Re: Please turn "Cannot use CONFIG_STACK_VALIDATION" into build error

[Jessica Yu]

+++ Josh Poimboeuf [13/02/17 12:41 -0600]:
> On Mon, Feb 13, 2017 at 12:07:09AM -0800, Marc MERLIN wrote:
> > Hi Josh,
> >
> > I'll start with the story as to why.
> > i've lost more hours than I care to list, because I was unable to build
> > the virtualbox kernel driver with newer kernels.
> > Sadly, it gives no useful debug info outside of
> > make[1]: *** No rule to make target '/tmp/vbox.0/linux/SUPDrv-linux.o', needed by '/tmp/vbox.0/vboxdrv.o'.  Stop.
> >
> > It took some pretty deep debugging to finally see this:
> >      Trying rule prerequisite 'tools/objtool/objtool'.
> >      Looking for a rule with intermediate file 'tools/objtool/objtool'.
> >       Avoiding implicit rule recursion.
> > which look quite inoccuous and don't look as errors at all.
> > When I filed a bug with the vbox folks, they were unable to find out why
> > the module refused to build on my kernel, and I was stuck with older
> > kernels as a result.
> >
> > Then, I had another module, bbswitch, to turn off the nvidia chip on my
> > laptop to save battery. That one also failed to build with newer
> > kernels, but thankfully made it more clear that the problem was related
> > to tools/objtool/objtool missing.
> >
> > But why was it missing? No idea...
> > I trace that down to CONFIG_STACK_VALIDATION which there seems to be no
> > menu option for, so I manually disable it in .config, rebuild, and it's
> > automatically re-enabled. Gah.
>
> Hm, that doesn't sound right.  Nothing automatically enables
> CONFIG_STACK_VALIDATION.  It should be disabled unless manually enabled.
> Maybe you got it confused with CONFIG_HAVE_STACK_VALIDATION, which is
> always enabled?
>
> BTW, there is a config option for it in the menu:
>
>  Kernel hacking
>    Compile-time checks and compiler options
>      Compile-time stack metadata validation
>
> > More hair pulling, and finally I make a typo
> > saruman:/usr/src/linux-block# make xonfig
> > Makefile:1044: "Cannot use CONFIG_STACK_VALIDATION, please install libelf-dev, libelf-devel or elfutils-libelf-devel"
> > scripts/kconfig/conf  --silentoldconfig Kconfig
> > Makefile:1044: "Cannot use CONFIG_STACK_VALIDATION, please install libelf-dev, libelf-devel or elfutils-libelf-devel"
> > make: *** No rule to make target 'xonfig'.  Stop.
> >
> > Sure enough, this was my problem, but I never saw the error message
> > because I build kernels with
> > make-kpkg --revision 1gandalf kernel-image
> > which does other stuff and hid that warning, which really should have
> > been a fatal error in my opinion.
> >
> > Given that
> > 1) CONFIG_STACK_VALIDATION seems silently auto enabled.
> > 2) without libelf-dev, the kernel will build but will leave a tree
> > missing objtool, which in turn causes (all?) 3rd party modules to fail
> > building.
>
> Yes, this is a bug.
>
> > 3) and that it's kind of non trivial to find out why if that happens,
> >
> > Would you consider making
> > "Cannot use CONFIG_STACK_VALIDATION, please install libelf-dev, libelf-devel or elfutils-libelf-devel"
> > a build error as opposed to a warning?
> > This sure would have saved me countless errors of debugging the wrong
> > things.
>
> Correct me if I'm wrong, but it sounds like make-kpkg suppressed stderr?
> If so, that should be fixed.
>
> When I try to build an OOT module with CONFIG_STACK_VALIDATION enabled
> and elfutils-libelf-devel missing (on Fedora), I get:
>
>  make: Entering directory '/home/jpoimboe/git/linux'
>  make[1]: Entering directory '/home/jpoimboe/ktest/output'
>    CC [M]  /home/jpoimboe/livepatch-test/1/livepatch2.o
>  /bin/sh: ./tools/objtool/objtool: No such file or directory
>  /home/jpoimboe/git/linux/scripts/Makefile.build:300: recipe for target '/home/jpoimboe/livepatch-test/1/livepatch2.o' failed
>  make[2]: *** [/home/jpoimboe/livepatch-test/1/livepatch2.o] Error 1
>  /home/jpoimboe/git/linux/Makefile:1490: recipe for target '_module_/home/jpoimboe/livepatch-test/1' failed
>  make[1]: *** [_module_/home/jpoimboe/livepatch-test/1] Error 2
>  make[1]: Leaving directory '/home/jpoimboe/ktest/output'
>  Makefile:150: recipe for target 'sub-make' failed
>  make: *** [sub-make] Error 2
>  make: Leaving directory '/home/jpoimboe/git/linux'
>
> It's not a perfect error message, but the
>
>  '/bin/sh: ./tools/objtool/objtool: No such file or directory'
>
> is at least a big clue.  I'm curious why you didn't see that.
>
> Anyway, the above libelf-dev warning is just a warning and not a build
> error because CONFIG_STACK_VALIDATION is enabled for allyesconfig, and
> it's not a severe enough problem to warrant breaking the build.
>
> Ideally the same warning should be printed when building OOT modules.
> I'll try to figure out if there's a way to do that it.

Btw, it looks like the libelf warning is inside an `ifeq ($(KBUILD_EXTMOD),)`
block, so it does not get applied to OOT modules. It would be
possible to add the warning to the corresponding KBUILD_EXTMOD (else)
block or somewhere else in the Makefile common to both cases (probably
more preferable). I tried the latter case and the warning prints for
all cases (vmlinux, intree, external modules).

Jessica