[PATCH 1/2] anon_inodes: use fops->owner for module refcount

From: Christian Borntraeger
Date: Tue Dec 02 2008 - 05:17:33 EST

There is an imbalance for anonymous inodes. If the fops->owner field is set,
the module reference count of owner is decreases on release.
("filp_close" --> "__fput" ---> "fops_put")

On the other hand, anon_inode_getfd does not increase the module reference
count of owner. This causes two problems:

- if owner is set, the module refcount goes negative
- if owner is not set, the module can be unloaded while code is running

This patch changes anon_inode_getfd to be symmetric regarding fops->owner

I have checked all existing users of anon_inode_getfd. Noone sets fops->owner,
thats why nobody has seen the module refcount negative. The refcounting was
tested with a patched and unpatched KVM module.(see patch 2/2) I also did an
epoll_open/close test.

Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
Reviewed-by: Davide Libenzi <davidel@xxxxxxxxxxxxxxx>
fs/anon_inodes.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

Index: kvm/fs/anon_inodes.c
--- kvm.orig/fs/anon_inodes.c
+++ kvm/fs/anon_inodes.c
@@ -79,9 +79,12 @@ int anon_inode_getfd(const char *name, c
if (IS_ERR(anon_inode_inode))
return -ENODEV;

+ if (fops->owner && !try_module_get(fops->owner))
+ return -ENOENT;
error = get_unused_fd_flags(flags);
if (error < 0)
- return error;
+ goto err_module;
fd = error;

@@ -128,6 +131,8 @@ err_dput:
+ module_put(fops->owner);
return error;
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/