From: Victor Duchovni (Victor.DuchovniMorganStanley.com)
Date: Wed Sep 19 2007 - 10:38:44 CDT

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On Wed, Sep 19, 2007 at 09:47:14AM -0400, Wietse Venema wrote:

> > For efficiency, such scanning can be skipped for body parts with a "QP"
> > or "base64" CTE (really anything other than 7bit or 8bit) because these
> > must be 7bit and will not be downgraded even if they are not. So large
> > base64 attachments should not impose a CPU penalty for inspecting the
> > high bit of every octet.
> >
> > Is this reasonable?
>
> Only if the workaround can be implemented without imposing an
> unreasonable burden on Postfix itself. Reasonable means that the
> workaround's impact is clearly confined to a small section of code.

I take that as a very qualified yes. The code in question would I think be
confined to the mime_state engine, which given a new flag (optionally, but
by default enabled in cleanup) would look for "true" 8bit octets in the
bodies of MIME entities with a CTE of "8bit". If no such octets are found,
the message-level encoding is 7bit, even if the sender (via BODY=8BITMIME
in MAIL FROM:) or CTE headers in one or more non-composite MIME parts
signaled possible 8bit content.

This is somewhat comparable in cost to a regexp check on each line of
body content, except that base64 and QP content is always uninspected
(cheaper) while 8bit content is inspected in its entirety (not just the
first $body_checks_size_limit octets, more expensive).

--
        Viktor.

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