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Date:   Tue, 15 Sep 2020 17:44:01 -0700
From:   Yonghong Song <yhs@...com>
To:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, <kernel-team@...com>,
        Andrii Nakryiko <andriin@...com>,
        Martin KaFai Lau <kafai@...com>
Subject: [PATCH bpf] bpf: fix a rcu warning for bpffs map pretty-print

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.

Reported-by: Alexei Starovoitov <ast@...nel.org>
Cc: Andrii Nakryiko <andriin@...com>
Cc: Martin KaFai Lau <kafai@...com>
Fixes: a26ca7c982cb ("bpf: btf: Add pretty print support to the basic arraymap")
Signed-off-by: Yonghong Song <yhs@...com>
---
 kernel/bpf/inode.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
index fb878ba3f22f..18f4969552ac 100644
--- a/kernel/bpf/inode.c
+++ b/kernel/bpf/inode.c
@@ -226,10 +226,12 @@ 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;
 }
 
-- 
2.24.1

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