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

[Arnd Bergmann]

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