Re: [RFC PATCH 07/11] mm/mempolicy: add userland mempolicy arg structure

[Gregory Price <gregory.price@memverge.com>]

On Thu, Dec 07, 2023 at 08:13:22AM +0100, Arnd Bergmann wrote:
> On Thu, Dec 7, 2023, at 01:27, Gregory Price wrote:
> > This patch adds the new user-api argument structure intended for
> > set_mempolicy2 and mbind2.
> >
> > struct mpol_args {
> >   /* Basic mempolicy settings */
> >   unsigned short mode;
> >   unsigned short mode_flags;
> >   unsigned long *pol_nodes;
> >   unsigned long pol_maxnodes;
> >
> >   /* get_mempolicy2: policy information (e.g. next interleave node) */
> >   int policy_node;
> >
> >   /* get_mempolicy2: memory range policy */
> >   unsigned long addr;
> >   int addr_node;
> >
> >   /* all operations: policy home node */
> >   unsigned long home_node;
> >
> >   /* mbind2: address ranges to apply the policy */
> >   const struct iovec __user *vec;
> >   size_t vlen;
> > };
> 
> This is not a great structure layout for a system call ABI,
> mostly because it requires adding a compat syscall handler
> to be usable from 32-bit tasks. It would be nice if this
> could be rewritten in a way that uses only fixed-length
> members (__u16, __u32, __aligned_u64), though that does
> require the use of u64_to_user_ptr() to replace the pointers
> and the reverse in userspace.
> 
> Aside from this, you should avoid holes in the data structure.
> On 64-bit architectures, the layout above has holes after
> policy_node and after addr_node.
> 
>       Arnd

doh, clearly i didn't stop to think about alignment. Good eye.
I'll redo this with __u/s members and fix the holes.

Didn't stop to think about compat pointers.  I don't think the
u64_to_user_ptr pattern is offensive, so i'll make that change.
At least I don't see what the other options are beyond compat.

Thanks
~Gregory