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[Bug 1805913] Re: readdir() returns NULL (errno=EOVERFLOW) for 32-bit us

From: Marcin Konarski
Subject: [Bug 1805913] Re: readdir() returns NULL (errno=EOVERFLOW) for 32-bit user-static qemu on 64-bit host
Date: Sun, 02 Feb 2020 21:01:45 -0000

This problem affected my virtual environment which I used (via qemu-
static) to build my project for RaspberryPI platform. After I upgraded
my virtual Raspbian to buster release `readdir` stopped working (as
described in this thread) due to mapping of 64 inode numbers to qemu
32bit ARM land. I needed this builder working and I found a workaround
in some obscure (2nd page of google result) blog.

Before the work around my virtual Raspbian was just a directory on one
of my ext4 partitions. To fix the issue I created image file with dd,
formatted with mkfs.ext4 it with `dir_index` option disabled and moved
my virual Raspbian onto that newly created filesystem. This fixed the
issue for me and my builder started again.

I am posting it here so `dir_index` trick can be easier to found for
others in this situation.

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

Status in QEMU:

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-

  # 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 
  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'
  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:

  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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