Do not check for existence of ${_dir}/kern/ , because for modules that set

.PATH to ${.CURDIR}/[...]/kern , the "exists" expression will fail for the
form exists(${.CURDIR}/[...]/kern/).  This appears to be happening because
make is searching for the argument to "exists" by using .PATH rather than a
relative search, because .PATH and the argument match at the beginning.
Additionally, make appears to consider a path that starts with ${.CURDIR}
as relative, even though it expands to an absolute path.

The reason that most people aren't seeing this problem is that the absolute
paths of /usr/src/sys and /sys are also searched, so as long as the kernel
source can be found in at least one of those places, no problems surface.
This problem was inadvertently introduced on 1 December 2000, with the
addition of the sysvipc modules.

share/mk/bsd.kmod.mk

@@ -2,11 +2,11 @@
 
 # Search for kernel source tree in standard places.
 .for _dir in ${.CURDIR}/../.. ${.CURDIR}/../../.. ${.CURDIR}/../../../.. /sys /usr/src/sys
-.if !defined(SYSDIR) && exists(${_dir}/kern/) && exists(${_dir}/conf/)
+.if !defined(SYSDIR) && exists(${_dir}/conf/kmod.mk)
 SYSDIR=	${_dir}
 .endif
 .endfor
-.if !defined(SYSDIR) || !exists(${SYSDIR}/kern/) || !exists(${SYSDIR}/conf/)
+.if !defined(SYSDIR) || !exists(${SYSDIR}/kern) || !exists(${SYSDIR}/conf/)
 .error "can't find kernel source tree"
 .endif
 

sys/conf/kmod.mk

@@ -166,7 +166,7 @@ beforedepend ${OBJS}: ${_ILINKS}
 SYSDIR=	${_dir}
 .endif
 .endfor
-.if !defined(SYSDIR) || !exists(${SYSDIR}/kern/)
+.if !defined(SYSDIR) || !exists(${SYSDIR}/kern)
 .error "can't find kernel source tree"
 .endif