[Regression 4.15] Can't kill CONFIG_UNWINDER_ORC with fire or plague.

[Paul Gortmaker]

There is a regression new to 4.15 that happens in a rather common
workflow which results in the frustrating situation where the user has
clearly disabled UNWINDER_ORC in their ".config" file, and yet they
still get the immediate false error saying they need libelf-dev since
"CONFIG_UNWINDER_ORC=y" is still set, and hence they can not build.

The regression requires UNWINDER_ORC=y (now the default in commit
fc72ae40e303) followed by the user subsequently disabling it, which is
common if the user doesn't have libelf-dev and doesn't care about ORC.

This happens because the recently added test in the top level Makefile
assumes that ,config data will be mapped into include/config/auto.conf
either by the end user running "make oldconfig" or the Makefile itself
detecting that "silentoldconfig" should be run.  As the simple
reproducer below shows, this clearly does not happen as expected.

Note that the test for CONFIG_STACK_VALIDATION and libelf-dev in the
same place in the Makefile is broken in the same way, but since it is a
warning and not an error, nobody cared about the bogus false positives.

Since there is no way I'm going to debug Makefile stuff on Christmas
Day, I figured that I should at least report it intstead.  The work
around is to save your .config and run a "make distclean" or manually
clobber the stale include/config/auto.conf or similar.  But a frustrated
user wouldn't know that w/o the root cause...


    --------------------- reproducer ------------------------

paul@gw:~/git/linux-head$ git describe 
v4.15-rc5
paul@gw:~/git/linux-head$ make distclean 
paul@gw:~/git/linux-head$ make defconfig
  HOSTCC  scripts/basic/fixdep
  HOSTCC  scripts/kconfig/conf.o
  SHIPPED scripts/kconfig/zconf.tab.c
  SHIPPED scripts/kconfig/zconf.lex.c
  HOSTCC  scripts/kconfig/zconf.tab.o
  HOSTLD  scripts/kconfig/conf
*** Default configuration is based on 'x86_64_defconfig'
#
# configuration written to .config
#
paul@gw:~/git/linux-head$ make
scripts/kconfig/conf  --silentoldconfig Kconfig
Makefile:926: *** "Cannot generate ORC metadata for CONFIG_UNWINDER_ORC=y, please install libelf-dev, libelf-devel or elfutils-libelf-devel".  Stop.
paul@gw:~/git/linux-head$ vi .config  # disable ORC in favour of FRAME_POINTER
paul@gw:~/git/linux-head$ make oldconfig
scripts/kconfig/conf  --oldconfig Kconfig
#
# configuration written to .config
#
paul@gw:~/git/linux-head$ grep UNWINDER .config
# CONFIG_UNWINDER_ORC is not set
CONFIG_UNWINDER_FRAME_POINTER=y
paul@gw:~/git/linux-head$ make
Makefile:926: *** "Cannot generate ORC metadata for CONFIG_UNWINDER_ORC=y, please install libelf-dev, libelf-devel or elfutils-libelf-devel".  Stop.
paul@gw:~/git/linux-head$ ls -l .config include/config/auto.conf
-rw-rw-r-- 1 paul paul 115953 Dec 25 22:48 .config
-rw-rw-r-- 1 paul paul  33069 Dec 25 22:46 include/config/auto.conf
paul@gw:~/git/linux-head$ grep UNWINDER include/config/auto.conf
CONFIG_UNWINDER_ORC=y
paul@gw:~/git/linux-head$ 

    --------------------- reproducer ------------------------