On Mon, Mar 25, 2019 at 11:11:23AM +0000, Al Viro wrote:
> On Mon, Mar 25, 2019 at 10:15:40AM +0100, Daniel Borkmann wrote:
> > > So we have 5 broken cases, all with the same kind of fix: move freeing
> > > into the RCU-delayed part of ->destroy_inode(); for debugfs and bpf
> > > that requires adding ->alloc_inode()/->destroy_inode(), rather than
> > > relying upon the defaults from fs/inode.c
> >
> > I believe I copied that logic from one of the other fs'es back then, sigh.
> > Thanks for the analysis and hints for fixing. Would the below (only compile
> > tested for now) look reasonable to you? I believe there is no other way
> > around than to add our own inode cache, but so be it. The freeing of the
> > i_link is then RCU-deferred in bpf_destroy_inode_deferred():
>
> It looks like it would suffice, but it's way too much boilerplate for my
> taste ;-/
>
> Most of your headache here comes from messing with slab setup and the
> only reason for that is being unable to free stuff into inode_cachep.
> So grep for inode_cachep would be a good idea, and it digs up the
> following sucker:
> void free_inode_nonrcu(struct inode *inode)
> {
> kmem_cache_free(inode_cachep, inode);
> }
> EXPORT_SYMBOL(free_inode_nonrcu);
>
> IOW, you need nothing on ->alloc_inode() side and for ->destroy_inode()
> just do call_rcu() of a callback, that would kfree(inode->link) if
> it was a symlink, then call free_inode_nonrcu(inode).
>
> Considerably smaller patch that way...
AFAICS, it boils down to just this (also only compile-tested):
diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
index 2ada5e21dfa6..9437f88b6acf 100644
--- a/kernel/bpf/inode.c
+++ b/kernel/bpf/inode.c
@@ -80,6 +80,20 @@ static const struct inode_operations bpf_dir_iops;
static const struct inode_operations bpf_prog_iops = { };
static const struct inode_operations bpf_map_iops = { };
+static void bpf_destroy_inode_deferred(struct rcu_head *head)
+{
+ struct inode *inode = container_of(head, struct inode, i_rcu);
+
+ if (S_ISLNK(inode->i_mode))
+ kfree(inode->i_link);
+ free_inode_nonrcu(inode);
+}
+
+static void bpf_destroy_inode(struct inode *inode)
+{
+ call_rcu(&inode->i_rcu, bpf_destroy_inode_deferred);
+}
+
static struct inode *bpf_get_inode(struct super_block *sb,
const struct inode *dir,
umode_t mode)
@@ -561,8 +575,6 @@ static void bpf_evict_inode(struct inode *inode)
truncate_inode_pages_final(&inode->i_data);
clear_inode(inode);
- if (S_ISLNK(inode->i_mode))
- kfree(inode->i_link);
if (!bpf_inode_type(inode, &type))
bpf_any_put(inode->i_private, type);
}
@@ -584,6 +596,7 @@ static const struct super_operations bpf_super_ops = {
.drop_inode = generic_delete_inode,
.show_options = bpf_show_options,
.evict_inode = bpf_evict_inode,
+ .destroy_inode = bpf_destroy_inode,
};
enum {
