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

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