[PATCH] vfs: get_next_ino(), never inum=0

[hooanon05g@gmail.com]

From: "J. R. Okajima" <hooanon05g@gmail.com>

It is very rare for get_next_ino() to return zero as a new inode number
since its type is unsigned int, but it can surely happen eventually.

Interestingly, ls(1) and find(1) don't show a file whose inum is zero,
so people won't be able to find it. This issue may be harmful especially
for tmpfs.
On a very long lived and busy system, users may frequently create files
on tmpfs. And if unluckily he gets inum=0, then he cannot see its
filename. If he remembers its name, he may be able to use or unlink it
by its name since the file surely exists. Otherwise, the file remains on
tmpfs silently. No one can touch it. This behaviour looks like resource
leak.
As a worse case, if a dir gets inum=0 and a user creates several files
under it, then the leaked memory will increase since a user cannot see
the name of all files under the dir whose inum=0, regardless the inum of
the children.

There is another unpleasant effect when get_next_ino() wraps
around. When there is a file whose inum=100 on tmpfs, a new file may get
inum=100. I am not sure what will happen when the duplicated inums exist
on tmpfs. Anyway this is not a issue in get_next_ino(). It should be
fixed in mm/shmem.c if it is really necessary.

Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>
---
 fs/inode.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/inode.c b/fs/inode.c
index f96d2a6..a3e274a 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -848,7 +848,11 @@ unsigned int get_next_ino(void)
 	}
 #endif
 
-	*p = ++res;
+	res++;
+	/* never zero */
+	if (unlikely(!res))
+		res++;
+	*p = res;
 	put_cpu_var(last_ino);
 	return res;
 }
-- 
1.7.10.4