libc: link libsys as a auxiliary filter library

At runtime, when rtld loads libc it will also load libsys.  For each
symbol that is present in both, the libsys one will override the libc
one.  It continues to be the case that program need only link against
libc (usually implicitly).  The linkage to libsys is automatic.

Reviewed by:	kib, emaste, imp
Pull Request:	https://github.com/freebsd/freebsd-src/pull/908

Makefile.inc1

@@ -3125,10 +3125,12 @@ _startup_libs=	lib/csu
 _startup_libs+=	lib/libc
 _startup_libs+=	lib/libc_nonshared
 _startup_libs+=	lib/libcxxrt
+_startup_libs+=	lib/libsys
 
 _prereq_libs+=	lib/libgcc_eh lib/libgcc_s
 _startup_libs+=	lib/libgcc_eh lib/libgcc_s
 
+lib/libc__L: lib/libsys__L
 lib/libgcc_s__L: lib/libc__L
 lib/libgcc_s__L: lib/libc_nonshared__L
 lib/libcxxrt__L: lib/libgcc_s__L

UPDATING

@@ -27,6 +27,12 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 15.x IS SLOW:
 	world, or to merely disable the most expensive debugging functionality
 	at runtime, run "ln -s 'abort:false,junk:false' /etc/malloc.conf".)
 
+20240205:
+	For dynamically linked programs, system calls are now made from
+	libsys rather than libc.  No change in linkage is required as
+	libsys is an auxiliary filter for libc.  People building custom
+	images must ensure that libsys.so.7 is included.
+
 20240202:
 	Loader now also read configuration files listed in local_loader_conf_files.
 	Files listed here are the last ones read. And /boot/loader.conf.local was

lib/Makefile

@@ -123,7 +123,7 @@ SUBDIR_DEPEND_libbsnmp= ${_libnetgraph}
 SUBDIR_DEPEND_libc++:= libcxxrt
 # libssp_nonshared doesn't need to be linked into libc on every arch, but it is
 # small enough to build that this bit of serialization is likely insignificant.
-SUBDIR_DEPEND_libc= libcompiler_rt libssp_nonshared
+SUBDIR_DEPEND_libc= libsys libcompiler_rt libssp_nonshared
 SUBDIR_DEPEND_libcam= libsbuf
 SUBDIR_DEPEND_libcasper= libnv
 SUBDIR_DEPEND_libdevstat= libkvm

lib/libc/Makefile

@@ -62,6 +62,9 @@ CFLAGS+= -ftls-model=initial-exec
 #
 LDFLAGS+= -nodefaultlibs
 LIBADD+=	compiler_rt
+LIBADD+=	sys
+
+LDFLAGS+=-Wl,--auxiliary,libsys.so
 
 .if ${MK_SSP} != "no" && \
     (${LIBC_ARCH} == "i386" || ${LIBC_ARCH:Mpowerpc*} != "")

share/mk/src.libnames.mk

@@ -393,7 +393,7 @@ _DP_xo=		util
 _DP_ztest=	geom m nvpair umem zpool pthread avl zfs_core spl zutil zfs uutil icp
 # The libc dependencies are not strictly needed but are defined to make the
 # assert happy.
-_DP_c=		compiler_rt
+_DP_c=		sys compiler_rt
 # Use libssp_nonshared only on i386 and power*.  Other archs emit direct calls
 # to __stack_chk_fail, not __stack_chk_fail_local provided by libssp_nonshared.
 .if ${MK_SSP} != "no" && \