[PATCH 2/2] vfs: support statx(..., NULL, AT_EMPTY_PATH, ...)

[Mateusz Guzik <mjguzik@gmail.com>]

The newly used helper also checks for 0-sized buffers.

This avoids path lookup code, lockref management, memory allocation and
in case of NULL path userspace memory access (which can be quite
expensive with SMAP on x86_64).

statx with AT_EMPTY_PATH paired with "" or NULL argument as appropriate
issued on Sapphire Rapids (ops/s):
stock:     4231237
0-check:   5944063 (+40%)
NULL path: 6601619 (+11%/+56%)

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
---
 fs/internal.h    |  2 ++
 fs/stat.c        | 90 ++++++++++++++++++++++++++++++++++--------------
 io_uring/statx.c | 23 +++++++------
 3 files changed, 80 insertions(+), 35 deletions(-)

diff --git a/fs/internal.h b/fs/internal.h
index 1caa6a8f666f..0a018ebcaf49 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -244,6 +244,8 @@ extern const struct dentry_operations ns_dentry_operations;
 int getname_statx_lookup_flags(int flags);
 int do_statx(int dfd, struct filename *filename, unsigned int flags,
 	     unsigned int mask, struct statx __user *buffer);
+int do_statx_fd(int fd, unsigned int flags, unsigned int mask,
+		struct statx __user *buffer);
 
 /*
  * fs/splice.c:
diff --git a/fs/stat.c b/fs/stat.c
index 106684034fdb..1214826f3a36 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -214,6 +214,43 @@ int getname_statx_lookup_flags(int flags)
 	return lookup_flags;
 }
 
+static int vfs_statx_path(struct path *path, int flags, struct kstat *stat,
+			  u32 request_mask)
+{
+	int error = vfs_getattr(path, stat, request_mask, flags);
+
+	if (request_mask & STATX_MNT_ID_UNIQUE) {
+		stat->mnt_id = real_mount(path->mnt)->mnt_id_unique;
+		stat->result_mask |= STATX_MNT_ID_UNIQUE;
+	} else {
+		stat->mnt_id = real_mount(path->mnt)->mnt_id;
+		stat->result_mask |= STATX_MNT_ID;
+	}
+
+	if (path->mnt->mnt_root == path->dentry)
+		stat->attributes |= STATX_ATTR_MOUNT_ROOT;
+	stat->attributes_mask |= STATX_ATTR_MOUNT_ROOT;
+
+	/* Handle STATX_DIOALIGN for block devices. */
+	if (request_mask & STATX_DIOALIGN) {
+		struct inode *inode = d_backing_inode(path->dentry);
+
+		if (S_ISBLK(inode->i_mode))
+			bdev_statx_dioalign(inode, stat);
+	}
+
+	return error;
+}
+
+static int vfs_statx_fd(int fd, int flags, struct kstat *stat,
+			  u32 request_mask)
+{
+	CLASS(fd_raw, f)(fd);
+	if (!f.file)
+		return -EBADF;
+	return vfs_statx_path(&f.file->f_path, flags, stat, request_mask);
+}
+
 /**
  * vfs_statx - Get basic and extra attributes by filename
  * @dfd: A file descriptor representing the base dir for a relative filename
@@ -243,36 +280,13 @@ static int vfs_statx(int dfd, struct filename *filename, int flags,
 retry:
 	error = filename_lookup(dfd, filename, lookup_flags, &path, NULL);
 	if (error)
-		goto out;
-
-	error = vfs_getattr(&path, stat, request_mask, flags);
-
-	if (request_mask & STATX_MNT_ID_UNIQUE) {
-		stat->mnt_id = real_mount(path.mnt)->mnt_id_unique;
-		stat->result_mask |= STATX_MNT_ID_UNIQUE;
-	} else {
-		stat->mnt_id = real_mount(path.mnt)->mnt_id;
-		stat->result_mask |= STATX_MNT_ID;
-	}
-
-	if (path.mnt->mnt_root == path.dentry)
-		stat->attributes |= STATX_ATTR_MOUNT_ROOT;
-	stat->attributes_mask |= STATX_ATTR_MOUNT_ROOT;
-
-	/* Handle STATX_DIOALIGN for block devices. */
-	if (request_mask & STATX_DIOALIGN) {
-		struct inode *inode = d_backing_inode(path.dentry);
-
-		if (S_ISBLK(inode->i_mode))
-			bdev_statx_dioalign(inode, stat);
-	}
-
+		return error;
+	error = vfs_statx_path(&path, flags, stat, request_mask);
 	path_put(&path);
 	if (retry_estale(error, lookup_flags)) {
 		lookup_flags |= LOOKUP_REVAL;
 		goto retry;
 	}
-out:
 	return error;
 }
 
@@ -677,6 +691,29 @@ int do_statx(int dfd, struct filename *filename, unsigned int flags,
 	return cp_statx(&stat, buffer);
 }
 
+int do_statx_fd(int fd, unsigned int flags, unsigned int mask,
+	     struct statx __user *buffer)
+{
+	struct kstat stat;
+	int error;
+
+	if (mask & STATX__RESERVED)
+		return -EINVAL;
+	if ((flags & AT_STATX_SYNC_TYPE) == AT_STATX_SYNC_TYPE)
+		return -EINVAL;
+
+	/* STATX_CHANGE_COOKIE is kernel-only for now. Ignore requests
+	 * from userland.
+	 */
+	mask &= ~STATX_CHANGE_COOKIE;
+
+	error = vfs_statx_fd(fd, flags, &stat, mask);
+	if (error)
+		return error;
+
+	return cp_statx(&stat, buffer);
+}
+
 /**
  * sys_statx - System call to get enhanced stats
  * @dfd: Base directory to pathwalk from *or* fd to stat.
@@ -696,6 +733,9 @@ SYSCALL_DEFINE5(statx,
 	int ret;
 	struct filename *name;
 
+	if (flags == AT_EMPTY_PATH && vfs_empty_path(dfd, filename))
+		return do_statx_fd(dfd, flags, mask, buffer);
+
 	name = getname_flags(filename, getname_statx_lookup_flags(flags), NULL);
 	ret = do_statx(dfd, name, flags, mask, buffer);
 	putname(name);
diff --git a/io_uring/statx.c b/io_uring/statx.c
index abb874209caa..fe967ecb1762 100644
--- a/io_uring/statx.c
+++ b/io_uring/statx.c
@@ -23,6 +23,7 @@ struct io_statx {
 int io_statx_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 {
 	struct io_statx *sx = io_kiocb_to_cmd(req, struct io_statx);
+	struct filename *filename;
 	const char __user *path;
 
 	if (sqe->buf_index || sqe->splice_fd_in)
@@ -36,15 +37,14 @@ int io_statx_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 	sx->buffer = u64_to_user_ptr(READ_ONCE(sqe->addr2));
 	sx->flags = READ_ONCE(sqe->statx_flags);
 
-	sx->filename = getname_flags(path,
-				     getname_statx_lookup_flags(sx->flags),
-				     NULL);
-
-	if (IS_ERR(sx->filename)) {
-		int ret = PTR_ERR(sx->filename);
-
-		sx->filename = NULL;
-		return ret;
+	sx->filename = NULL;
+	if (!(sx->flags == AT_EMPTY_PATH && vfs_empty_path(sx->dfd, path))) {
+		filename = getname_flags(path,
+					 getname_statx_lookup_flags(sx->flags),
+					 NULL);
+		if (IS_ERR(filename))
+			return PTR_ERR(filename);
+		sx->filename = filename;
 	}
 
 	req->flags |= REQ_F_NEED_CLEANUP;
@@ -59,7 +59,10 @@ int io_statx(struct io_kiocb *req, unsigned int issue_flags)
 
 	WARN_ON_ONCE(issue_flags & IO_URING_F_NONBLOCK);
 
-	ret = do_statx(sx->dfd, sx->filename, sx->flags, sx->mask, sx->buffer);
+	if (sx->filename == NULL)
+		ret = do_statx_fd(sx->dfd, sx->flags, sx->mask, sx->buffer);
+	else
+		ret = do_statx(sx->dfd, sx->filename, sx->flags, sx->mask, sx->buffer);
 	io_req_set_res(req, ret, 0);
 	return IOU_OK;
 }
-- 
2.43.0