[PATCH 5.16 191/227] lockdep: Correct lock_classes index mapping

Greg Kroah-Hartman
2022-02-21T10:12:50Z

From: Cheng Jui Wang <cheng-jui.wang@mediatek.com>

commit 28df029d53a2fd80c1b8674d47895648ad26dcfb upstream.

A kernel exception was hit when trying to dump /proc/lockdep_chains after
lockdep report "BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!":

Unable to handle kernel paging request at virtual address 00054005450e05c3
...
00054005450e05c3] address between user and kernel address ranges
...
pc : [0xffffffece769b3a8] string+0x50/0x10c
lr : [0xffffffece769ac88] vsnprintf+0x468/0x69c
...
Call trace:
string+0x50/0x10c
vsnprintf+0x468/0x69c
seq_printf+0x8c/0xd8
print_name+0x64/0xf4
lc_show+0xb8/0x128
seq_read_iter+0x3cc/0x5fc
proc_reg_read_iter+0xdc/0x1d4

The cause of the problem is the function lock_chain_get_class() will
shift lock_classes index by 1, but the index don't need to be shifted
anymore since commit 01bb6f0af992 ("locking/lockdep: Change the range
of class_idx in held_lock struct") already change the index to start
from 0.

The lock_classes[-1] located at chain_hlocks array. When printing
lock_classes[-1] after the chain_hlocks entries are modified, the
exception happened.

The output of lockdep_chains are incorrect due to this problem too.

Fixes: f611e8cf98ec ("lockdep: Take read/write status in consideration when generate chainkey")
Signed-off-by: Cheng Jui Wang <cheng-jui.wang@mediatek.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20220210105011.21712-1-cheng-jui.wang@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/locking/lockdep.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -3462,7 +3462,7 @@ struct lock_class *lock_chain_get_class(
u16 chain_hlock = chain_hlocks[chain->base + i];
unsigned int class_idx = chain_hlock_class_idx(chain_hlock);

- return lock_classes + class_idx - 1;
+ return lock_classes + class_idx;
}

/*
@@ -3530,7 +3530,7 @@ static void print_chain_keys_chain(struc
hlock_id = chain_hlocks[chain->base + i];
chain_key = print_chain_key_iteration(hlock_id, chain_key);

- print_lock_name(lock_classes + chain_hlock_class_idx(hlock_id) - 1);
+ print_lock_name(lock_classes + chain_hlock_class_idx(hlock_id));
printk("\n");
}
}