Miklos Szeredi:
> One issue, though, is that stat("x") and fd=open("x"); fstat(fd) will
> return different file stats (the fstat one will return the stats for
> the original file). This may confuse some apps.
>
> This is a something a "true" union filesystem can do better that
> union-mounts or overlayfs.
Exactly.
That was one example I picked up and posted when I read overlayfs years
ago.
Recently I named it "name-based union". Of course AUFS is "inode-based
union." stat(2) operates upon a filename while fstat(2) operates upon a
file descriptor. Overlayfs (and probably UnionMount too) may return
different info for these two systemcalls even when they should be equal.
All the inode-based (or file descriptor based) operations can confuse
users in overlayfs, I am afraid. I was looking forward to see how
overlayfs solve this problem.
Overriding uid/gid is an idea, but obviously it will not match using
with system dirs such as /bin and /usr, since they often have suid
binaries. So it will be used only for user dirs which is unioned as
lower RO layer. It might be better to implement such feature into your
lower FS (or generic VFS feature) instead of overlayfs.
If I remember correctly, some non-unix FS such as VFAT already have such
feature.
J. R. Okajima
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