Message-ID: <20170213230852.GA14810@packer-debian-8-amd64.digitalocean.com>
Date:   Mon, 13 Feb 2017 18:08:52 -0500
From:   Jessica Yu <jeyu@...hat.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Marc MERLIN <marc@...lins.org>, linux-kernel@...r.kernel.org
Subject: Re: Please turn "Cannot use CONFIG_STACK_VALIDATION" into build error

+++ 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

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