[Bug 1908756] Re: Ubuntu 20.10 - elfutils unwinding broken for s390 compat

[Frank Heimes]

MP for hirsute is open

** Changed in: ubuntu-z-systems
       Status: New => In Progress

** Changed in: elfutils (Ubuntu Hirsute)
     Assignee: Skipper Bug Screeners (skipper-screen-team) => Frank Heimes (fheimes)

** Changed in: elfutils (Ubuntu Groovy)
     Assignee: (unassigned) => Skipper Bug Screeners (skipper-screen-team)

** Changed in: elfutils (Ubuntu Hirsute)
       Status: New => In Progress

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Title:
  Ubuntu 20.10 - elfutils unwinding broken for s390 compat

Status in Ubuntu on IBM z Systems:
  In Progress
Status in elfutils package in Ubuntu:
  In Progress
Status in elfutils source package in Groovy:
  New
Status in elfutils source package in Hirsute:
  In Progress

Bug description:
  SRU Bug Template:
  =================

  [Impact]

   * There is an endianess problem in pid_memory_read that impacts
  s390x.

   * Due to this elfutils biarch test fails on s390x doing unwinding for
  a 32 bit process.

  [Fix]

   * e4d985a3c1c873f77d20fa0cd421458cc2824996 e4d985a3 "IBM Z: Fix endianess problem in pid_memory_read"
   
  [Test Case]

   * Have an Ubuntu Server 20.10 system or newer installed on LPAR, z/VM or KVM that comes with elfutils 0.181 or 0.182.
   
   * Run the test script: 'run-backtrace-native-biarch.sh'
   
   * It either core dumps without the patch  in place - look for "/test-subr.sh: line 84: 376822 Aborted (core dumped)" or succeeds.
   
   [Where problems could occur]
   
   * If the translation is not done right (shift of the upper 4 bytes + down on big endian 64 bit targets) the situation is broken in the same way, and things stay the same.
   
   * But in worst case the changes in 'pid_memory_read' of /libdwfl/linux-pid-attach.c could have a negative impact even on architectures other than s390x, in case the check for the endiness is wrong.
   
   * But the changes are quite traceable and the additional code that fixes the endianess problem is really only active on a big endian architecture.
   
   [Other]
   
   * The fix is upstream accepted with elfutils > 0.182.
  __________

  ---Problem Description---
  libdw unwinding fails for 32 bit binaries.

  Elfutils biarch test currently fails on s390x doing unwinding for a 32
  bit process.

  FAIL: run-backtrace-native-biarch.sh
  ====================================

  0x557e7000      0x557eb000      /root/elfutils/tests/backtrace-child-biarch
  0x7dba4000      0x7dd51000      /usr/lib/libc-2.32.so
  0x7dd53000      0x7dd70000      /usr/lib/libpthread-2.32.so
  0x7dd7f000      0x7dda6000      /usr/lib/ld-2.32.so
  TID 376858:
  # 0 0x7dd6668a          raise
  # 1 0x7fd0ef60 - 1      <null>
  /root/elfutils/tests/backtrace: dwfl_thread_getframes: No DWARF information found
  backtrace: backtrace.c:114: callback_verify: Assertion `symname != NULL && strcmp (symname, "sigusr2") == 0' failed.
  ./test-subr.sh: line 84: 376822 Aborted                 (core dumped) LD_LIBRARY_PATH="${built_library_path}${LD_LIBRARY_PATH:+:}$LD_LIBRARY_PATH" $VALGRIND_CMD "$@"
  backtrace-child-biarch: no main
  FAIL run-backtrace-native-biarch.sh (exit status: 1)

  Patch has already been posted and upstream committed:
  https://sourceware.org/pipermail/elfutils-devel/2020q4/003149.html

  Please pick up the following commit for the next release:
  commit e4d985a3c1c873f77d20fa0cd421458cc2824996
  Author: Andreas Krebbel <krebbel at linux.ibm.com>
  Date:   Thu Nov 19 20:32:24 2020 +0100

      IBM Z: Fix endianess problem in pid_memory_read

      The cached reads lack the big endian adjustments done in the fallback
      path.

      Signed-off-by: Andreas Krebbel <krebbel at linux.ibm.com>

  The patch applies cleanly ontop of elfutils_0.181-1.

  ====
  strace right now is not built with unwinding support for IBM Z although both variants should work - libdw and libunwind.

  Other distros use libdw from elfutils while Ubuntu on x86 appears to
  use libunwind. libdw and libunwind do support IBM Z.

  It would be good to build strace with unwinding support for Z.

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