You could change /etc/fstab:

tmpfs       /dev/shm          tmpfs   defaults,mode=01777        0 0

and initscripts (rc.sysinit) should be able to remount according to /etc/fstab with:

while read line; do 
     eval set -- $line
     [ -d "$2" ] && mount $2 -o remount
done < /etc/fstab

That would be a fairly big RFE; I'd just rather have dracut do it, and have the normal mount -a set it in the non-dracut case.

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