[Bug 2004039] Re: libunwind 1.6.2-0 assumes 4k page sizes and crashes on systems with bigger page sizes

[Simon Quigley]

LGTM, sponsored, thank you!

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Title:
  libunwind 1.6.2-0 assumes 4k page sizes and crashes on systems with
  bigger page sizes

Status in libunwind package in Ubuntu:
  New
Status in libunwind source package in Kinetic:
  New

Bug description:
  [ Impact ]

   * On kernels with page size > 4K Xorg (and presumably other applications
     relying on libunwind) crashes on startup. This affects anyone
     running the official arm64 generic-64k kernel or custom non 4k kernels
     (as used by e.g. apple silicon).

     The exact error I am seeing in the logs is:

  Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: (EE) 0: /usr/lib/xorg/Xorg (OsLookupColor+0x188) [0xaaab456ca998]
  Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: (EE) unw_get_proc_info failed: no unwind info found [-10]
  Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: (EE)
  Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: (EE) Segmentation fault at address 0x0
  Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: (EE)
  Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: Fatal server error:
  Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: (EE) Caught signal 11 (Segmentation fault). Server aborting
  Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: (EE)
  Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: (EE)
  Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: Please consult the The X.Org Foundation support
  Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]:          at http://wiki.x.org
  Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]:  for help.
  Jan 30 11:16:20 ubuntu /usr/libexec/gdm-x-session[3199]: (EE)

    I have not found a workaround other than using wayland (which has other
    limitations). To reproduce use a kernel configured with a page size of
    16K (CONFIG_ARM64_16K_PAGES=y or CONFIG_ARM64_64K_PAGES=y or) and try
    to start "Ubuntu on Xorg" in gdm.

  [ Test Plan ]

   * Make sure Xorg doesn't crash on 4K, 16K and 64K kernels.

  [ Where problems could occur ]

   * We will have to make sure the fixed version still works with 4k
     kernels. The patch is already widely in use so the risk seems low if
     we test properly.

  [ Other Info ]

   * The lunar version ships the bug fix synced from debian

   * Debian bug: https://bugs.debian.org/cgi-
  bin/bugreport.cgi?bug=1026217

   * Upstream fix:
  https://github.com/libunwind/libunwind/commit/e85b65cec757ef589f28957d0c6c21c498a03bdf

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