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SubjectRe: WARNING with LBR + precise_ip=2 + bpf_get_stackid()
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> On Jun 18, 2020, at 3:35 PM, Andi Kleen <ak@linux.intel.com> wrote:
>
>> We noticed that this only happens with precise_ip >= 2. This is caused by
>> setup_pebs_fixed_sample_data() using pens->real_ip:
>>
>> set_linear_ip(regs, pebs->real_ip);
>
> With precise_ip == 1 the IP will be one instruction behind.
> That's the only difference to 2 or 3.
>
> So something about the actual instruction confuses the unwinder.
>
> I would check if there is anything special about these IPs:
>
>> 0xfffffe00004d1f78 entry_SYSCALL_64
>> 0xfffffe00004d1fa0 entry_SYSCALL_64
>> 0xfffffe00004d1fd8 entry_SYSCALL_64

I don't have this system at the moment. For the system I have, I got:

[36000.163945] WARNING: can't access registers at error_entry+0x74/0xe0
[36000.243879] WARNING: can't access registers at error_entry+0x65/0xe0
[36000.334288] WARNING: can't access registers at syscall_return_via_sysret+0x12/0x7f
[36000.540940] WARNING: can't access registers at syscall_return_via_sysret+0x6/0x7f
[58419.640387] WARNING: can't access registers at syscall_return_via_sysret+0xb/0x7f
[58423.082418] WARNING: can't access registers at syscall_return_via_sysret+0xf/0x7f
[58423.155705] WARNING: can't access registers at syscall_return_via_sysret+0x6/0x7f

And

(gdb) x/5i error_entry+0x65
0xffffffff81c01075 <error_entry+101>: pop %r12
0xffffffff81c01077 <error_entry+103>: mov %rsp,%rdi
0xffffffff81c0107a <error_entry+106>: callq 0xffffffff81aa7a30 <sync_regs>
0xffffffff81c0107f <error_entry+111>: mov %rax,%rsp
0xffffffff81c01082 <error_entry+114>: push %r12
(gdb) x/5i error_entry+0x74
0xffffffff81c01084 <error_entry+116>: retq
0xffffffff81c01085 <error_entry+117>: nop
0xffffffff81c01086 <error_entry+118>: nop
0xffffffff81c01087 <error_entry+119>: nop
0xffffffff81c01088 <error_entry+120>: retq
(gdb) x/5i syscall_return_via_sysret+0xb
0xffffffff81c000ec <entry_SYSCALL_64+236>: pop %r10
0xffffffff81c000ee <entry_SYSCALL_64+238>: pop %r9
0xffffffff81c000f0 <entry_SYSCALL_64+240>: pop %r8
0xffffffff81c000f2 <entry_SYSCALL_64+242>: pop %rax
0xffffffff81c000f3 <entry_SYSCALL_64+243>: pop %rsi
(gdb) x/5i syscall_return_via_sysret+0xa
0xffffffff81c000eb <entry_SYSCALL_64+235>: pop %rsi
0xffffffff81c000ec <entry_SYSCALL_64+236>: pop %r10
0xffffffff81c000ee <entry_SYSCALL_64+238>: pop %r9
0xffffffff81c000f0 <entry_SYSCALL_64+240>: pop %r8
0xffffffff81c000f2 <entry_SYSCALL_64+242>: pop %rax
(gdb) x/5i syscall_return_via_sysret+0x12
0xffffffff81c000f3 <entry_SYSCALL_64+243>: pop %rsi
0xffffffff81c000f4 <entry_SYSCALL_64+244>: pop %rdx
0xffffffff81c000f5 <entry_SYSCALL_64+245>: pop %rsi
0xffffffff81c000f6 <entry_SYSCALL_64+246>: mov %rsp,%rdi
0xffffffff81c000f9 <entry_SYSCALL_64+249>: mov %gs:0x6004,%rsp
(gdb) x/5i syscall_return_via_sysret+0x6
0xffffffff81c000e7 <entry_SYSCALL_64+231>: pop %r12
0xffffffff81c000e9 <entry_SYSCALL_64+233>: pop %rbp
0xffffffff81c000ea <entry_SYSCALL_64+234>: pop %rbx
0xffffffff81c000eb <entry_SYSCALL_64+235>: pop %rsi
0xffffffff81c000ec <entry_SYSCALL_64+236>: pop %r10

I am not sure whether there is anything special. Maybe "pop"?

Thanks,
Song

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