lkml.org 
[lkml]   [2008]   [Oct]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH] x86_64: Implement personality ADDR_LIMIT_32BIT
    On Mon, Oct 06, 2008 at 08:13:19AM +0200, Andi Kleen wrote:
    > "Kirill A. Shutemov" <kirill@shutemov.name> writes:
    > >
    > >>
    > >> but more generally, we already have ADDR_LIMIT_3GB support on x86.
    > >
    > > Does ADDR_LIMIT_3GB really work?
    >
    > As Arjan pointed out it only takes effect on exec()
    >
    > andi@basil:~/tsrc> cat tstack2.c
    > #include <stdio.h>
    > int main(void)
    > {
    > void *p = &p;
    > printf("%p\n", &p);
    > return 0;
    > }
    > andi@basil:~/tsrc> gcc -m32 tstack2.c -o tstack2
    > andi@basil:~/tsrc> ./tstack2
    > 0xff807d70
    > andi@basil:~/tsrc> linux32 --3gb ./tstack2
    > 0xbfae2840

    Which kernel do you use?
    Does it work only when compiled with -m32?

    $ cat 1.c
    #include <stdio.h>
    int main(void)
    {
    void *p = &p;
    printf("%p\n", &p);
    return 0;
    }
    $ gcc 1.c
    $ linux32 --3gb ./a.out
    0x7fffa667e7b8

    > >> Why
    > >> should support for ADDR_LIMIT_32BIT be added?
    > >
    > > It's useful for user mode qemu when you try emulate 32-bit target on
    > > x86_64. For example, if shmat(2) return addres above 32-bit, target will
    > > get SIGSEGV on access to it.
    >
    > The traditional way in mmap() to handle this is to give it a search
    > hint < 4GB and then free the memory again/fail if the result was >4GB.

    mmap() has MAP_32BIT flag on x86_64.

    > Unfortunately that doesn't work for shmat() because the address argument
    > is not a search hint, but a fixed address.
    >
    > I presume you need this for the qemu syscall emulation. For a standard
    > application I would just recommend to use mmap with tmpfs instead
    > (sysv shm is kind of obsolete). For shmat() emulation the cleanest way
    > would be probably to add a new flag to shmat() that says that address
    > is a search hint, not a fixed address. Then implement it the way recommended
    > above.

    I prefer one handle to switch application to 32-bit address mode. Why is it
    wrong?

    --
    Regards, Kirill A. Shutemov
    + Belarus, Minsk
    + ALT Linux Team, http://www.altlinux.com/
    [unhandled content-type:application/pgp-signature]
    \
     
     \ /
      Last update: 2008-10-06 10:19    [W:0.026 / U:29.376 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site