[PATCH v2] modpost: Fix secondary errors seen if a single module build fails

[Guenter Roeck <linux@roeck-us.net>]

Commit ea4054a23 (modpost: handle huge numbers of modules) added
support for building a large number of modules.

Unfortunately, the commit changed the semantics of the makefile: Instead of
passing only existing object files to modpost, make now passes all expected
object files. If make was started with option -i, this results in a modpost
error if a single file failed to build.

Example with the current btrfs build falure on m68k:

fs/btrfs/btrfs.o: No such file or directory
make[1]: [__modpost] Error 1 (ignored)

This error is followed by lots of errors such as:

m68k-linux-gcc: error: arch/m68k/emu/nfcon.mod.c: No such file or directory
m68k-linux-gcc: fatal error: no input files
compilation terminated.
make[1]: [arch/m68k/emu/nfcon.mod.o] Error 1 (ignored)

This doesn't matter much for normal builds, but it is annoying for builds
started with "make -i" due to the large number of secondary errors.
Those errors unnececessarily clog any error log and make it difficult
to find the real errors in the build.

Fix the problem by only passing existing object files to modpost.

Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
v2: Use "xargs -r ls 2>/dev/null" instead of a shell while loop
    to filter for existing files.
    Not sure if this is much better, but it is an alternative.
    If anyone has a better solution, please let me know.
    [Another option would be to use 'find' (no parameters) instead
     of 'ls', but that would still require the error redirect.]

 scripts/Makefile.modpost |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
index 8dcdca2..edc4394 100644
--- a/scripts/Makefile.modpost
+++ b/scripts/Makefile.modpost
@@ -81,7 +81,8 @@ modpost = scripts/mod/modpost                    \
 
 # We can go over command line length here, so be careful.
 quiet_cmd_modpost = MODPOST $(words $(filter-out vmlinux FORCE, $^)) modules
-      cmd_modpost = $(MODLISTCMD) | sed 's/\.ko$$/.o/' | $(modpost) -s -T -
+      cmd_modpost = $(MODLISTCMD) | sed 's/\.ko$$/.o/' | \
+	xargs -r ls 2>/dev/null | $(modpost) -s -T -
 
 PHONY += __modpost
 __modpost: $(modules:.ko=.o) FORCE
-- 
1.7.9.7