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From: phil@amdcad.UUCP
Newsgroups: comp.arch
Subject: Re: catering to bad code
Message-ID: <15014@amdcad.UUCP>
Date: Sat, 28-Feb-87 22:02:06 EST
Article-I.D.: amdcad.15014
Posted: Sat Feb 28 22:02:06 1987
Date-Received: Sun, 1-Mar-87 17:25:14 EST
References: <14833@amdcad.UUCP> <14837@amdcad.UUCP> <436@cpocd2.UUCP> <64@celerity.UUCP>
Reply-To: phil@amdcad.UUCP (Phil Ngai)
Distribution: world
Organization: Advanced Micro Devices, Sunnyvale, California
Lines: 31

I'm glad to see some people are mildly interested in this subject. :-)
I didn't make very clear the purpose of the Unix system I'm designing.
It is intended to be a throw-away. We only want to bring it up to show
that Unix can be brought up, and to see how fast it runs.  We don't
intend to sell this software as a product. As such, our only interest
is in doing it fast and to be able to run as much existing software as
possible. 

I think the best thing to do is make accessing null pointers a
segmentation violation by default and be able to allow it as needed.
Perhaps we could discuss the best method of doing this, so as to try
to provide a common mechanism. HP's method, for example sounds
interesting. Should the choice be made at link time, at load time, at
system configuration time, or something else? Should there be special
magic numbers to ask the kernel to load with null pointers returning 0
on a data read? Etc. 

As an unlikely example, suppose you bought a binary from a vendor who
used a permissive kernel and yours was strict. 

Someone mentioned that just returning 0 for the first 2 or 4 bytes was
not enough, as there is code that accesses structure members with null
structure pointers. (how perverse can you get?) Is this really a
problem? Have many people seen this? 

-- 
 I'd rather be compatible than right.

 Phil Ngai +1 408 982 7840
 UUCP: {ucbvax,decwrl,hplabs,allegra}!amdcad!phil
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