Bug 1785652

Description of problem:

mksquashfs sometimes fails with an error:

  Parallel mksquashfs: Using 2 processors
  Creating 4.0 filesystem on /tmp/test.sqs, block size 131072.
  [===================================================================|] 1/1 100%Write failed because Bad file descriptor
  FATAL ERROR:Failed to write to output filesystem

This error occurs occasionally, and was noticed by us in automated testing of the Singularity container runtime, which uses squashfs format images extensively, and therefore generates many squashfs images during tests. These tests fail only on RHEL8.

Version-Release number of selected component (if applicable):

Red Hat Enterprise Linux release 8.1 (Ootpa)
squashfs-tools-4.3-19.el8.x86_64
kernel 4.18.0-147.el8.x86_64


How reproducible:

On an EC2 RHEL8.1 instance (t3.small), this can be triggered consistently withing 10,000 iterations of `mkqsuashfs` on a single file. On a VM running under VirtualBox it has been consistently triggered within 1000 iterations.


Steps to Reproduce:

Create test script as test_mksquashfs.sh

#!/bin/sh
for i in $(seq 1 100000); do
	rm -f /tmp/test.sqs
	if ! mksquashfs $1 /tmp/test.sqs -noappend >/tmp/output 2>&1; then
		echo "Failed - iteration $i"
		cat /tmp/output
	fi
done

Run script, creating squashfs from a file, e.g:

./test_mksquashfs.sh /etc/passwd



Actual results:

[ec2-user@ip-172-31-14-236 ~]$ ./test_mksquashfs.sh /etc/passwd
Failed - iteration 6190
Parallel mksquashfs: Using 2 processors
Creating 4.0 filesystem on /tmp/test.sqs, block size 131072.
[===================================================================|] 1/1 100%Write failed because Bad file descriptor
FATAL ERROR:Failed to write to output filesystem


Exportable Squashfs 4.0 filesystem, gzip compressed, data block size 131072
	compressed data, compressed metadata, compressed fragments, compressed xattrs
	duplicates are removed
Filesystem size 0.82 Kbytes (0.00 Mbytes)
	55.61% of uncompressed filesystem size (1.47 Kbytes)
Inode table size 51 bytes (0.05 Kbytes)
	56.67% of uncompressed inode table size (90 bytes)
Directory table size 26 bytes (0.03 Kbytes)
	92.86% of uncompressed directory table size (28 bytes)
Xattr table size 52 bytes (0.05 Kbytes)
	100.00% of uncompressed xattr table size (52 bytes)
Number of duplicate files found 0
Number of inodes 2
Number of files 1
Number of fragments 1
Number of symbolic links  0
Number of device nodes 0
Number of fifo nodes 0
Number of socket nodes 0
Number of directories 1
Number of ids (unique uids + gids) 2
Number of uids 2


Expected results:

Completes loop without any error.


Additional info:

Cannot be reproduced on an Ubuntu install running squashfs-tools 4.3

Does not reproduce using squashfs-tools 4.4 built from source