventional. Most syscalls that take lengths do > > > >> > so with the native register size (`unsigned long`, `size_t`), rather > > > >> > than u64. If you can point to a recent trend away from this by > > > >> > indicating some commits that added new syscalls with u64, I'd be happy > > > >> > to be shown otherwise. But AFAIK, that's not the way it's done. > > > >> > > > >> See clone3 and struct clone_args. > > For system calls that take structs as arguments we use u64 in the struct > for proper alignment so we can extend structs without regressing old > kernels. We have a few of those extensible struct system calls. > > But we don't really have a lot system calls that pass u64 as a pointer > outside of a structure so far. Neither as register and nor as pointer > iirc. Right, the __u64_aligned business seemed to be mostly about extensibility. > > > > The struct is one thing. But actually, clone3 takes a `size_t`: > > > > > > > > SYSCALL_DEFINE2(clone3, struct clone_args __user *, uargs, size_t, size) > > > > > > > > I take from this that I too should use `size_t` rather than `unsigned > > > > long.` And it doesn't seem like there's any compat clone3. > > > > > > But vgetrandom_alloc does not use unsigned long, but unsigned long *. > > > You need to look at the contents for struct clone_args for comparison. > > > > Ah! I see what you mean; that's a good point. The usual register > > clearing thing isn't going to happen because these are addresses. > > > > I still am somewhat hesitant, though, because `size_t` is really the > > "proper" type to be used. Maybe the compat syscall thing is just a > > necessary evil? > > I think making this a size_t is fine. We haven't traditionally used u32 > for sizes. All syscalls that pass structs versioned by size use size_t. > So I would recommend to stick with that. This isn't quite a struct versioned by size. This is: void *vgetrandom_alloc([inout] size_t *num, [out] size_t *size_per_each, unsigned int flags); You give it an input 'num' and some flags (currently flags=0), and it gives you back an output 'num' size, an output 'size_per_each' size, and an opaque pointer value mapping as its return value. I do like the idea of keeping size_t so that the type is "right". But the other arguments are equally compelling as well, so not sure. Jason