[PATCH v3 1/6] fsnotify: add pre-content hooks on mmap()

[Amir Goldstein <amir73il@gmail.com>]

Pre-content hooks in page faults introduces potential deadlock of HSM
handler in userspace with filesystem freezing.

The requirement with pre-content event is that for every accessed file
range an event covering at least this range will be generated at least
once before the file data is accesses.

In preparation to disabling pre-content event hooks on page faults,
add pre-content hooks at mmap() variants for the entire mmaped range,
so HSM can fill content when user requests to map a portion of the file.

Note that exec() variant also calls vm_mmap_pgoff() internally to map
code sections, so pre-content hooks are also generated in this case.

Link: https://lore.kernel.org/linux-fsdevel/7ehxrhbvehlrjwvrduoxsao5k3x4aw275patsb3krkwuq573yv@o2hskrfawbnc/
Suggested-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 include/linux/fsnotify.h | 21 +++++++++++++++++++++
 mm/util.c                |  3 +++
 2 files changed, 24 insertions(+)

diff --git a/include/linux/fsnotify.h b/include/linux/fsnotify.h
index 6a33288bd6a1f..83d3ac97f8262 100644
--- a/include/linux/fsnotify.h
+++ b/include/linux/fsnotify.h
@@ -170,6 +170,21 @@ static inline int fsnotify_file_area_perm(struct file *file, int perm_mask,
 	return fsnotify_path(&file->f_path, FS_ACCESS_PERM);
 }
 
+/*
+ * fsnotify_mmap_perm - permission hook before mmap of file range
+ */
+static inline int fsnotify_mmap_perm(struct file *file, int prot,
+				     const loff_t off, size_t len)
+{
+	/*
+	 * mmap() generates only pre-content events.
+	 */
+	if (!file || likely(!FMODE_FSNOTIFY_HSM(file->f_mode)))
+		return 0;
+
+	return fsnotify_pre_content(&file->f_path, &off, len);
+}
+
 /*
  * fsnotify_truncate_perm - permission hook before file truncate
  */
@@ -223,6 +238,12 @@ static inline int fsnotify_file_area_perm(struct file *file, int perm_mask,
 	return 0;
 }
 
+static inline int fsnotify_mmap_perm(struct file *file, int prot,
+				     const loff_t off, size_t len)
+{
+	return 0;
+}
+
 static inline int fsnotify_truncate_perm(const struct path *path, loff_t length)
 {
 	return 0;
diff --git a/mm/util.c b/mm/util.c
index b6b9684a14388..8c965474d329f 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -23,6 +23,7 @@
 #include <linux/processor.h>
 #include <linux/sizes.h>
 #include <linux/compat.h>
+#include <linux/fsnotify.h>
 
 #include <linux/uaccess.h>
 
@@ -569,6 +570,8 @@ unsigned long vm_mmap_pgoff(struct file *file, unsigned long addr,
 	LIST_HEAD(uf);
 
 	ret = security_mmap_file(file, prot, flag);
+	if (!ret)
+		ret = fsnotify_mmap_perm(file, prot, pgoff >> PAGE_SHIFT, len);
 	if (!ret) {
 		if (mmap_write_lock_killable(mm))
 			return -EINTR;
-- 
2.34.1