[RFC PATCH 1/2] fs/xattr: add *at family syscalls

[Casey Schaufler]

On 8/31/2022 3:17 PM, Al Viro wrote:
> [linux-arch Cc'd for ABI-related stuff]

The LSM list <linux-security-module at vger.kernel.org> should be on
this thread as SELinux isn't the only security module that uses xattrs
extensively.

>
> On Tue, Aug 30, 2022 at 05:28:39PM +0200, Christian Göttsche wrote:
>> Add the four syscalls setxattrat(), getxattrat(), listxattrat() and
>> removexattrat() to enable extended attribute operations via file
>> descriptors.  This can be used from userspace to avoid race conditions,
>> especially on security related extended attributes, like SELinux labels
>> ("security.selinux") via setfiles(8).
>>
>> Use the do_{name}at() pattern from fs/open.c.
>> Use a single flag parameter for extended attribute flags (currently
>> XATTR_CREATE and XATTR_REPLACE) and *at() flags to not exceed six
>> syscall arguments in setxattrat().
> 	I've no problems with the patchset aside of the flags part;
> however, note that XATTR_CREATE and XATTR_REPLACE are actually exposed
> to the network - the values are passed to nfsd by clients.
> See nfsd4_decode_setxattr() and
>         BUILD_BUG_ON(XATTR_CREATE != SETXATTR4_CREATE);
> 	BUILD_BUG_ON(XATTR_REPLACE != SETXATTR4_REPLACE);
> in encode_setxattr() on the client side.
>
> 	Makes me really nervous about constraints like that.  Sure,
> AT_... flags you are using are in the second octet and these are in
> the lowest one, but...