On Sun, 22 Sep 2024, Kees Cook wrote:
> From: Erick Archer <erick.archer@xxxxxxxxxxx>
>
> One-element arrays as fake flex arrays are deprecated[1] as the kernel
> has switched to C99 flexible-array members instead. This case, however,
> has more complexity because it is a flexible array of flexible arrays
> and this patch needs to be ready to enable the new compiler flag
> -Wflex-array-member-not-at-end (coming in GCC-14) globally.
>
> So, define a new struct type for the single reports:
>
> struct report {
> uint16_t size;
> struct hostif_msg_hdr msg;
> } __packed;
>
> but without the payload (flex array) in it. And add this payload to the
> "hostif_msg" structure. This way, in the "report_list" structure we can
> declare a flex array of single reports which now do not contain another
> flex array.
>
> struct report_list {
> [...]
> struct report reports[];
> } __packed;
>
> Therefore, the "struct hostif_msg" is now made up of a header and a
> payload. And the "struct report" uses only the "hostif_msg" header.
> The perfect solution would be for the "report" structure to use the
> whole "hostif_msg" structure but this is not possible due to nested
> flexible arrays. Anyway, the end result is equivalent since this patch
> does attempt to change the behaviour of the code.
>
> Now as well, we have more clarity after the cast from the raw bytes to
> the new structures. Refactor the code accordingly to use the new
> structures.
>
> Also, use "container_of()" whenever we need to retrieve a pointer to
> the flexible structure, through which we can access the flexible array
> if needed.
>
> Link: https://www.kernel.org/doc/html/next/process/deprecated.html#zero-length-and-one-element-arrays [1]
> Closes: https://github.com/KSPP/linux/issues/333
> Signed-off-by: Erick Archer <erick.archer@xxxxxxxxxxx>
> Link: https://lore.kernel.org/r/AS8PR02MB723760CB93942370E92F00638BF72@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> [kees: tweaked commit log and dropped struct_size() uses]
> Signed-off-by: Kees Cook <kees@xxxxxxxxxx>
> ---
> v2: - update based on feedback
> rfc: https://lore.kernel.org/lkml/AS8PR02MB723760CB93942370E92F00638BF72@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
Applied, thanks.
--
Jiri Kosina
SUSE Labs
