2023-12-19 00:26:53

by Gustavo A. R. Silva

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

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 <[email protected]>
---
fs/bcachefs/bcachefs_ioctl.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/bcachefs/bcachefs_ioctl.h b/fs/bcachefs/bcachefs_ioctl.h
index 21f81b16f24e..4b8fba754b1c 100644
--- a/fs/bcachefs/bcachefs_ioctl.h
+++ b/fs/bcachefs/bcachefs_ioctl.h
@@ -397,7 +397,7 @@ struct bch_ioctl_fsck_offline {
__u64 flags;
__u64 opts; /* string */
__u64 nr_devs;
- __u64 devs[0];
+ __u64 devs[] __counted_by(nr_devs);
};

/*
--
2.34.1

2023-12-19 01:31:52

by Kent Overstreet

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

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 <[email protected]>

Applied