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    SubjectPER_LINUX32, Was: [PATCH v2 21/31] arm64: 32-bit (compat) applications support
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    On Thursday 16 August 2012, Will Deacon wrote:
    > On Wed, Aug 15, 2012 at 03:34:04PM +0100, Arnd Bergmann wrote:
    > > On Tuesday 14 August 2012, Catalin Marinas wrote:
    > > > +asmlinkage int compat_sys_personality(compat_ulong_t personality)
    > > > +{
    > > > + int ret;
    > > > +
    > > > + if (personality(current->personality) == PER_LINUX32 &&
    > > > + personality == PER_LINUX)
    > > > + personality = PER_LINUX32;
    > > > + ret = sys_personality(personality);
    > > > + if (ret == PER_LINUX32)
    > > > + ret = PER_LINUX;
    > > > + return ret;
    > > > +}
    > >
    > > Where did you get this from?
    > >
    > > You should not need compat_sys_personality, just call the native function.
    >
    > Hmm, but in that case an aarch32 application doing a personality(PER_LINUX)
    > syscall will start seeing the wrong uname.

    Coming back at this topic, I noticed another issue. Jiri Kosina
    has recently posted patches to fix this function in the other architectures
    in order to mask out the other personality bits, which is a correct fix,
    but the above function is odd for other reasons.

    * On MIPS, it is used only for compat tasks, like you have it above.
    * On PA-RISC, it is used for native 32 bit tasks and for compat 32 bit tasks,
    but not for native 64 bit ones.
    * On IA64, it was used for compat tasks (support for which has since
    been removed from the kernel), plus all 32 bit tasks would start with
    PER_LINUX32.
    * On PowerPC, Sparc and s390, it is used for native 64 bit tasks and for
    compat 32 bit tasks, but not for native 32 bit ones.
    * On Tile, it was never used.
    * On x86_64, it used to be defined (copied from ia64) but not used
    throughout the git history.

    The semantics of the function are also interesting: The intention seems
    to be that to a compat task, PER_LINUX32 would appear as PER_LINUX.
    The effect is that any process can set PER_LINUX32 but it can never
    be unset except by a 64 bit MIPS or PA-RISC task.

    Since x86_64 does not implement this behavior at all, I suspect that
    there are now lots of things depending on not having it, while all
    the other architectures might also have some (even predating the
    x86_64 port) use cases that depend on depend on not being able to
    observe PER_LINUX32 in 32 bit compat tasks.

    I think we should try to agree on how this is all supposed to work
    and use common code, either put the ppc/sparc/s390 version into
    sys_personality, or remove all of them and just do what x86 and tile
    do, using the regular sys_personality for all tasks.

    Arnd


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