Re: 2001_05_09 patch against 2.4.4

From: Chris Wright (chrisat_private)
Date: Mon May 14 2001 - 21:06:14 PDT

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    * Chris Evans (chrisat_private) wrote:
    > Hi,
    > I'm not so sure you want to add special cased code in mmap that "knows"
    > MAP_SHARED, PROT_WRITE && !MAP_ANON is equivalent to a write operation.
    It's kind of already there isn't it?  The check !(file->f_mode &
    FMODE_WRITE).  Not that that makes it right to add this check...
    > The "pushing hooks to be as late as possible" sounds like it may be the
    > way forward, but wouldn't that mean looking at "struct
    > address_space_operations": readpage, writepage?
    Thanks, I hadn't taken the time to really look at this yet.  My only concern
    is overhead.  Isn't readpage going to get called _a lot_ when reading a
    > Unfortunately, (and ICBW), this doesn't handle non-pagecache entities like
    > block/character devices. Or directories. Ho hum.
    Ugh!  Back to the page cache/buffer cache problem.  If we are lucky we can
    punt on some of these issues. AFAIU, Linus wants to push block i/o to the
    page cache for 2.5 (and this project is targeting 2.5...;-)
    > BTW, I don't think your current hooks would catch a sendfile() operation,
    > which IIRC goes through the unhooked readpage() rather than the hooked
    > read() or mmap().
    I think you are right.  We mean to catch sendfile (i.e. it's on the
    hotlist), but it just hasn't been added yet.
    > In fact is there much value to attempting to hook read()/write()
    > operations at all? Shouldn't access be controlled at open() time? (or
    > bind() time for sockets). Something like MAC should work with just that. far as the list archives go, this is akin to a can of worms. ;-)
    Basically, because you can pass open file descriptors between processes
    (via fork or sendmsg/recvmsg for example) you can't exclusively rely on
    open() (and bind() is not "required" for socket communication).
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