[2/4] fs/notify: don't put file handle buffer on stack.

Commit Message

NeilBrown Dec. 11, 2017, 6:04 a.m. UTC

A file handle buffer is not tiny, and could need to be larger in future,
so it isn't safe to allocate one on the stack.  Instead, we need to
kmalloc().

There is no way to return an error status from a ->show_fdinfo()
function, so if the kmalloc fails, we silently exclude the filehandle
from the output.  As it is at the end of line, this shouldn't
upset parsing too much.  In any case, it can only fail when the
process is being killed by the OOM killer, so the file will never
be parsed anyway.

Signed-off-by: NeilBrown <[e-mail]>
---
 fs/notify/fdinfo.c |   47 +++++++++++++++++++++++++----------------------
 1 file changed, 25 insertions(+), 22 deletions(-)

Comments

Amir Goldstein Dec. 11, 2017, 6:47 a.m. UTC | #1

On Mon, Dec 11, 2017 at 8:04 AM, NeilBrown <[e-mail]> wrote:
> A file handle buffer is not tiny, and could need to be larger in future,
> so it isn't safe to allocate one on the stack.  Instead, we need to
> kmalloc().
>
> There is no way to return an error status from a ->show_fdinfo()
> function, so if the kmalloc fails, we silently exclude the filehandle
> from the output.  As it is at the end of line, this shouldn't
> upset parsing too much.

I think that is a bold assumption to make about parsers ;)
Anyway, the second reasoning is stronger, so may as well drop this one.
There is probably a single userspace user for this which is CRIU,
for which this fdinfo file handle was added, so you could possibly say
that this change does not upset this user (?).

> In any case, it can only fail when the
> process is being killed by the OOM killer, so the file will never
> be parsed anyway.
>
> Signed-off-by: NeilBrown <[e-mail]>

Patch

diff --git a/fs/notify/fdinfo.c b/fs/notify/fdinfo.c
index d1135ed61229..7347f295bc0f 100644
--- a/fs/notify/fdinfo.c
+++ b/fs/notify/fdinfo.c
@@ -23,54 +23,56 @@ 
 
 static void show_fdinfo(struct seq_file *m, struct file *f,
 			void (*show)(struct seq_file *m,
-				     struct fsnotify_mark *mark))
+				     struct fsnotify_mark *mark,
+				     struct fid *fh))
 {
 	struct fsnotify_group *group = f->private_data;
 	struct fsnotify_mark *mark;
+	struct fid *fh = kmalloc(MAX_HANDLE_SZ, GFP_KERNEL);
 
 	mutex_lock(&group->mark_mutex);
 	list_for_each_entry(mark, &group->marks_list, g_list) {
-		show(m, mark);
+		show(m, mark, fh);
 		if (seq_has_overflowed(m))
 			break;
 	}
 	mutex_unlock(&group->mark_mutex);
+	kfree(fh);
 }
 
 #if defined(CONFIG_EXPORTFS)
-static void show_mark_fhandle(struct seq_file *m, struct inode *inode)
+static void show_mark_fhandle(struct seq_file *m, struct inode *inode,
+			      struct fid *fhbuf)
 {
-	struct {
-		struct file_handle handle;
-		u8 pad[MAX_HANDLE_SZ];
-	} f;
 	int size, ret, i;
+	unsigned char *bytes;
 
-	f.handle.handle_bytes = sizeof(f.pad);
-	size = f.handle.handle_bytes >> 2;
+	if (!fhbuf)
+		return;
+	size = MAX_HANDLE_SZ >> 2;
 
-	ret = exportfs_encode_inode_fh(inode, (struct fid *)f.handle.f_handle, &size, 0);
+	ret = exportfs_encode_inode_fh(inode, fhbuf, &size, 0);
 	if ((ret == FILEID_INVALID) || (ret < 0))
 		return;
 
-	f.handle.handle_type = ret;
-	f.handle.handle_bytes = size * sizeof(u32);
-
-	seq_printf(m, "fhandle-bytes:%x fhandle-type:%x f_handle:",
-		   f.handle.handle_bytes, f.handle.handle_type);
+	seq_printf(m, "fhandle-bytes:%zx fhandle-type:%x f_handle:",
+		   size * sizeof(u32), ret);
 
-	for (i = 0; i < f.handle.handle_bytes; i++)
-		seq_printf(m, "%02x", (int)f.handle.f_handle[i]);
+	bytes = (unsigned char *)(fhbuf->raw);
+	for (i = 0; i < size * sizeof(u32); i++)
+		seq_printf(m, "%02x", bytes[i]);
 }
 #else
-static void show_mark_fhandle(struct seq_file *m, struct inode *inode)
+static void show_mark_fhandle(struct seq_file *m, struct inode *inode,
+			      struct fid *fhbuf)
 {
 }
 #endif
 
 #ifdef CONFIG_INOTIFY_USER
 
-static void inotify_fdinfo(struct seq_file *m, struct fsnotify_mark *mark)
+static void inotify_fdinfo(struct seq_file *m, struct fsnotify_mark *mark,
+			   struct fid *fhbuf)
 {
 	struct inotify_inode_mark *inode_mark;
 	struct inode *inode;
@@ -91,7 +93,7 @@  static void inotify_fdinfo(struct seq_file *m, struct fsnotify_mark *mark)
 		seq_printf(m, "inotify wd:%x ino:%lx sdev:%x mask:%x ignored_mask:%x ",
 			   inode_mark->wd, inode->i_ino, inode->i_sb->s_dev,
 			   mask, mark->ignored_mask);
-		show_mark_fhandle(m, inode);
+		show_mark_fhandle(m, inode, fhbuf);
 		seq_putc(m, '\n');
 		iput(inode);
 	}
@@ -106,7 +108,8 @@  void inotify_show_fdinfo(struct seq_file *m, struct file *f)
 
 #ifdef CONFIG_FANOTIFY
 
-static void fanotify_fdinfo(struct seq_file *m, struct fsnotify_mark *mark)
+static void fanotify_fdinfo(struct seq_file *m, struct fsnotify_mark *mark,
+			    struct fid *fhbuf)
 {
 	unsigned int mflags = 0;
 	struct inode *inode;
@@ -121,7 +124,7 @@  static void fanotify_fdinfo(struct seq_file *m, struct fsnotify_mark *mark)
 		seq_printf(m, "fanotify ino:%lx sdev:%x mflags:%x mask:%x ignored_mask:%x ",
 			   inode->i_ino, inode->i_sb->s_dev,
 			   mflags, mark->mask, mark->ignored_mask);
-		show_mark_fhandle(m, inode);
+		show_mark_fhandle(m, inode, fhbuf);
 		seq_putc(m, '\n');
 		iput(inode);
 	} else if (mark->connector->flags & FSNOTIFY_OBJ_TYPE_VFSMOUNT) {