uaccess: add copy_struct_to_user helper

uaccess: add copy_struct_to_user helper

This is based on copy_struct_from_user(), but there is one additional
case to consider when creating a syscall that returns an
extensible-struct to userspace -- how should data in the struct that
cannot fit into the userspace struct be handled (ksize > usize)?

There are three possibilies:

 1. The interface is like sched_getattr(2), where new information will
    be silently not provided to userspace. This is probably what most
    interfaces will want to do, as it provides the most possible
    backwards-compatibility.

 2. The interface is like lsm_list_modules(2), where you want to return
    an error like -EMSGSIZE if not providing information could result in
    the userspace program making a serious mistake (such as one that
    could lead to a security problem) or if you want to provide some
    flag to userspace so they know that they are missing some
    information.

 3. The interface is like statx(2), where there some kind of a request
    mask that indicates what data userspace would like. One could
    imagine that statx2(2) (using extensible structs) would want to
    return -EMSGSIZE if the user explicitly requested a field that their
    structure is too small to fit, but not return an error if the field
    was not explicitly requested. This is kind of a mix between (1) and
    (2) based on the requested mask.

The copy_struct_to_user() helper includes a an extra argument that is
used to return a boolean flag indicating whether there was a non-zero
byte in the trailing bytes that were not copied to userspace. This can
be used in the following ways to handle all three cases, respectively:

 1. Just pass NULL, as you don't care about this case.

 2. Return an error (say -EMSGSIZE) if the argument was set to true by
    copy_struct_to_user().

 3. If the argument was set to true by copy_struct_to_user(), check if
    there is a flag that implies a field larger than usize.

    This is the only case where callers of copy_struct_to_user() should
    check usize themselves. This will probably require scanning an array
    that specifies what flags were added for each version of the flags
    struct and returning an error if the request mask matches any of the
    flags that were added in versions of the struct that are larger than
    usize.

At the moment we don't have any users of (3), so this patch doesn't
include any helpers to make the necessary scanning easier, but it should
be fairly easy to add some if necessary.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Link: https://lore.kernel.org/r/20241010-extensible-structs-check_fields-v3-1-d2833dfe6edd@cyphar.com
Signed-off-by: Christian Brauner <brauner@kernel.org>