[RFC PATCH v2] fs/xattr: add *at family syscalls

[Christian Brauner]

On Thu, May 11, 2023 at 05:08:02PM +0200, Christian Göttsche wrote:
> 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.
> 
> Add XATTR flags to the private namespace of AT_* flags.
> 
> 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().
> 
> Previous approach ("f*xattr: allow O_PATH descriptors"): https://lore.kernel.org/all/20220607153139.35588-1-cgzones@googlemail.com/
> v1 discussion: https://lore.kernel.org/all/20220830152858.14866-2-cgzones@googlemail.com/
> 
> Signed-off-by: Christian Göttsche <cgzones at googlemail.com>
> CC: x86 at kernel.org
> CC: linux-alpha at vger.kernel.org
> CC: linux-kernel at vger.kernel.org
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> CC: linux-mips at vger.kernel.org
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> CC: linux-sh at vger.kernel.org
> CC: sparclinux at vger.kernel.org
> CC: linux-fsdevel at vger.kernel.org
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> CC: linux-arch at vger.kernel.org
> CC: linux-api at vger.kernel.org
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> CC: selinux at vger.kernel.org
> ---

Fwiw, your header doesn't let me see who the mail was directly sent to
so I'm only able to reply to lists which is a bit pointless...

> v2:
>   - squash syscall introduction and wire up commits
>   - add AT_XATTR_CREATE and AT_XATTR_REPLACE constants

> +#define AT_XATTR_CREATE	        0x1	/* setxattrat(2): set value, fail if attr already exists */
> +#define AT_XATTR_REPLACE	0x2	/* setxattrat(2): set value, fail if attr does not exist */

We really shouldn't waste any AT_* flags for this. Otherwise we'll run
out of them rather quickly. Two weeks ago we added another AT_* flag
which is up for merging for v6.5 iirc and I've glimpsed another AT_*
flag proposal in one of the talks at last weeks Vancouver conference
extravaganza.

Even if we reuse 0x200 for AT_XATTR_CREATE (like we did for AT_EACCESS
and AT_REMOVEDIR) we still need another bit for AT_XATTR_REPLACE.

Plus, this is really ugly since AT_XATTR_{CREATE,REPLACE} really isn't
in any way related to lookup and we're mixing it in with lookup
modifying flags.

So my proposal for {g,s}etxattrat() would be:

struct xattr_args {
        __aligned_u64 value;
        __u32 size;
        __u32 cmd;
};

So everything's nicely 64bit aligned in the struct. Use the @cmd member
to set either XATTR_REPLACE or XATTR_CREATE and treat it as a proper
enum and not as a flag argument like the old calls did.

So then we'd have:

setxattrat(int dfd, const char *path, const char __user *name,
           struct xattr_args __user *args, size_t size, unsigned int flags)
getxattrat(int dfd, const char *path, const char __user *name,
           struct xattr_args __user *args, size_t size, unsigned int flags)

The current in-kernel struct xattr_ctx would be renamed to struct
kernel_xattr_args and then we do the usual copy_struct_from_user()
dance:

struct xattr_args args;
err = copy_struct_from_user(&args, sizeof(args), uargs, usize);

and then go on to handle value/size for setxattrat()/getxattrat()
accordingly.

getxattr()/setxattr() aren't meaningfully filterable by seccomp already
so there's not point in not using a struct.

If that isn't very appealing then another option is to add a new flag
namespace just for setxattrat() similar to fspick() and move_mount()
duplicating the needed lookup modifying flags.
Thoughts?