Messages in this thread | | | From | Song Liu <> | Subject | Re: WARNING with LBR + precise_ip=2 + bpf_get_stackid() | Date | Thu, 18 Jun 2020 23:21:11 +0000 |
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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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