[Bug 1907298] Re: glibc 2.32-0ubuntu5 ADT test failure with linux 5.10.0-7.8

[Seth Forshee]

Looking through powerpc changes between Linux 5.8 and 5.10, I see a
handful of changes to signal-related code. This in particular jumps out
as potentially affecting the register state for signal handling:

commit 0138ba5783ae0dcc799ad401a1e8ac8333790df9
Author: Nicholas Piggin <npiggin at gmail.com>
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 at ha
        addi 2,2,.TOC.-.LCF0 at 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 <npiggin at gmail.com>
    Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
    Link: https://lore.kernel.org/r/20200511101952.1463138-1-npiggin@gmail.com

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https://bugs.launchpad.net/bugs/1907298

Title:
  glibc 2.32-0ubuntu5 ADT test failure with linux 5.10.0-7.8

Status in glibc package in Ubuntu:
  New

Bug description:
  Testing failed on:
      ppc64el: https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-hirsute-canonical-kernel-team-bootstrap/hirsute/ppc64el/g/glibc/20201208_192946_04f11@/log.gz

  ----------
  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
  ----------

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