Currently the lstat(2) calls in sqashfs-tools (mksquashfs.c specifically) don't handle EINTR. They should as lstat(2) is subject to being interrupted.
Please find attached a patch to handle this. Unfortunately the patch is against 4.0 since that's what's in RHEL 6.2 currently and therefore what I am working with.
handling EINTR from lstat(2)