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file_chmod in unionfs, copy-on-write

From: Moritz Schulte
Subject: file_chmod in unionfs, copy-on-write
Date: Thu, 21 Nov 2002 17:20:23 +0100
User-agent: Gnus/5.090008 (Oort Gnus v0.08) Emacs/21.2 (i686-pc-linux-gnu)


  just out of curiosity: how should the yet-to-be-written unionfs
implement copy-on-write for file_chmod()?

  unionfs usually gives out ports to the underlying files; when in
copy-on-write mode, it should give out a port to which the user can
safely write without modifying the `real' data.  In my understanding
this works, since unionfs notices at open time that the user intents
to write to the file (the O_WRITE flag is specified).

  But for file_chmod there is no O_WRITE needed, therefore unionfs is
not able to detect this `write'.

moritz@duesseldorf.ccc.de - http://duesseldorf.ccc.de/~moritz/
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