[5.7-rc5 x86 regression] ORC unwinder generates unreliable traces

[Tetsuo Handa]

Hello.

syzbot is troubled by CONFIG_UNWINDER_ORC=y .

As of commit d2f8825ab78e4c18 ("Merge tag 'for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost"),
sched_show_task() is defunctional (all threads' traces are marked as 'unreliable').

[   43.975229] sysrq: Show State
[   43.978609]   task                        PC stack   pid father
[   43.981856] systemd         S11664     1      0 0x00000000
[   43.984118] Call Trace:
[   43.985626]  ? __schedule+0x27d/0x770
[   43.987738]  ? schedule+0x5a/0xd0
[   43.989509]  ? schedule_hrtimeout_range_clock+0x11c/0x130
[   43.991702]  ? ep_poll+0x461/0x4f0
[   43.993384]  ? _raw_write_unlock_irq+0x1f/0x30
[   43.995343]  ? ep_poll+0x472/0x4f0
[   43.997053]  ? wait_woken+0xa0/0xa0
[   43.998763]  ? do_epoll_wait+0xcd/0xf0
[   44.000494]  ? __x64_sys_epoll_wait+0x15/0x20
[   44.002385]  ? do_syscall_64+0x45/0x1d0
[   44.004158]  ? entry_SYSCALL_64_after_hwframe+0x49/0xb3

Since v5.7-rc5 is bad and v5.7-rc4 is good, the culprit is commit c14cab2688d09b85
("Merge tag 'x86-urgent-2020-05-10' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip")
which contains

    Pull x86 fixes from Thomas Gleixner:
     "A set of fixes for x86:
(...snipped...)
       - A series of fixes for objtool and the ORC unwinder addressing
         variety of small issues:

in the log.

----------
Kernel 5.7.0-rc6-00069-gd2f8825ab78e on an x86_64

localhost login: [   43.975229][    C6] sysrq: Show State
[   43.978609][    C6]   task                        PC stack   pid father
[   43.981856][    C6] systemd         S11664     1      0 0x00000000
[   43.984118][    C6] Call Trace:
[   43.985626][    C6]  ? __schedule+0x27d/0x770
[   43.987738][    C6]  ? schedule+0x5a/0xd0
[   43.989509][    C6]  ? schedule_hrtimeout_range_clock+0x11c/0x130
[   43.991702][    C6]  ? ep_poll+0x461/0x4f0
[   43.993384][    C6]  ? _raw_write_unlock_irq+0x1f/0x30
[   43.995343][    C6]  ? ep_poll+0x472/0x4f0
[   43.997053][    C6]  ? wait_woken+0xa0/0xa0
[   43.998763][    C6]  ? do_epoll_wait+0xcd/0xf0
[   44.000494][    C6]  ? __x64_sys_epoll_wait+0x15/0x20
[   44.002385][    C6]  ? do_syscall_64+0x45/0x1d0
[   44.004158][    C6]  ? entry_SYSCALL_64_after_hwframe+0x49/0xb3

----------
Kernel 5.7.0-rc5 on an x86_64

localhost login: [   33.662344][    C6] sysrq: Show State
[   33.663434][    C6]   task                        PC stack   pid father
[   33.665144][    C6] systemd         S11968     1      0 0x00000000
[   33.666724][    C6] Call Trace:
[   33.667603][    C6]  ? __schedule+0x27d/0x770
[   33.668692][    C6]  ? schedule+0x5a/0xd0
[   33.669834][    C6]  ? schedule_hrtimeout_range_clock+0x11c/0x130
[   33.671397][    C6]  ? ep_poll+0x29e/0x4d0
[   33.672494][    C6]  ? _raw_write_unlock_irq+0x1f/0x30
[   33.673811][    C6]  ? ep_poll+0x45c/0x4d0
[   33.674839][    C6]  ? wait_woken+0xa0/0xa0
[   33.676000][    C6]  ? do_epoll_wait+0xcd/0xf0
[   33.677184][    C6]  ? __x64_sys_epoll_wait+0x15/0x20
[   33.678433][    C6]  ? do_syscall_64+0x45/0x1d0
[   33.679637][    C6]  ? entry_SYSCALL_64_after_hwframe+0x49/0xb3

----------
Kernel 5.7.0-rc4 on an x86_64

localhost login: [   35.448925][    C6] sysrq: Show State
[   35.450252][    C6]   task                        PC stack   pid father
[   35.452227][    C6] systemd         S11944     1      0 0x00000000
[   35.454001][    C6] Call Trace:
[   35.454983][    C6]  ? __schedule+0x27d/0x770
[   35.456288][    C6]  schedule+0x5a/0xd0
[   35.457393][    C6]  schedule_hrtimeout_range_clock+0x11c/0x130
[   35.459106][    C6]  ? ep_poll+0x40b/0x4c0
[   35.460306][    C6]  ? _raw_write_unlock_irq+0x1f/0x30
[   35.461840][    C6]  ep_poll+0x38f/0x4c0
[   35.463017][    C6]  ? wake_up_q+0x90/0x90
[   35.464230][    C6]  do_epoll_wait+0xcd/0xf0
[   35.465464][    C6]  __x64_sys_epoll_wait+0x15/0x20
[   35.466878][    C6]  do_syscall_64+0x45/0x1d0
[   35.468073][    C6]  entry_SYSCALL_64_after_hwframe+0x49/0xb3
[   35.469657][    C6] RIP: 0033:0x7f1f191b0e93
[   35.470910][    C6] Code: Bad RIP value.
[   35.472046][    C6] RSP: 002b:00007fffc4c25c88 EFLAGS: 00000246 ORIG_RAX: 00000000000000e8
[   35.474303][    C6] RAX: ffffffffffffffda RBX: 00007fffc4c25c90 RCX: 00007f1f191b0e93
[   35.476852][    C6] RDX: 0000000000000035 RSI: 00007fffc4c25c90 RDI: 0000000000000004
[   35.479254][    C6] RBP: 00007fffc4c26000 R08: 9c23bff2734474f6 R09: 000055ec811556e0
[   35.481458][    C6] R10: 00000000ffffffff R11: 0000000000000246 R12: 0000000000000001
[   35.483643][    C6] R13: ffffffffffffffff R14: 0000000000000000 R15: 000055ec81fd23b0

----------