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[Bug 1805913] Re: readdir() returns NULL (errno=EOVERFLOW) for 32-bit us
From: |
Manuel Reimer |
Subject: |
[Bug 1805913] Re: readdir() returns NULL (errno=EOVERFLOW) for 32-bit user-static qemu on 64-bit host |
Date: |
Fri, 27 Mar 2020 15:19:09 -0000 |
I seem to have found another workaround. Knowing now what causes this my
guess was: If I make the qemu-arm-static on the host a 32 bit binary and
get "multilib" running to make my 64 bit Linux installation run this,
then in theory this incompatibility should not happen. If it would, then
32 bit x86 applications whould run into the same problem.
And at least according to my tries, I did so far, this seems to be the
case. I was able to reproduce this with svn (no checkout possible from
32 bit armv7h). If the qemu-arm-static binary is a 32 bit x86
application, then SVN checkouts work well now.
So until there is a better solution it seems to be a good idea to make
the emulation layer run through multilib for 32 bit target
architectures, so the host kernel can switch to its 32 bit backwards
compatibility mode.
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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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