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Path: sserve!newshost.anu.edu.au!munnari.oz.au!news.Hawaii.Edu!ames!agate!doc.ic.ac.uk!uknet!pavo.csi.cam.ac.uk!camcus!pc123
From: pc123@cus.cam.ac.uk (Pete Chown)
Subject: Bug in mmap()? (and fix)
Message-ID: <PC123.93May7010930@bootes.cus.cam.ac.uk>
Sender: news@infodev.cam.ac.uk (USENET news)
Nntp-Posting-Host: bootes.cus.cam.ac.uk
Organization: U of Cambridge, England
Date: Fri, 7 May 1993 00:09:38 GMT
Lines: 44
I have found what I think must be a bug in the mmap() code. The idea
of mmap(), in case you don't know, is that you can map files into
virtual memory; you have a choice of three styles of operation:
1. Map the file in, and write any changes made back to disc.
2. Map the file in, but discard any changes made.
3. Copy the whole file before doing anything.
For the first of these, you need write access to the file, but for 2
and 3 you only need read access. Unfortunately you are currently
refused permission for all of them unless you have write access. The
following diff corrects this behaviour, and should be applied to
/sys/vm/vm_mmap.c.
This may sound obscure, but unfortunately this call is sure to be used
by whoever ends up doing the shared libraries. I'm afraid I don't
know who is handling the patchkit now, but perhaps they would like to
have to a look at this diff and add it to the kit if it looks alright.
That way, the fix should have worked into everyone's systems before
anyone is trying to distribute a shared library package.
201,205c201,209
< maxprot = VM_PROT_EXECUTE;
< if (fp->f_flag & FREAD)
< maxprot |= VM_PROT_READ;
< if (fp->f_flag & FWRITE)
< maxprot |= VM_PROT_WRITE;
---
> if(flags & MAP_SHARED) {
> maxprot = VM_PROT_EXECUTE;
> if (fp->f_flag & FREAD)
> maxprot |= VM_PROT_READ;
> if (fp->f_flag & FWRITE)
> maxprot |= VM_PROT_WRITE;
> } else {
> maxprot = VM_PROT_ALL;
> }
--
---------------------------------------------+ "A tight hat can be stretched.
Pete Chown, pc123@phx.cam.ac.uk (Internet) | First damp the head with steam
pc123@uk.ac.cam.phx (Janet :-) -+ from a boiling kettle."