Re: [PATCH] usb: gadget: uvc: don't pack struct uvcg_extension_unit_descriptor

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Sat, Aug 01, 2026 at 06:37:47PM +0200, Rituparna Warwatkar wrote:
> kmemleak reports the baSourceID and bmControls arrays allocated by the
> UVC extension-unit configfs attributes as leaked, e.g.:
> 
>   BUG: memory leak
>   unreferenced object 0xffff888114fee2c0 (size 8):
>     __kmalloc_noprof
>     uvcg_extension_ba_source_id_store
>     configfs_write_iter
>     vfs_write
>     ksys_write
> 
> The arrays are not actually leaked: they are reachable through
> xu->desc.baSourceID / xu->desc.bmControls and are freed when the
> extension unit is removed.  The problem is that struct
> uvcg_extension_unit_descriptor is marked __packed, so these two heap
> pointers are stored at unaligned offsets (22 and 31).  kmemleak only
> scans memory on pointer-aligned boundaries, so it never sees the
> pointers and reports the arrays as unreferenced.
> 
> Unlike the UAPI struct uvc_extension_unit_descriptor, this is a purely
> in-memory staging structure: baSourceID and bmControls are pointers,
> not inline arrays, and the wire descriptor is assembled field by field
> in UVC_COPY_XU_DESCRIPTOR().  Nothing relies on the packed layout, so
> the __packed attribute is unnecessary and only serves to misalign the
> pointers.
> 
> Drop __packed so the pointers are naturally aligned and visible to
> kmemleak, silencing the false positive.
> 
> Fixes: 0525210c9840 ("usb: gadget: uvc: Allow definition of XUs in configfs")
> Reported-by: syzbot+54927260acba030187a6@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=54927260acba030187a6
> Signed-off-by: Rituparna Warwatkar <rwarwatkar@gmail.com>
> ---
>  drivers/usb/gadget/function/uvc_configfs.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/usb/gadget/function/uvc_configfs.h b/drivers/usb/gadget/function/uvc_configfs.h
> index 9391614135e..5a882afbce4 100644
> --- a/drivers/usb/gadget/function/uvc_configfs.h
> +++ b/drivers/usb/gadget/function/uvc_configfs.h
> @@ -176,7 +176,7 @@ struct uvcg_extension_unit_descriptor {
>         u8 bControlSize;
>         u8 *bmControls;
>         u8 iExtension;
> -} __packed;
> +};

Why not rearrange the structure so that there is no space in it now
instead?

thanks,

greg k-h