Re: [PATCH] ublk_drv: fix NULL pointer dereference in ublk_ctrl_start_recovery()

[Ming Lei <ming.lei@redhat.com>]

On Thu, Jun 06, 2024 at 04:05:33PM +0800, Li Nan wrote:
> 
> 
> 在 2024/6/6 12:48, Changhui Zhong 写道:
> 
> [...]
> 
> > > 
> > > Hi Changhui,
> > > 
> > > The hang is actually expected because recovery fails.
> > > 
> > > Please pull the latest ublksrv and check if the issue can still be
> > > reproduced:
> > > 
> > > https://github.com/ublk-org/ublksrv
> > > 
> > > BTW, one ublksrv segfault and two test cleanup issues are fixed.
> > > 
> > > Thanks,
> > > Ming
> > > 
> > 
> > Hi,Ming and Nan
> > 
> > after applying the new patch and pulling the latest ublksrv,
> > I ran the test for 4 hours and did not observe any task hang.
> > the test results looks good！
> > 
> > Thanks，
> > Changhui
> > 
> > 
> > .
> 
> Thanks for you test!
> 
> However, I got a NULL pointer dereference bug with ublksrv. It is not

BTW, your patch isn't related with generic/004 which won't touch
recovery code path.

> introduced by this patch. It seems io was issued after deleting disk. And
> it can be reproduced by:
> 
>   while true; do make test T=generic/004; done

We didn't see that when running such test with linus tree, and usually
Changhui run generic test for hours.

> 
> [ 1524.286485] running generic/004
> [ 1529.110875] blk_print_req_error: 109 callbacks suppressed
...
> [ 1541.171010] BUG: kernel NULL pointer dereference, address: 0000000000000000
> [ 1541.171734] #PF: supervisor write access in kernel mode
> [ 1541.172271] #PF: error_code(0x0002) - not-present page
> [ 1541.172798] PGD 0 P4D 0
> [ 1541.173065] Oops: Oops: 0002 [#1] PREEMPT SMP
> [ 1541.173515] CPU: 0 PID: 43707 Comm: ublk Not tainted
> 6.9.0-next-20240523-00004-g9bc7e95c7323 #454
> [ 1541.174417] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> 1.16.1-2.fc37 04/01/2014
> [ 1541.175311] RIP: 0010:io_fallback_tw+0x252/0x300

This one looks one io_uring issue.

Care to provide which line of source code points to by 'io_fallback_tw+0x252'?

gdb> l *(io_fallback_tw+0x252)


Thanks,
Ming