Date:   Tue, 14 Feb 2017 08:06:57 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Jessica Yu <jeyu@...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

On Tue, Feb 14, 2017 at 07:59:38AM -0600, Josh Poimboeuf wrote:
> On Mon, Feb 13, 2017 at 06:08:52PM -0500, Jessica Yu wrote:
> > +++ 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).
> 
> Aha, that explains it, thanks.  Will look into fixing it.

Yep, that was an easy fix.  Marc, can you test the following patch?


diff --git a/Makefile b/Makefile
index 8e223e0..3e794d4 100644
--- a/Makefile
+++ b/Makefile
@@ -908,6 +908,17 @@ mod_sign_cmd = true
 endif
 export mod_sign_cmd
 
+ifdef CONFIG_STACK_VALIDATION
+  has_libelf := $(call try-run,\
+		echo "int main() {}" | $(HOSTCC) -xc -o /dev/null -lelf -,1,0)
+  ifeq ($(has_libelf),1)
+    objtool_target := tools/objtool FORCE
+  else
+    $(warning "Cannot use CONFIG_STACK_VALIDATION, please install libelf-dev, libelf-devel or elfutils-libelf-devel")
+    SKIP_STACK_VALIDATION := 1
+    export SKIP_STACK_VALIDATION
+  endif
+endif
 
 ifeq ($(KBUILD_EXTMOD),)
 core-y		+= kernel/ certs/ mm/ fs/ ipc/ security/ crypto/ block/
@@ -1035,18 +1046,6 @@ prepare0: archprepare gcc-plugins
 # All the preparing..
 prepare: prepare0 prepare-objtool
 
-ifdef CONFIG_STACK_VALIDATION
-  has_libelf := $(call try-run,\
-		echo "int main() {}" | $(HOSTCC) -xc -o /dev/null -lelf -,1,0)
-  ifeq ($(has_libelf),1)
-    objtool_target := tools/objtool FORCE
-  else
-    $(warning "Cannot use CONFIG_STACK_VALIDATION, please install libelf-dev, libelf-devel or elfutils-libelf-devel")
-    SKIP_STACK_VALIDATION := 1
-    export SKIP_STACK_VALIDATION
-  endif
-endif
-
 PHONY += prepare-objtool
 prepare-objtool: $(objtool_target)
 

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