[Bug 233339] genassym.o build race

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=233339

            Bug ID: 233339
           Summary: genassym.o build race
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: markj at FreeBSD.org

I occasionally hit a kernel build failure when building a kernel from scratch
(i.e., no -DKERNFAST or anything like that).  It manifests like this:

nm: 'genassym.o': No such file or directory
nm: 'genassym.o': No such file or directory
/usr/home/markj/src/freebsd-dev/sys/amd64/linux32/linux32_support.s:38:2:
error: expected relocatable expression
 movl $-EFAULT,%eax
 ^

At this point, if I go into the objdir for linux.ko, I see an empty assym.inc
and no genassym.o.  In kmod.mk we have:

480 assym.inc: genassym.o
481 offset.inc: genoffset.o
482 assym.inc: ${SYSDIR}/kern/genassym.sh
483         sh ${SYSDIR}/kern/genassym.sh genassym.o > ${.TARGET}
484 genassym.o: ${SYSDIR}/${MACHINE}/${MACHINE}/genassym.c offset.inc
485 genassym.o: ${SRCS:Mopt_*.h}
486         ${CC} -c ${CFLAGS:N-flto:N-fno-common} \
487             ${SYSDIR}/${MACHINE}/${MACHINE}/genassym.c
488 offset.inc: ${SYSDIR}/kern/genoffset.sh genoffset.o
489         sh ${SYSDIR}/kern/genoffset.sh genoffset.o > ${.TARGET}
490 genoffset.o: ${SYSDIR}/kern/genoffset.c
491 genoffset.o: ${SRCS:Mopt_*.h}
492         ${CC} -c ${CFLAGS:N-flto:N-fno-common} \
493             ${SYSDIR}/kern/genoffset.c

So, we *should* have compiled genassym.c before invoking genassym.sh (which is
where the nm(1) error above comes from), but that's not happening.  The problem
seems to be that the dependency is satisfied by the kernel's genassym.o, which
is generated very early during the kernel build, so when building modules we
don't wait for the module's genassym.o to be created.

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