kbuild: create *.mod with full directory path and remove MODVERDIR

While descending directories, Kbuild produces objects for modules,
but do not link final *.ko files; it is done in the modpost.

To keep track of modules, Kbuild creates a *.mod file in $(MODVERDIR)
for every module it is building. Some post-processing steps read the
necessary information from *.mod files. This avoids descending into
directories again. This mechanism was introduced in 2003 or so.

Later, commit 551559e13af1 ("kbuild: implement modules.order") added
modules.order. So, we can simply read it out to know all the modules
with directory paths. This is easier than parsing the first line of
*.mod files.

$(MODVERDIR) has a flat directory structure, that is, *.mod files
are named only with base names. This is based on the assumption that
the module name is unique across the tree. This assumption is really
fragile.

Stephen Rothwell reported a race condition caused by a module name
conflict:

  https://lkml.org/lkml/2019/5/13/991

In parallel building, two different threads could write to the same
$(MODVERDIR)/*.mod simultaneously.

Non-unique module names are the source of all kind of troubles, hence
commit 3a48a91901c5 ("kbuild: check uniqueness of module names")
introduced a new checker script.

However, it is still fragile in the build system point of view because
this race happens before scripts/modules-check.sh is invoked. If it
happens again, the modpost will emit unclear error messages.

To fix this issue completely, create *.mod with full directory path
so that two threads never attempt to write to the same file.

$(MODVERDIR) is no longer needed.

Since modules with directory paths are listed in modules.order, Kbuild
is still able to find *.mod files without additional descending.

I also killed cmd_secanalysis; scripts/mod/sumversion.c computes MD4 hash
for modules with MODULE_VERSION(). When CONFIG_DEBUG_SECTION_MISMATCH=y,
it occurs not only in the modpost stage, but also during directory
descending, where sumversion.c may parse stale *.mod files. It would emit
'No such file or directory' warning when an object consisting a module is
renamed, or when a single-obj module is turned into a multi-obj module or
vice versa.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Nicolas Pitre <nico@fluxnic.net>

1 files changed, 1 insertions, 11 deletions

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 4ac4ca21a30af..cde5675340ba0 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -353,23 +353,13 @@ config DEBUG_SECTION_MISMATCH
 	  which results in the code/data being placed in specific sections.
 	  The section mismatch analysis is always performed after a full
 	  kernel build, and enabling this option causes the following
-	  additional steps to occur:
+	  additional step to occur:
 	  - Add the option -fno-inline-functions-called-once to gcc commands.
 	    When inlining a function annotated with __init in a non-init
 	    function, we would lose the section information and thus
 	    the analysis would not catch the illegal reference.
 	    This option tells gcc to inline less (but it does result in
 	    a larger kernel).
-	  - Run the section mismatch analysis for each module/built-in.a file.
-	    When we run the section mismatch analysis on vmlinux.o, we
-	    lose valuable information about where the mismatch was
-	    introduced.
-	    Running the analysis for each module/built-in.a file
-	    tells where the mismatch happens much closer to the
-	    source. The drawback is that the same mismatch is
-	    reported at least twice.
-	  - Enable verbose reporting from modpost in order to help resolve
-	    the section mismatches that are reported.
 
 config SECTION_MISMATCH_WARN_ONLY
 	bool "Make section mismatch errors non-fatal"