[PATCH v8 8/8] livepatch: Detect offset for the ftrace location during build

Wed Feb 17 14:08:41 AEDT 2016

On Tue, 2016-02-16 at 14:57 +0100, Petr Mladek wrote:
> 
> Some dugging has shown an Oops in the fucntion int_to_scsilun()
> called from ibmvscsi_queuecommand(). So, I rebooted and
> did the following test:
> 
> $> echo ibmvscsi_queuecommand >/sys/kernel/debug/tracing/set_ftrace_filter
> $> echo function > /sys/kernel/debug/tracing/current_tracer 
> $> echo 1 > /sys/kernel/debug/tracing/tracing_on
> $> cat /sys/kernel/debug/tracing/trace
> # tracer: function
> #
> # entries-in-buffer/entries-written: 7/7   #P:4
> #
> #                              _-----=> irqs-off
> #                             / _----=> need-resched
> #                            | / _---=> hardirq/softirq
> #                            || / _--=> preempt-depth
> #                            ||| /     delay
> #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
> #              | |       |   ||||       |         |
>             bash-3488  [000] ....   100.278622: ibmvscsi_queuecommand <-scsi_dispatch_cmd
>      kworker/1:2-223   [001] ....   101.048569: ibmvscsi_queuecommand <-scsi_dispatch_cmd
>      kworker/1:2-223   [001] ....   103.048575: ibmvscsi_queuecommand <-scsi_dispatch_cmd
>      jbd2/sda3-8-1021  [003] ....   104.008645: ibmvscsi_queuecommand <-scsi_dispatch_cmd
>      jbd2/sda3-8-1021  [003] ....   104.008883: ibmvscsi_queuecommand <-scsi_dispatch_cmd
>           <idle>-0     [000] ..s.   104.017672: ibmvscsi_queuecommand <-scsi_dispatch_cmd
>           <idle>-0     [003] ..s.   104.017771: ibmvscsi_queuecommand <-scsi_dispatch_cmd
> 
> It means that ibmvscsi_queuecommand can be traced. Then I did
> 
> c79:/sys/kernel/debug/tracing # echo int_to_scsilun >set_ftrace_filter
> 
> BANG!
> 
> Unable to handle kernel paging request for data at address 0xd00000000108b148
> Faulting instruction address: 0xd000000000bde35c
> Oops: Kernel access of bad area, sig: 11 [#1]
> SMP NR_CPUS=2048 NUMA pSeries
> Modules linked in: af_packet(E) dm_mod(E) e1000(E) rtc_generic(E) ext4(E) crc16(E) mbcache(E) jbd2(E) sr_mod(E) cdrom(E) sd_mod(E) ibmvscsi(E) scsi_transport_srp(E) sg(E) scsi_mod(E) autofs4(E)
> CPU: 1 PID: 223 Comm: kworker/1:2 Tainted: G            E   4.5.0-rc2-11-default+ #90
> Workqueue: events_freezable_power_ disk_events_workfn
> task: c0000000f7d99aa0 ti: c0000000f7cb8000 task.ti: c0000000f7cb8000
> NIP: d000000000bde35c LR: d000000000bcffec CTR: d000000000bcffe0
> REGS: c0000000f7cbb3b0 TRAP: 0300   Tainted: G            E    (4.5.0-rc2-11-default+)
> MSR: 8000000100009033 <SF,EE,ME,IR,DR,RI,LE,TM[E]>  CR: 24c82220  XER: 00000000
> CFAR: d000000000bdd144 DAR: d00000000108b148 DSISR: 40000000 SOFTE: 0 
> GPR00: d0000000010a21cc c0000000f7cbb630 d0000000010aeda0 0000000000008200 
> GPR04: c0000000fa0b33fc 000000000000014a c0000000fa0b3408 0000000000000010 
> GPR08: 0000000000000008 c00000000370a4e0 0000000000000000 d00000000108b128 
> GPR12: d000000000bcffe0 c000000007e80300 c0000000000d8b18 c0000000fe04ba80 
> GPR16: 0000000000000000 c0000000f7b6f208 0000000000000000 0000000000010000 
> GPR20: c0000000f7b6f144 c0000000f7b6f140 d000000000bcbcf0 0000000000000000 
> GPR24: 0000000000000200 0000000000000000 0000000000000001 c0000000f7b6f810 
> GPR28: c0000000f7b6f000 c0000000f7b6f000 c0000000fa0b33a0 c0000000fe837e00 
> NIP [d000000000bde35c] scsi_inq_str+0x21b0/0x41ac [scsi_mod]
> LR [d000000000bcffec] int_to_scsilun+0xc/0x60 [scsi_mod]
> Call Trace:
> [c0000000f7cbb630] [d0000000010a21cc] ibmvscsi_queuecommand+0x10c/0x4e0 [ibmvscsi] (unreliable)
> [c0000000f7cbb6e0] [d000000000bcbea8] scsi_dispatch_cmd+0xe8/0x2c0 [scsi_mod]
> [c0000000f7cbb760] [d000000000bceb0c] scsi_request_fn+0x50c/0x8b0 [scsi_mod]
> [c0000000f7cbb850] [c000000000407280] __blk_run_queue+0x60/0x90
> [c0000000f7cbb880] [c000000000413640] blk_execute_rq_nowait+0x100/0x1a0
> [c0000000f7cbb8d0] [c000000000413768] blk_execute_rq+0x88/0x170
> [c0000000f7cbb9b0] [d000000000bca048] scsi_execute+0x108/0x1d0 [scsi_mod]
> [c0000000f7cbba20] [d000000000bca2e8] scsi_execute_req_flags+0xc8/0x150 [scsi_mod]
> [c0000000f7cbbae0] [d0000000012209e4] sr_check_events+0xb4/0x340 [sr_mod]
> [c0000000f7cbbb90] [d0000000011c00b4] cdrom_check_events+0x44/0x80 [cdrom]
> [c0000000f7cbbbc0] [d000000001220fa4] sr_block_check_events+0x44/0x60 [sr_mod]
> [c0000000f7cbbbe0] [c0000000004222f8] disk_check_events+0x78/0x1b0
> [c0000000f7cbbc50] [c0000000000d0610] process_one_work+0x1a0/0x480
> [c0000000f7cbbce0] [c0000000000d0998] worker_thread+0xa8/0x5c0
> [c0000000f7cbbd80] [c0000000000d8c24] kthread+0x114/0x140
> [c0000000f7cbbe30] [c000000000009538] ret_from_kernel_thread+0x5c/0xa4
> Instruction dump:
> 396bc360 f8410018 e98b0020 7d8903a6 4e800420 00000000 00000000 0044fb30 
> c0000000 3d62fffe 396bc388 60000000 <e98b0020> 7d8903a6 4e800420 00000000 
> ---[ end trace 3b830c669dd7adb5 ]---
> 
> 
> Note that ibmvscsi_queuecommand() handle TOC and int_to_scsilun()
> does not handle TOC
> 
> $> objdump -hdr  drivers/scsi/ibmvscsi/ibmvscsi.ko
> 00000000000020c0 <ibmvscsi_queuecommand>:
>     20c0:       00 00 4c 3c     addis   r2,r12,0
>                         20c0: R_PPC64_REL16_HA  .TOC.
>     20c4:       00 00 42 38     addi    r2,r2,0
>                         20c4: R_PPC64_REL16_LO  .TOC.+0x4
>     20c8:       a6 02 08 7c     mflr    r0
>     20cc:       10 00 01 f8     std     r0,16(r1)
>     20d0:       01 00 00 48     bl      20d0 <ibmvscsi_queuecommand+0x10>
>                         20d0: R_PPC64_REL24     _mcount
>     20d4:       a6 02 08 7c     mflr    r0
> 
> 
> $> objdump -hdr  drivers/scsi/scsi_mod.ko
> [...]
> 000000000000ffe0 <int_to_scsilun>:
>     ffe0:       a6 02 08 7c     mflr    r0
>     ffe4:       10 00 01 f8     std     r0,16(r1)
>     ffe8:       01 00 00 48     bl      ffe8 <int_to_scsilun+0x8>
>                         ffe8: R_PPC64_REL24     _mcount
>     ffec:       a6 02 08 7c     mflr    r0
>     fff0:       10 00 01 f8     std     r0,16(r1)
>     fff4:       e1 ff 21 f8     stdu    r1,-32(r1)
> 
> 
> I am able to trace int_to_scsilun() if I add a global call
> into this fucntion.
> 
> I could trace any function when everything is built into the kernel.
> 
> 
> I am not sure where the problem is but even normal tracing does not
> work with this patchset and when -pg -mprofile-kernel is used.

When you call into ibmvscsi_queuecommand() it sets up r2 to hold the TOC for
ibmvscsi.ko.

It then calls int_to_scsilun(). int_to_scsilun() does no TOC setup, and tries
to call _mcount. It can't call _mcount() directly (because int_to_scsilun() is
in a module), it has to go via a stub.

That stub uses r2 to find the location of itself, but it only works if r2 holds
the TOC for scsi_mod.ko. In this case r2 still contains ibmvscsi.ko's TOC.

So when the stub calculates the address of itself it gets the wrong value, and
then when it does a load using that address it either faults (as in your case),
or you get some bogus value and jump into a random piece of code.

cheers