bcachefs: Hold read lock in bch2_snapshot_tree_oldest_subvol()
Syzbot reports a problem that a warning is triggered due to suspicious use of rcu_dereference_check(). That is triggered by a call of bch2_snapshot_tree_oldest_subvol(). The cause of the warning is that inside bch2_snapshot_tree_oldest_subvol(), snapshot_t() is called which calls rcu_dereference() that requires a read lock to be held. Also, the call of bch2_snapshot_tree_next() eventually calls snapshot_t(). To fix this, call rcu_read_lock() before calling snapshot_t(). Then, release the lock after the termination of the while loop. Reported-by: <syzbot+f7c41a878676b72c16a6@syzkaller.appspotmail.com> Signed-off-by: Ahmed Ehab <bottaawesome633@gmail.com> Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
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@ -469,6 +469,7 @@ static u32 bch2_snapshot_tree_oldest_subvol(struct bch_fs *c, u32 snapshot_root)
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u32 id = snapshot_root;
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u32 subvol = 0, s;
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rcu_read_lock();
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while (id) {
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s = snapshot_t(c, id)->subvol;
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@ -477,6 +478,7 @@ static u32 bch2_snapshot_tree_oldest_subvol(struct bch_fs *c, u32 snapshot_root)
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id = bch2_snapshot_tree_next(c, id);
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}
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rcu_read_unlock();
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return subvol;
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}
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