Re: [PATCH] nfc: virtual_ncidev: Add missing ioctl compat handler

[Simon Horman <horms@kernel.org>]

On Fri, Sep 04, 2026 at 06:42:52PM +0200, Chris Gellermann wrote:
> The compat handler for ioctls to the virtual nci device is missing. So,
> nci-specific ioctls of a compat task return with -1 and errno set to
> ENOTTY. Add a handler.
> 
> The handling of an ioctl() call of a compat task to get the index of
> virtual nci device (IOCTL_GET_NCIDEV_IDX) lands in the default case of
> the ioctl compat handler (see fs/ioctl.c):
> 
> COMPAT_SYSCALL_DEFINE3(ioctl, ...)
> {
> 	...
> 	default:
>        		error = do_vfs_ioctl(fd_file(f), fd, cmd, ...);
> 		if (error != -ENOIOCTLCMD)
> 			break;
> 
> 		if (fd_file(f)->f_op->compat_ioctl)
> 			error = fd_file(f)->f_op->compat_ioctl(fd_file(f), cmd, arg);
> 		if (error == -ENOIOCTLCMD)
> 			error = -ENOTTY;
> 	...
> }
> 
> There, do_vfs_ioctl() returns -ENOIOCTLCMD and compat_ioctl is not
> set for virtual_ncidev_fops, i.e. f_op->compat_ioctl == NULL. So, the
> ioctl() syscall returns with -1 and errno set to ENOTTY to the compat
> task.
> 
> To fix this, use the compat_ptr_ioctl helper for compat handling here.
> It shall be used for ioctls that "either ignore the argument or pass a
> pointer to a compatible data type". The driver's sole ioctl takes a user
> void pointer and copies nfc_dev->idx to it, a 4-byte integer across all
> ABIs.
> 
> This issue has been found by running the nci_dev kernel selftest as
> rv64 binary on top of a CHERI kernel, where the ioctl() ends up in
> the ioctl compat handler, similar to a 32-bit application on top of a
> 64-bit kernel.
> 
> Fixes: e624e6c3e777 ("nfc: Add a virtual nci device driver")
> Signed-off-by: Chris Gellermann <christian.gellermann@codasip.com>

Reviewed-by: Simon Horman <horms@kernel.org>