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    SubjectRe: make PROT_WRITE imply PROT_READ
    On 6/23/06, Arjan van de Ven <arjan@infradead.org> wrote:
    > you ask for it, and the kernel is supposed to deliver the best behavior
    > it can.

    The kernel should provide

    - a stable, reliable interface

    - a consistent interface at least accross architectures, maybe even platforms


    Providing write-only support for memory falls into none of these
    categories. When Jason and I discussed this my position actually was
    to disallow PROT_WRITE without PROT_READ completely, making it an
    error of mmap and mprotect. That's perfectly legal according to POSIX
    and it will teach those who write broken code like this.

    Jason's proposal to implicitly set PROT_READ is the second best
    solution. It gets rid of the error cases which a developer might not
    notice, either because of lucky circumstances or details of the
    architecture.

    The reality is that almost no platform can really implement write-only
    memory. RISC architectures implement sub-word write as word reads,
    modify, word write. And even for CISC archs the compiler, for
    instance, might decide to read memory, be it for prefetching or
    explicit write-combining.

    I'm strongly in favor of adding of adding one of the two possible
    patches (forbidding, implicitly setting PROT_READ). This will help
    preventing these kinds of programming mistakes from spreading and we
    already _know_ that there are such programs out there.
    -
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