[PATCH RFC 08/20] fs: add generic_llseek_cookie()

[Christian Brauner <brauner@kernel.org>]

This is similar to generic_file_llseek() but allows the caller to
specify a cookie that will be updated to indicate that a seek happened.
Caller's requiring that information in their readdir implementations can
use that.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/read_write.c    | 44 ++++++++++++++++++++++++++++++++++++++++++++
 include/linux/fs.h |  2 ++
 2 files changed, 46 insertions(+)

diff --git a/fs/read_write.c b/fs/read_write.c
index ad93b72cc378..47f7b4e32a53 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -179,6 +179,50 @@ generic_file_llseek_size(struct file *file, loff_t offset, int whence,
 }
 EXPORT_SYMBOL(generic_file_llseek_size);
 
+/**
+ * generic_llseek_cookie - versioned llseek implementation
+ * @file:	file structure to seek on
+ * @offset:	file offset to seek to
+ * @whence:	type of seek
+ * @cookie:	cookie to update
+ *
+ * See generic_file_llseek for a general description and locking assumptions.
+ *
+ * In contrast to generic_file_llseek, this function also resets a
+ * specified cookie to indicate a seek took place.
+ */
+loff_t generic_llseek_cookie(struct file *file, loff_t offset, int whence,
+			     u64 *cookie)
+{
+	struct inode *inode = file->f_mapping->host;
+	loff_t maxsize = inode->i_sb->s_maxbytes;
+	loff_t eof = i_size_read(inode);
+	int ret;
+
+	if (WARN_ON_ONCE(!cookie))
+		return -EINVAL;
+
+	ret = must_set_pos(file, &offset, whence, eof);
+	if (ret < 0)
+		return ret;
+	if (ret == 0)
+		return offset;
+
+	if (whence == SEEK_CUR) {
+		/*
+		 * f_lock protects against read/modify/write race with
+		 * other SEEK_CURs. Note that parallel writes and reads
+		 * behave like SEEK_SET.
+		 */
+		guard(spinlock)(&file->f_lock);
+		return vfs_setpos_cookie(file, file->f_pos + offset, maxsize,
+					 cookie);
+	}
+
+	return vfs_setpos_cookie(file, offset, maxsize, cookie);
+}
+EXPORT_SYMBOL(generic_llseek_cookie);
+
 /**
  * generic_file_llseek - generic llseek implementation for regular files
  * @file:	file structure to seek on
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 58c91a52cad1..3e6b3c1afb31 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3202,6 +3202,8 @@ extern loff_t vfs_setpos(struct file *file, loff_t offset, loff_t maxsize);
 extern loff_t generic_file_llseek(struct file *file, loff_t offset, int whence);
 extern loff_t generic_file_llseek_size(struct file *file, loff_t offset,
 		int whence, loff_t maxsize, loff_t eof);
+loff_t generic_llseek_cookie(struct file *file, loff_t offset, int whence,
+			     u64 *cookie);
 extern loff_t fixed_size_llseek(struct file *file, loff_t offset,
 		int whence, loff_t size);
 extern loff_t no_seek_end_llseek_size(struct file *, loff_t, int, loff_t);

-- 
2.45.2