lkml.org 
[lkml]   [2021]   [Apr]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[Patch] blk-wbt: fix a divide-by-zero error in rwb_arm_timer()
Date
From: Cong Wang <cong.wang@bytedance.com>

We hit a divide error in rwb_arm_timer() and crash dump shows
rqd->scale_step is 16777215 (0xffffff in hex), so the expression
"(rqd->scale_step + 1) << 8)" is 0x100000000, which is just beyond
32-bit integer range, hence it is truncated to 0 and int_sqrt(0)
returns 0 too, so we end up passing 0 as a divisor to div_u64().

Looking at the assembly code generated:

add $0x1,%edi
shl $0x8,%edi
movslq %edi,%rdi
mov 0x10(%rbx),%rdi
xor %edx,%edx
mov %eax,%ecx
shl $0x4,%rdi
mov %rdi,%rax
div %rcx

we notice that the left shift is still using 32 bit register %edi,
because the type of rqd->scale_step is 'int'. But actually int_sqrt()
takes 'long' as a parameter, so the temporary result should fit well
at least on x86_64. Fix this by explicitly casting the expression to
u64 and call int_sqrt64() to avoid any ambiguity on 32 bit.

After this patch, the assembly code looks correct:

add $0x1,%edi
movslq %edi,%rdi
shl $0x8,%rdi
mov 0x10(%rbx),%rdi
xor %edx,%edx
mov %eax,%ecx
shl $0x4,%rdi
mov %rdi,%rax
div %rcx

Fixes: e34cbd307477 ("blk-wbt: add general throttling mechanism")
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Fam Zheng <fam.zheng@bytedance.com>
Cc: Xiongchun Duan <duanxiongchun@bytedance.com>
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
---
block/blk-wbt.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-wbt.c b/block/blk-wbt.c
index 42aed0160f86..5157ca86574f 100644
--- a/block/blk-wbt.c
+++ b/block/blk-wbt.c
@@ -337,7 +337,7 @@ static void rwb_arm_timer(struct rq_wb *rwb)
* though.
*/
rwb->cur_win_nsec = div_u64(rwb->win_nsec << 4,
- int_sqrt((rqd->scale_step + 1) << 8));
+ int_sqrt64((u64)(rqd->scale_step + 1) << 8));
} else {
/*
* For step < 0, we don't want to increase/decrease the
--
2.25.1
\
 
 \ /
  Last update: 2021-04-18 06:42    [W:0.036 / U:0.048 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site