[Qemu-devel] [Bug 1749393] Re: sbrk() not working under qemu-user with a PIE-compiled binary?

[Matthias Klose]

** Also affects: qemu (Ubuntu)
   Importance: Undecided
       Status: New

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Title:
  sbrk() not working under qemu-user with a PIE-compiled binary?

Status in QEMU:
  New
Status in qemu package in Ubuntu:
  New

Bug description:
  In Debian unstable, we recently switched bash to be a PIE-compiled
  binary (for hardening). Unfortunately this resulted in bash being
  broken when run under qemu-user (for all target architectures, host
  being amd64 for me).

  $ sudo chroot /srv/chroots/sid-i386/ qemu-i386-static /bin/bash
  bash: xmalloc: .././shell.c:1709: cannot allocate 10 bytes (0 bytes allocated)

  bash has its own malloc implementation based on sbrk():
  https://git.savannah.gnu.org/cgit/bash.git/tree/lib/malloc/malloc.c

  When we disable this internal implementation and rely on glibc's
  malloc, then everything is fine. But it might be that glibc has a
  fallback when sbrk() is not working properly and it might hide the
  underlying problem in qemu-user.

  This issue has also been reported to the bash upstream author and he 
suggested that the issue might be in qemu-user so I'm opening a ticket here. 
Here's the discussion with the bash upstream author:
  https://lists.gnu.org/archive/html/bug-bash/2018-02/threads.html#00080

  You can find the problematic bash binary in that .deb file:
  
http://snapshot.debian.org/archive/debian/20180206T154716Z/pool/main/b/bash/bash_4.4.18-1_i386.deb

  The version of qemu I have been using is 2.11 (Debian package qemu-
  user-static version 1:2.11+dfsg-1) but I have had reports that the
  problem is reproducible with older versions (back to 2.8 at least).

  Here are the related Debian bug reports:
  https://bugs.debian.org/889869
  https://bugs.debian.org/865599

  It's worth noting that bash used to have this problem (when compiled as a PIE 
binary) even when run directly but then something got fixed in the kernel and 
now the problem only appears when run under qemu-user:
  https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1518483

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