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Unsafe assumptions (Re: Mail delivery...)
From: Olaf Titz <olaf () bigred inka de>
Date: Sat, 19 May 2001 14:07:47 +0200

local delivery agent(s).  After all that's all you've got with "*.lock"
files, since they too are only advisory locks.  Putting them into the
kernel simply makes it possible to eliminate the risk of a mode 01777
spool directory.  (The risk is already quite low of course if you
pre-create all mailbox spool files, and especially if you write careful
lock validation code in the local delivery agent.  Kernel locks simply
make the code for safe local delivery less complex.)

Not quite. Any scheme which relies on pre-existing mailboxes would
also have to make sure that the owner of the mailbox cannot remove it.
This means not only standard MUAs but also "rm", "mv"[1], accidental
mistakes or user-installed MUAs. As I see it this is pretty much
impossible to guarantee.

So reliance on pre-existing mailboxes is inherently unsafe because it
relies on assumptions which can not be guaranteed, regardless of
useradd programs etc.

Another reason why mail delivery into the home directory, although
requiring root privileges (rsp. setuid capability), causes less
headache overall.

Olaf

[1] Didn't you ever filter out the few good messages out of a 10MB
mailbox full of looped bounces with sed after moving it into your home
and then remove the whole junk at once instead of waiting for the MUA
to do several minutes of filtering? I did.


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