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    SubjectRe: mmap bug?
    On Fri, 29 May 1998, Natarajan Krishnaswami wrote:

    > [someone] wrote:
    > >> It seems that attempts to mmap a region of size zero causes
    > >> mmap to return zero. Is this a bug?
    > >>
    > >> Since it doesn't return -1, it looks like success, but of
    > >> course dereferencing this NULL pointer causes a crash.
    >
    > Zero is a multiple of the page size, so there is no partial page mapped:
    > the map is empty. Returning a pointer which may not be dereferenced
    > seems reasonable. However, mmap is explicitly forbidden from returning
    > 0, and SIGBUS would probably be the correct signal to deliver if an
    > address in a 0-length map were dereferenced.

    Thank you.

    I was surprised at the number of people prepared to tell me not to
    mmap zero length areas. Here's a trivial patch to fix this, although
    I don't know if this will affect many programs.


    --- mm/mmap.c.old Fri May 29 19:48:18 1998
    +++ mm/mmap.c Fri May 29 19:48:45 1998
    @@ -165,7 +165,7 @@
    int correct_wcount = 0, error;

    if ((len = PAGE_ALIGN(len)) == 0)
    - return addr;
    + return -ENXIO;

    if (len > TASK_SIZE || addr > TASK_SIZE-len)
    return -EINVAL;
    Cheers,
    Matthew.


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