[Qemu-devel] [Bug 1779955] Re: qemu linux-user requires read permissions on memory passed to syscalls that should only need write access

[Ke Liu]

Thanks Peter for your information.

I was hit by the bug when trying to compile bazel (a build system open-
sourced by Google: https://bazel.build) and the code is at
https://github.com/bazelbuild/bazel/blob/master/third_party/ijar/mapped_file_unix.cc#L116

Of course I can send PR to fix that in bazel source, but I would say it
is not true that "in practice no guest binaries set up memory that is
only writable and can't be read". It is legitimate requirement and
coding practice.

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https://bugs.launchpad.net/bugs/1779955

Title:
  qemu linux-user requires read permissions on memory passed to syscalls
  that should only need write access

Status in QEMU:
  Confirmed

Bug description:
  When read() function takes an mmap'ed address as output buffer, it
  returns EFAULT. The expected behavior is it should just work.

  The following code works for qemu-system-arm, but not for qemu-arm-
  static.

  QEMU version affected: latest release 2.12.0.

  Steps to reproduce (please substitute /path/to/qemu-arm-static with
  the path of the binary, and /tmp/a.cpp with the example source code
  attached):

  # First register binfmt_misc
  [hidden]$ docker run --rm --privileged multiarch/qemu-user-static:register 
--reset

  # Compile the code and run
  [hidden]$ docker run --rm -it -v /tmp/a.cpp:/tmp/a.cpp -v 
/path/to/qemu-arm-static:/usr/bin/qemu-arm-static arm32v7/ubuntu:18.04 bash -c 
'{ apt update -y && apt install -y g++; } >& /dev/null && g++ -std=c++14 
/tmp/a.cpp -o /tmp/a.out && echo hehe > /tmp/haha.txt && /tmp/a.out'
  ofd=3
  ftruncate=0
  mmap=0xff3f5000
  fd=4
  0xff3f5023 -1 14

  The expected result in qemu-system-arm as well as natively on x86_64 host:
  hidden$ ./a.out
  ofd=3
  ftruncate=0
  mmap=0xb6fb7000
  fd=4
  0xb6fb7023 5 0

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