Comment 5 for bug 1907298

Linux 5.10 includes an optimization that changes signal handling breaking sigcontext_get_pc, which is detected by misc/tst-sigcontext-get_pc:

commit 0138ba5783ae0dcc799ad401a1e8ac8333790df9
Author: Nicholas Piggin <email address hidden>
Date: Mon May 11 20:19:52 2020 +1000

    powerpc/64/signal: Balance return predictor stack in signal trampoline

    Returning from an interrupt or syscall to a signal handler currently
    begins execution directly at the handler's entry point, with LR set to
    the address of the sigreturn trampoline. When the signal handler
    function returns, it runs the trampoline. It looks like this:

        # interrupt at user address xyz
        # kernel stuff... signal is raised
        rfid
        # void handler(int sig)
        addis 2,12,.TOC.-.LCF0@ha
        addi 2,2,.TOC.-.LCF0@l
        mflr 0
        std 0,16(1)
        stdu 1,-96(1)
        # handler stuff
        ld 0,16(1)
        mtlr 0
        blr
        # __kernel_sigtramp_rt64
        addi r1,r1,__SIGNAL_FRAMESIZE
        li r0,__NR_rt_sigreturn
        sc
        # kernel executes rt_sigreturn
        rfid
        # back to user address xyz

    Note the blr with no matching bl. This can corrupt the return
    predictor.

    Solve this by instead resuming execution at the signal trampoline
    which then calls the signal handler. qtrace-tools link_stack checker
    confirms the entire user/kernel/vdso cycle is balanced after this
    patch, whereas it's not upstream.

    Alan confirms the dwarf unwind info still looks good. gdb still
    recognises the signal frame and can step into parent frames if it
    break inside a signal handler.

    Performance is pretty noisy, not a very significant change on a POWER9
    here, but branch misses are consistently a lot lower on a
    microbenchmark:

     Performance counter stats for './signal':

           13,085.72 msec task-clock # 1.000 CPUs utilized
      45,024,760,101 cycles # 3.441 GHz
      65,102,895,542 instructions # 1.45 insn per cycle
      11,271,673,787 branches # 861.372 M/sec
          59,468,979 branch-misses # 0.53% of all branches

           12,989.09 msec task-clock # 1.000 CPUs utilized
      44,692,719,559 cycles # 3.441 GHz
      65,109,984,964 instructions # 1.46 insn per cycle
      11,282,136,057 branches # 868.585 M/sec
          39,786,942 branch-misses # 0.35% of all branches

    Signed-off-by: Nicholas Piggin <email address hidden>
    Signed-off-by: Michael Ellerman <email address hidden>
    Link: https://<email address hidden>

---

----------
FAIL: misc/tst-sigcontext-get_pc
original exit status 1
info: address in signal handler: 0x737faa11db44
info: call stack entry 0: 0x737faa311f58
info: call stack entry 1: 0x737faa3404c4
info: call stack entry 2: 0x0
info: call stack entry 3: 0x737faa312144
info: call stack entry 4: 0x737faa312870
info: call stack entry 5: 0x737faa313264
info: call stack entry 6: 0x737faa311c40
info: call stack entry 7: 0x737faa0f9e5c
info: call stack entry 8: 0x737faa0fa040
error: ../sysdeps/unix/sysv/linux/tst-sigcontext-get_pc.c:60: not true: found
error: 1 test failures