Re: [PATCH][next] bcachefs: Replace zero-length array with flex-array member and use __counted_by

[Kent Overstreet]

On Mon, Dec 18, 2023 at 06:24:53PM -0600, Gustavo A. R. Silva wrote:
> Fake flexible arrays (zero-length and one-element arrays) are
> deprecated, and should be replaced by flexible-array members.
> So, replace zero-length array with a flexible-array member in
> `struct bch_ioctl_fsck_offline`.
> 
> Also annotate array `devs` with `__counted_by()` to prepare for the
> coming implementation by GCC and Clang of the `__counted_by` attribute.
> Flexible array members annotated with `__counted_by` can have their
> accesses bounds-checked at run-time via `CONFIG_UBSAN_BOUNDS` (for
> array indexing) and `CONFIG_FORTIFY_SOURCE` (for strcpy/memcpy-family
> functions).
> 
> This fixes the following -Warray-bounds warnings:
> fs/bcachefs/chardev.c: In function 'bch2_ioctl_fsck_offline':
> fs/bcachefs/chardev.c:363:34: warning: array subscript 0 is outside array bounds of '__u64[0]' {aka 'long long unsigned int[]'} [-Warray-bounds=]
>   363 |         if (copy_from_user(devs, &user_arg->devs[0], sizeof(user_arg->devs[0]) * arg.nr_devs)) {
>       |                                  ^~~~~~~~~~~~~~~~~~
> In file included from fs/bcachefs/chardev.c:5:
> fs/bcachefs/bcachefs_ioctl.h:400:33: note: while referencing 'devs'
>   400 |         __u64                   devs[0];
> 
> This results in no differences in binary output.
> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>

Applied