linux-kernel - Re: kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with nmi

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Date:   Wed, 27 Jan 2021 19:57:56 +0200
From:   Nikolay Borisov <nborisov@...e.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with
 nmi_enter()")



On 27.01.21 г. 17:24 ч., Masami Hiramatsu wrote:
> On Thu, 28 Jan 2021 00:13:53 +0900
> Masami Hiramatsu <mhiramat@...nel.org> wrote:
> 
>> Hi Nikolay,
>>
>> On Wed, 27 Jan 2021 15:43:29 +0200
>> Nikolay Borisov <nborisov@...e.com> wrote:
>>
>>> Hello,
>>>
>>> I'm currently seeing latest Linus' master being somewhat broken w.r.t
>>> krpobes. In particular I have the following test-case:
>>>
>>> #!/bin/bash
>>>
>>> mkfs.btrfs -f /dev/vdc &> /dev/null
>>> mount /dev/vdc /media/scratch/
>>>
>>> bpftrace -e 'kprobe:btrfs_sync_file {printf("kprobe: %s\n", kstack());}'
>>> &>bpf-output &
>>> bpf_trace_pid=$!
>>>
>>> # force btrfs_sync_file to be called
>>> sleep 2
>>> xfs_io -f -c "pwrite 0 4m" -c "fsync" /media/scratch/file5
>>>
>>> kill $bpf_trace_pid
>>> sleep 1
>>>
>>> grep -q kprobe bpf-output
>>> retval=$?
>>> rm -f bpf-output
>>> umount /media/scratch
>>>
>>> exit $retval
>>>
>>> It traces btrfs_sync_file which is called when fsync is executed on a
>>> btrfs file, however I don't see the stacktrace being printed i.e the
>>> kprobe doesn't fire at all. The following alternative program:
>>>
>>> bpftrace -e 'tracepoint:btrfs:btrfs_sync_file {printf("tracepoint:
>>> %s\n", kstack());} kprobe:btrfs_sync_file {printf("kprobe: %s\n",
>>> kstack());}'
>>>
>>> only prints the stack from the tracepoint and not from the kprobe, given
>>> that the tracepoint is called from the btrfs_sync_file function.
>>
>> Thank you for reporting!
>>
>> If you don't mind, could you confirm it with ftrace (tracefs)?
>> bpftrace etc. involves too many things. It is better to test with
>> simpler way to test it.
>> I'm not familer with the bpftrace, but I think you can check it with
>>
>> # cd /sys/kernel/debug/tracing
>> # echo p:myevent btrfs_sync_file >> kprobe_events
>> # echo stacktrace > events/kprobes/myevent/trigger
>>  (or echo 1 > options/stacktrace , if trigger file doesn't exist)
> 
> Of course, also you have to enable the event.
>  # echo 1 > events/kprobes/myevent/enable
> 
> And check the results
> 
>  # cat trace
> 
> 
>> Could you also share your kernel config, so that we can reproduce it?
> 

I've attached the config and indeed with the scenario you proposed it
seems to works. I see:

       xfs_io-20280   [000] d.Z.  9900.748633: myevent:
(btrfs_sync_file+0x0/0x580)
          xfs_io-20280   [000] d.Z.  9900.748647: <stack trace>
 => kprobe_trace_func
 => kprobe_dispatcher
 => kprobe_int3_handler
 => exc_int3
 => asm_exc_int3
 => btrfs_sync_file
 => do_fsync
 => __x64_sys_fsync
 => do_syscall_64
 => entry_SYSCALL_64_after_hwframe


<snip>

View attachment "config-bpftrace" of type "text/plain" (77813 bytes)