Re: [PATCH v6 2/5] fs: split off setxattr_setup function from setxattr

[Christian Brauner <christian.brauner@ubuntu.com>]

On Wed, Dec 22, 2021 at 01:01:24PM -0800, Stefan Roesch wrote:
> This splits of the setup part of the function
> setxattr in its own dedicated function called
> setxattr_setup.
> 
> This makes it possible to call this function
> from io_uring in the pre-processing of an
> xattr request.
> 
> Signed-off-by: Stefan Roesch <shr@fb.com>
> ---

I like the introduction of struct xattr_ctx.
But I would prefer if we called this setxattr_prepare() to mirror
setattr_prepare() and change the signature to:

int setxattr_setup(struct user_namespace *mnt_userns,
                   const char __user *name,
		   struct xattr_ctx *ctx,
		   void **xattr_val);

Since NULL is a success condition I think it makes more sense to have an
error returned and the value be a return argument. So sm like
(uncompiled and untested):

int setxattr_prepare(struct user_namespace *mnt_userns, const char __user *name,
		     struct xattr_ctx *ctx, void **xattr_val)
{
	void *kvalue = NULL;
	int error;

	if (ctx->flags & ~(XATTR_CREATE | XATTR_REPLACE))
		return -EINVAL;

	error = strncpy_from_user(ctx->kname, name, ctx->kname_sz);
	if (error == 0 || error == ctx->kname_sz)
		return -ERANGE;
	if (error < 0)
		return error;

	if (ctx->size) {
		if (ctx->size > XATTR_SIZE_MAX)
			return -E2BIG;

		kvalue = kvmalloc(ctx->size, GFP_KERNEL);
		if (!kvalue)
			return -ENOMEM;

		if (copy_from_user(kvalue, ctx->value, ctx->size)) {
			kvfree(kvalue);
			return -EFAULT;
		}

		if ((strcmp(ctx->kname, XATTR_NAME_POSIX_ACL_ACCESS) == 0) ||
		    (strcmp(ctx->kname, XATTR_NAME_POSIX_ACL_DEFAULT) == 0))
			posix_acl_fix_xattr_from_user(mnt_userns, kvalue, ctx->size);
	}

	*xattr_val = kvalue;
	return 0;
}