This is a note to let you know that I've just added the patch titled
bpf: Fix a rcu warning for bpffs map pretty-print
to the 4.19-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
bpf-fix-a-rcu-warning-for-bpffs-map-pretty-print.patch
and it can be found in the queue-4.19 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 78087bd0fa74175d58235fc3a46bd91ad75f6bb9
Author: Yonghong Song <yhs@xxxxxx>
Date: Tue Sep 15 17:44:01 2020 -0700
bpf: Fix a rcu warning for bpffs map pretty-print
[ Upstream commit ce880cb825fcc22d4e39046a6c3a3a7f6603883d ]
Running selftest
./btf_btf -p
the kernel had the following warning:
[ 51.528185] WARNING: CPU: 3 PID: 1756 at kernel/bpf/hashtab.c:717 htab_map_get_next_key+0x2eb/0x300
[ 51.529217] Modules linked in:
[ 51.529583] CPU: 3 PID: 1756 Comm: test_btf Not tainted 5.9.0-rc1+ #878
[ 51.530346] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.9.3-1.el7.centos 04/01/2014
[ 51.531410] RIP: 0010:htab_map_get_next_key+0x2eb/0x300
...
[ 51.542826] Call Trace:
[ 51.543119] map_seq_next+0x53/0x80
[ 51.543528] seq_read+0x263/0x400
[ 51.543932] vfs_read+0xad/0x1c0
[ 51.544311] ksys_read+0x5f/0xe0
[ 51.544689] do_syscall_64+0x33/0x40
[ 51.545116] entry_SYSCALL_64_after_hwframe+0x44/0xa9
The related source code in kernel/bpf/hashtab.c:
709 static int htab_map_get_next_key(struct bpf_map *map, void *key, void *next_key)
710 {
711 struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
712 struct hlist_nulls_head *head;
713 struct htab_elem *l, *next_l;
714 u32 hash, key_size;
715 int i = 0;
716
717 WARN_ON_ONCE(!rcu_read_lock_held());
In kernel/bpf/inode.c, bpffs map pretty print calls map->ops->map_get_next_key()
without holding a rcu_read_lock(), hence causing the above warning.
To fix the issue, just surrounding map->ops->map_get_next_key() with rcu read lock.
Fixes: a26ca7c982cb ("bpf: btf: Add pretty print support to the basic arraymap")
Reported-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Signed-off-by: Yonghong Song <yhs@xxxxxx>
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Acked-by: Andrii Nakryiko <andriin@xxxxxx>
Cc: Martin KaFai Lau <kafai@xxxxxx>
Link: https://lore.kernel.org/bpf/20200916004401.146277-1-yhs@xxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
index c04815bb15cc1..11fade89c1f38 100644
--- a/kernel/bpf/inode.c
+++ b/kernel/bpf/inode.c
@@ -207,10 +207,12 @@ static void *map_seq_next(struct seq_file *m, void *v, loff_t *pos)
else
prev_key = key;
+ rcu_read_lock();
if (map->ops->map_get_next_key(map, prev_key, key)) {
map_iter(m)->done = true;
- return NULL;
+ key = NULL;
}
+ rcu_read_unlock();
return key;
}
