Re: kbuild: Invalid configuration generated for dependent choices by'conf' but not by 'mconf', with reason

From: Ulf Magnusson
Date: Sat Dec 08 2012 - 18:12:44 EST

On Thu, Dec 6, 2012 at 1:07 AM, Ulf Magnusson <ulfalizer.lkml2@xxxxxxxxx> wrote:
> Hi,
> Consider the following Kconfig file:
> config MODULES
> def_bool y
> choice
> tristate "C1"
> config A
> tristate "A"
> config B
> tristate "B"
> endchoice
> choice
> bool "C2" if A
> config C
> bool "C"
> config D
> bool "D"
> endchoice
> When this Kconfig file is used with a configuration file containing
> just "CONFIG_A=y", conf generates the following invalid configuration:
> # CONFIG_B is not set
> # CONFIG_D is not set
> mconf on the other hand generates the correct configuration:
> # CONFIG_B is not set
> # CONFIG_D is not set
> To see this, compare the .config files generated by the following commands:
> $ scripts/kconfig/conf --defconfig=bug_config bug_Kconfig
> $ cp bug_config .config
> $ scripts/kconfig/mconf bug_Kconfig
> The root cause of the problem is the sym_calc_value(csym) call at the
> end of conf_set_all_new_symbols(). Since the second 'choice' depends
> on A, that call causes the value of A to be calculated. But this also
> causes SYMBOL_VALID to be set on A, preventing it from being
> reevaluated later when the configuration is written in conf_write().
> This reevaluation is required to get the correct value since only by
> then will the choice have SYMBOL_DEF_USER set on it (set in
> set_all_choice_values()).
> Some possible fixes:
> (1) Add another sym_clear_all_valid() call at the end of
> conf_set_all_new_symbols(). This is kinda wasteful.
> (2) Just randomize_choice_values() seems to actually need the
> sym_calc_value(csym). It could probably be moved into that function,
> perhaps doing the additional sym_clear_all_valid() there unless
> there's something neater you could do.
> It would probably be safer if someone more familiar with the
> randomization stuff wrote the actual patch.
> Linux version is 3.7.0-rc4, though this bug has likely been around a while.
> /Ulf

This bug causes an incorrect configuration to be generated for
Blackfin with the following defconfigs by the way:


For example:

$ make ARCH=blackfin CM-BF537U_defconfig
$ cp .config ._config
$ cp arch/blackfin/configs/CM-BF537U_defconfig .config
$ make ARCH=blackfin menuconfig
*save and exit*
$ diff -u .config ._config
--- .config 2012-12-08 08:46:07.598588422 +0100
+++ ._config 2012-12-08 08:44:03.566584623 +0100
@@ -997,9 +997,9 @@
# CONFIG_USB_MV_UDC is not set
# CONFIG_USB_M66592 is not set
# CONFIG_USB_NET2272 is not set
# CONFIG_USB_ZERO is not set
# CONFIG_USB_ETH_EEM is not set
# CONFIG_USB_G_NCM is not set

The defconfig itself contains "CONFIG_USB_ETH=y", which 'conf'
incorrectly truncates.

I noticed the bug because it causes the only test suite failures in
Kconfiglib ( with Linus'
latest kernel, in the tests that compare output between Kconfiglib and
the scripts/kconfig/*conf utilities. (I'm in the process of updating
Kconfiglib. I have no intention to get it into the kernel - just a
handy tool.)

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