Re: [PATCH 1/2] vfs: Annotate struct fdtable's fd field with __counted_by_ptr

[Kees Cook]

On Mon, Aug 10, 2026 at 08:41:12PM +0000, Bill Wendling wrote:
> The 'struct fdtable' holds the file descriptor table information,
> including the current file descriptor array 'fd' and its size 'max_fds'.
> To harden the kernel against out-of-bounds accesses, we can annotate the
> 'fd' pointer field with the '__counted_by_ptr' attribute, referencing
> 'max_fds'.
> 
> The compiler uses the '__counted_by_ptr' attribute to track the
> size of the memory allocated for the pointer field, enabling
> runtime bounds checks under KASAN and fortified functions. There are
> three places where a 'struct fdtable' is initialized, and in all of
> them, 'max_fds' is set before the 'fd' pointer is accessed or assigned
> in all allocation and initialization places.
> 
> No accesses to 'fd' occur before 'max_fds' is set, preventing any
> potential runtime false-positives or panics due to uninitialized count
> fields.
> 
> This patch was generated by CodeMender and checked by submitter.
> 
> Cc: codemender-patching+linux@xxxxxxxxxx
> Signed-off-by: Bill Wendling <morbo@xxxxxxxxxx>
> ---
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Christian Brauner <brauner@xxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Kees Cook <kees@xxxxxxxxxx>
> Cc: "Gustavo A. R. Silva" <gustavoars@xxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: linux-hardening@xxxxxxxxxxxxxxx
> ---
>  include/linux/fdtable.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
> index c45306a9f007..3a5c88291125 100644
> --- a/include/linux/fdtable.h
> +++ b/include/linux/fdtable.h
> @@ -25,7 +25,7 @@
>  
>  struct fdtable {
>  	unsigned int max_fds;
> -	struct file __rcu **fd;      /* current fd array */
> +	struct file __rcu **fd __counted_by_ptr(max_fds);      /* current fd array */
>  	unsigned long *close_on_exec;
>  	unsigned long *open_fds;
>  	unsigned long *full_fds_bits;

I see the alloc_fdtable test, that's one max_fds assignment, but I also
see dup_fd(), which does the assignment after the newf allocation, so
that looks safe too, but it might be nice to add that allocation path to
the tests too?

-- 
Kees Cook