Date: Wed, 2 Sep 1998 12:14:40 +1000 From: Bruce Evans <bde@zeta.org.au> To: jdp@polstra.com, reilly@zeta.org.au Cc: current@FreeBSD.ORG Subject: Re: ELF binaries size Message-ID: <199809020214.MAA20550@godzilla.zeta.org.au>
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>The mapping of the data in the address space is such that the first >data page is mapped after the last text page. So in the address >space, it looks like this: > > +---+---+---+---+ >text | | | | : | > +---+---+---+---+ > +---+---+---+---+ >data | : | | | | > +---+---+---+---+ > >Here, addresses as seen from the program advance horizontally. The >tail end of the last text page in your address space will contain the >beginning of the data. The beginning of the first data page will >likewise contain the end of the text. > >This does not imply that the program can modify its text. Remember, >the pages are mapped copy-on-write. The data segment can write into >the leading portion of its first page, but any changes it makes will >remain invisible to the text segment. I think it implies that elf wastes a full page of memory (the space between the ':'s above) most of the time (unless the ':'s are on a page boundary), while aout only wastes an average of half a page (the space between the text ':' and the end of the page). >> Is this a security problem? > >I don't think so. Do you? Maybe if text is supposed to be unreadable. The system would have to clear the part of the first data page before the ':' to prevent leakage. FreeBSD doesn't seem to do this. Bruce To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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