linux-kernel - Re: Please turn "Cannot use CONFIG_STACK

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Date:   Mon, 13 Feb 2017 12:41:06 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Marc MERLIN <marc@...lins.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: Please turn "Cannot use CONFIG_STACK_VALIDATION" into build error

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.

-- 
Josh