On Fri, Apr 26, 2024, at 18:20, Christian Göttsche wrote:

From: Christian Göttsche <redacted>

Add the four syscalls setxattrat(), getxattrat(), listxattrat() and
removexattrat().  Those can be used to operate on extended attributes,
especially security related ones, either relative to a pinned directory
or on a file descriptor without read access, avoiding a
/proc/<pid>/fd/<fd> detour, requiring a mounted procfs.

One use case will be setfiles(8) setting SELinux file contexts
("security.selinux") without race conditions and without a file
descriptor opened with read access requiring SELinux read permission.

Use the do_{name}at() pattern from fs/open.c.

Pass the value of the extended attribute, its length, and for
setxattrat(2) the command (XATTR_CREATE or XATTR_REPLACE) via an added
struct xattr_args to not exceed six syscall arguments and not
merging the AT_* and XATTR_* flags.

Signed-off-by: Christian Göttsche <redacted>
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I checked that the syscalls are all well-formed regarding
argument types, number of arguments and (absence of)
compat handling, and that they are wired up correctly
across architectures

I did not look at the actual implementation in detail.

Reviewed-by: Arnd Bergmann <arnd@arndb.de>