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

[Al Viro]

[linux-arch Cc'd for ABI-related stuff]

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...