[Bug 1805913] Re: readdir() returns NULL (errno=EOVERFLOW) for 32-bit user-static qemu on 64-bit host

[Peter Maydell]

Yes, using a 32-bit host QEMU process will also work. You might run into
a few guest programs that don't work with that -- a 64-bit QEMU process
allows us to give the guest the full address space it might need, while
a 32-bit QEMU process means that QEMU itself must share with the guest,
so if the guest uses a lot of virtual memory or is picky about where it
maps things then it might fail to mmap() things where it wants them. But
it's probably overall the least-bad workaround at the current time.

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https://bugs.launchpad.net/bugs/1805913

Title:
  readdir() returns NULL (errno=EOVERFLOW) for 32-bit user-static qemu
  on 64-bit host

Status in QEMU:
  Confirmed

Bug description:
  This can be simply reproduced by compiling and running the attached C
  code (readdir-bug.c) under 32-bit user-static qemu, such as qemu-arm-
  static:

  # Setup docker for user-static binfmt
  docker run --rm --privileged multiarch/qemu-user-static:register --reset
  # Compile the code and run (readdir for / is fine, so create a new directory 
/test).
  docker run -v /path/to/qemu-arm-static:/usr/bin/qemu-arm-static -v 
/path/to/readdir-bug.c:/tmp/readdir-bug.c -it --rm arm32v7/ubuntu:18.10 bash -c 
'{ apt update && apt install -y gcc; } >&/dev/null && mkdir -p /test && cd 
/test && gcc /tmp/readdir-bug.c && ./a.out'
  dir=0xff5b4150
  readdir(dir)=(nil)
  errno=75: Value too large for defined data type

  Do remember to replace the /path/to/qemu-arm-static and /path/to
  /readdir-bug.c to the actual paths of the files.

  The root cause is in glibc:
  
https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/getdents.c;h=6d09a5be7057e2792be9150d3a2c7b293cf6fc34;hb=a5275ba5378c9256d18e582572b4315e8edfcbfb#l87

  By C standard, the return type of readdir() is DIR*, in which the
  inode number and offset are 32-bit integers, therefore, glibc calls
  getdents64() and check if the inode number and offset fits the 32-bit
  range, and reports EOVERFLOW if not.

  The problem here is for 32-bit user-static qemu running on 64-bit
  host, getdents64 simply passing through the inode number and offset
  from underlying getdents64 syscall (from 64-bit kernel), which is very
  likely to not fit into 32-bit range. On real hardware, the 32-bit
  kernel creates 32-bit inode numbers, therefore works properly.

  The glibc code makes sense to do the check to be conformant with C
  standard, therefore ideally it should be a fix on qemu side. I admit
  this is difficult because qemu has to maintain a mapping between
  underlying 64-bit inode numbers and 32-bit inode numbers, which would
  severely hurt the performance. I don't expect this could be fix
  anytime soon (or even there would be a fix), but it would be
  worthwhile to surface this issue.

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