On Thu, Nov 24, 2022 at 01:24:42PM +0100, Jason A. Donenfeld wrote: > Hi Florian, > > On Thu, Nov 24, 2022 at 01:15:24PM +0100, Florian Weimer wrote: > > * Jason A. Donenfeld: > > > > > Hi Florian, > > > > > > On Thu, Nov 24, 2022 at 06:25:39AM +0100, Florian Weimer wrote: > > >> * Jason A. Donenfeld: > > >> > > >> > Hi Florian, > > >> > > > >> > On Wed, Nov 23, 2022 at 11:46:58AM +0100, Florian Weimer wrote: > > >> >> * Jason A. Donenfeld: > > >> >> > > >> >> > + * The vgetrandom() function in userspace requires an opaque state, which this > > >> >> > + * function provides to userspace, by mapping a certain number of special pages > > >> >> > + * into the calling process. It takes a hint as to the number of opaque states > > >> >> > + * desired, and returns the number of opaque states actually allocated, the > > >> >> > + * size of each one in bytes, and the address of the first state. > > >> >> > + */ > > >> >> > +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. Passing them as a register arg is problematic because of 32bit arches. But passing as pointer should be fine but it is indeed uncommon. > > > > > > 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? We try to avoid adding new compat-requiring syscalls like the plague usually. (At least for new syscalls that don't need to inherit behavior from earlier syscalls they are a revisions of.) > > The other direction would be making this a u32, since 640k ought to be > enough for anybody and such, but maybe that'd be a mistake too. 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. 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.
