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    SubjectRe: [PATCH 1/4] module: add syscall to load module from fd
    On Tue, Oct 23, 2012 at 1:42 PM, Kees Cook <keescook@chromium.org> wrote:
    > On Mon, Oct 22, 2012 at 9:08 PM, Lucas De Marchi
    > <lucas.demarchi@profusion.mobi> wrote:
    >> On Tue, Oct 23, 2012 at 1:40 AM, Kees Cook <keescook@chromium.org> wrote:
    >>> On Mon, Oct 22, 2012 at 7:37 PM, Lucas De Marchi
    >>> <lucas.demarchi@profusion.mobi> wrote:
    >>>> On Mon, Oct 22, 2012 at 5:39 AM, Rusty Russell <rusty@rustcorp.com.au> wrote:
    >>>>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
    >>>>>>> FIX: add flags arg to sys_finit_module()
    >>>>>>>
    >>>>>>> Thanks to Michael Kerrisk for keeping us honest.
    >>>>>>
    >>>>>> w00t! Thanks, Rusty ;-).
    >>>>>>
    >>>>>> Acked-by: Michael Kerrisk <mtk.manpages@gmail.com>
    >>>>>
    >>>>> Here's the version I ended up with when I added two flags.
    >>>>>
    >>>>> Lucas, is this useful to you?
    >>>>>
    >>>>> BTW Michael: why aren't the syscall man pages in the kernel source?
    >>>>>
    >>>>> Thanks,
    >>>>> Rusty.
    >>>>>
    >>>>> module: add flags arg to sys_finit_module()
    >>>>>
    >>>>> Thanks to Michael Kerrisk for keeping us honest. These flags are actually
    >>>>> useful for eliminating the only case where kmod has to mangle a module's
    >>>>> internals: for overriding module versioning.
    >>>>>
    >>>>> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
    >>>
    >>> Acked-by: Kees Cook <keescook@chromium.org>
    >>>
    >>>> I wonder if we shouldn't get a new init_module2() as well, adding the
    >>>> flags parameter. Of course this would be in another patch.
    >>>>
    >>>> My worries are that for compressed modules we still need to use
    >>>> init_module() and then --force won't work with signed modules.
    >>>
    >>> For those cases, I think it should remain up to userspace to do the
    >>> decompress and use init_module(). The code I'd written for patching
    >>> module-init-tools basically just kept the fd around if it didn't need
    >>> to mangle the module, and it would use finit_module (written before
    >>> the flags argument was added):
    >>>
    >>> /* request kernel linkage */
    >>> - ret = init_module(module->data, module->len, opts);
    >>> + if (fd < 0)
    >>> + ret = init_module(module->data, module->len, opts);
    >>> + else {
    >>> + ret = finit_module(fd, opts);
    >>> + if (ret != 0 && errno == ENOSYS)
    >>> + ret = init_module(module->data, module->len, opts);
    >>> + }
    >>> if (ret != 0) {
    >>>
    >>> (And yes, I realize kmod is what'll actually be getting this logic.
    >>> This was for my testing in Chrome OS, which is still using
    >>> module-init-tools.)
    >>
    >> sure... but do you realize this will fail in case kernel is checking
    >> module signature and we passed --force to modprobe (therefore mangled
    >> the decompressed memory area)?
    >
    > Hm, yeah, userspace mangling of a module plus signing would fail.
    > Seems like mangling and signing aren't compatible. Doing it in
    > kernel-space (as now written for finit_module) solves that, but it
    > means that now compression isn't possible if you need both signing and
    > mangling.
    >
    > I'm not a user of signing, compression, or mangling, so I'm probably a
    > bit unimaginative here. It seems like the case for needing all three
    > is pretty uncommon. (e.g. if you're doing compression, you're probably
    > building embedded images, which means you're unlikely to need
    > --force.)


    Some desktop distros ship compressed modules by default. I received
    feedback from distros some months ago that this is basically because
    of the disk space, not performance. However some measurements I did
    in a regular laptop with spinning disk showed a small advantage
    performance-wise, too (with SSD is another story and uncompressed
    wins by a large margin)

    Since this only affects users of --force option, I think it only
    affects module developers, who could uncompress the module, call
    depmod again, and modprobe it. ( Mixing compressed and uncompressed
    modules used not to work in module-init-tools and earlier versions of
    kmod wrt dependencies, but now it should be a seamless operation )


    Lucas De Marchi


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