Contents

Introduction

The NetBSD installation is not big, but it can be made smaller. Here are a few tricks to make the base of user space, the C standard library libc, a bit smaller for dynamically linked systems. These trials were done on a cross compiled NetBSD current ARMv6 branch.

First the user space is built with default options. Then the cross compiler script $TOOLDIR/bin/nbmake-evbarm was used to clean and rebuild the libc with special options. The new libc binary was then copied to the target file system and a smoke test of booting the ARM development board was done. If /sbin/init and /bin/sh managed with the new libc, the test was a success while everything else was a failure.

The result is a crippled libc and /lib/libc.so.12.159 file size reduction from 1164 to 692 kilobytes. Run time memory usage is harder to predict since it depends on what parts of in memory libc are actually used by the processes, but at least text and data sections reported by the size utility give some idea.

Build options

Default -O2, file size 1164025

-O2 optimization is used by default and the libc file size after a ./build.sh -U -m evbarm build is:

-r--r--r-- 1 test test 1164025 2008-04-18 08:23 obj/destdir.evbarm/lib/libc.so.12.159

Sections reported by size utility:

   text    data     bss     dec     hex filename
 931608   25728   64332 1021668   f96e4 obj/libc.so.12.159

-O1, file size 1159845

If the libc is build with CFLAGS=-O1, the ELF shared object file size is:

-rw-r--r-- 1 test test 1159845 Apr 19 09:20 lib/libc.so.12.159

Sections reported by size utility:

   text    data     bss     dec     hex filename
 927436   25728   64332 1017496   f8698 obj/libc.so.12.159

Linker strip, file size 1065624

If the -O1 build is stripped and size optimized by the linker with LDFLAGS=-Wl,-O1\ -Wl,-s, the file size reduces to:

-rwxr-xr-x 1 test test 1065624 2008-04-19 13:28 obj/libc.so.12.159 

Sections reported by size utility:

   text    data     bss     dec     hex filename
 923316   25728   64332 1013376   f7680 obj/libc.so.12.159

-Os, file size 1094281

The gcc compiler can optimize binaries for size with CFLAGS=-Os:

-rwxr-xr-x 1 test test 1094281 2008-04-19 10:56 obj/libc.so.12.159

Sections reported by size utility:

   text    data     bss     dec     hex filename
 861864   25736   64332  951932   e867c obj/libc.so.12.159

Manual strip, file size 1004180

The binary can then be stripped manually:

$ $TOOLDIR/bin/arm--netbsdelf-strip -s obj/libc.so.12.159 
$ ls -l obj/libc.so.12.159
-rwxr-xr-x 1 test test 1004180 2008-04-19 11:02 obj/libc.so.12.159

Sections reported by size utility:

   text    data     bss     dec     hex filename
 861864   25736   64332  951932   e867c obj/libc.so.12.159

Linker strip, file size 1000060

The -Os compiled binary is smaller with linker based strip and optimization where LDFLAGS=-Wl,-O1\ -Wl,-s than with a manual strip:

-rwxr-xr-x 1 test test 1000060 2008-04-19 12:07 obj/libc.so.12.159

Sections reported by size utility:

   text    data     bss     dec     hex filename
 857744   25736   64332  947812   e7664 obj/libc.so.12.159

Feature removal

In addition to compiler flags CFLAGS=-Os LDFLAGS=-Wl,-O1\ -Wl,-s, special feature flags can be used to strip features out of libc and reduce its size. Some feature flags, as documented by BUILDING and share/mk/bsd.README, are supported for the whole user space.

SCCS version strings, file size 953136

SCCS version strings are normally embedded into object file, but they be removed by following changes in lib/libc/Makefile:

--- lib/libc/Makefile.inc       3 Jun 2007 17:36:08 -0000       1.3 
+++ lib/libc/Makefile.inc       19 Apr 2008 11:01:23 -0000
@@ -24,7 +24,8 @@
 .include <bsd.own.mk>

 WARNS=4
-CPPFLAGS+=     -D_LIBC -DLIBC_SCCS -DSYSLIBC_SCCS -D_REENTRANT
+#CPPFLAGS+=    -D_LIBC -DLIBC_SCCS -DSYSLIBC_SCCS -D_REENTRANT
+CPPFLAGS+=     -D_LIBC -D_REENTRANT

 .if (${USE_HESIOD} != "no")
 CPPFLAGS+=     -DHESIOD

The resulting libc binary finally goes below the one megabyte mark:

-rwxr-xr-x 1 test test 953136 2008-04-19 13:54 obj/libc.so.12.159

Sections reported by size utility:

   text    data     bss     dec     hex filename
 857744   25736   64332  947812   e7664 obj/libc.so.12.159

Hesiod name service, file size 942468

Hesiod), a DNS based database service, support can be removed from libc with USE_HESIOD=no MKHESIOD=no build variables and result is:

-rwxr-xr-x 1 test test 942468 2008-04-19 14:16 obj/libc.so.12.159

Sections reported by size utility:

   text    data     bss     dec     hex filename
 847625   25252   62180  935057   e4491 obj/libc.so.12.159

Yellow Pages (YP), file size 917368

Yellow Pages (YP) or Network Information Service (NIS) directory service support can be removed with USE_YP=no MKYP=no variables:

-rwxr-xr-x 1 test test 917368 2008-04-19 14:29 obj/libc.so.12.159

Sections reported by size utility:

   text    data     bss     dec     hex filename
 824328   24488   58944  907760   dd9f0 obj/libc.so.12.159

IPv6 support, file size 909272

IPv6 support can be removed with USE_INET6=no:

-rwxr-xr-x 1 test test 909272 2008-04-19 14:48 obj/libc.so.12.159

Sections reported by size utility:

   text    data     bss     dec     hex filename
 816537   24368   58944  899849   dbb09 obj/libc.so.12.159

Stack smashing protection (SSP), file size 894764

SSP buffer overflow protection from the GCC compiler can be disabled with USE_SSP=no and the libc binary size goes below 900k:

-rwxr-xr-x 1 test test 894764 2008-04-19 15:02 obj/libc.so.12.159

Sections reported by size utility:

   text    data     bss     dec     hex filename
 802029   24368   58944  885341   d825d obj/libc.so.12.159

Remote procedure call (RPC), file size 806036

RPC support can be disabled with MKRPC=no variable and a patch like this:

--- lib/libc/Makefile 9 Jan 2008 01:33:52 -0000 1.131.4.1 +++ lib/libc/Makefile 23 Apr 2008 13:04:42 -0000 @@ -74,7 +80,10 @@ .endif .include "${.CURDIR}/regex/Makefile.inc" .include "${.CURDIR}/resolv/Makefile.inc" +MKRPC?= yes +.if (${MKRPC} != "no") .include "${.CURDIR}/rpc/Makefile.inc" +.endif .include "${.CURDIR}/ssp/Makefile.inc" .include "${.CURDIR}/stdio/Makefile.inc" .include "${.CURDIR}/stdlib/Makefile.inc"

As a result the libc size goes down to 806 kilobytes:

-rw-r--r-- 1 test test 806036 2008-04-23 16:00 lib/libc.so.12.159

Sections reported by size utility:

   text    data     bss     dec     hex filename
 717964   22624   58500  799088   c3170 obj/libc.so.12.159

Execution profiling control, file size 801720

Profiling control can be removed from libc with MKGMON=no and a patch like:

--- lib/libc/Makefile 9 Jan 2008 01:33:52 -0000 1.131.4.1 +++ lib/libc/Makefile 24 Apr 2008 08:07:08 -0000 @@ -58,7 +58,10 @@ .include "${.CURDIR}/dlfcn/Makefile.inc" .include "${.CURDIR}/gdtoa/Makefile.inc" .include "${.CURDIR}/gen/Makefile.inc" +MKGMON?= yes +.if (${MKGMON} != "no") .include "${.CURDIR}/gmon/Makefile.inc" +.endif .include "${.CURDIR}/hash/Makefile.inc" .include "${.CURDIR}/iconv/Makefile.inc" .include "${.CURDIR}/inet/Makefile.inc"

And libc size goes down around 4 kilobytes:

-rwxr-xr-x 1 test test 801720 2008-04-24 10:57 obj/libc.so.12.159 

Sections reported by size utility:

   text    data     bss     dec     hex filename
 713959   22500   58436  794895   c210f obj/libc.so.12.159

Citrus I18N, file size 767560

Citrus I18N support can be removed with MKCITRUS=no, lib/libc/locale/setlocale.c patch and a makefile patch:

--- Makefile 9 Jan 2008 01:33:52 -0000 1.131.4.1 +++ Makefile 25 Apr 2008 07:51:20 -0000 @@ -53,12 +53,18 @@

 .include "${.CURDIR}/../../common/lib/libc/Makefile.inc"
 .include "${.CURDIR}/db/Makefile.inc"
+MKCITRUS?= yes
+.if (${MKCITRUS} != "no")
 .include "${.CURDIR}/citrus/Makefile.inc"
+.endif
 .include "${.CURDIR}/compat-43/Makefile.inc"
 .include "${.CURDIR}/dlfcn/Makefile.inc"
 .include "${.CURDIR}/gdtoa/Makefile.inc"

The libc binary is now below 800 kilobytes:

-rwxr-xr-x 1 test test 767560 2008-04-25 10:01 obj/libc.so.12.159 

Sections reported by size utility:

   text    data     bss     dec     hex filename
 685150   18696   56968  760814   b9bee obj/libc.so.12.159

MD2, RMD160, SHA1, SHA2, MD4 and MD5 cryptographic hash functions, file size 723780

All cryptographic hash functions can be removed from the libc with MKMD2=no MKRMD160=no MKSHA1=no MKSHA2=no MKMD=no and lib/libc/hash/Makefile.inf and lib/libc/Makefile patches:

--- lib/libc/hash/Makefile.inc 27 Oct 2006 18:29:21 -0000 1.11 +++ lib/libc/hash/Makefile.inc 30 Apr 2008 09:10:21 -0000 @@ -4,8 +4,20 @@ # hash functions .PATH: ${ARCHDIR}/hash ${.CURDIR}/hash

+MKMD2?= yes
+.if (${MKMD2} != "no")
 .include "${.CURDIR}/hash/md2/Makefile.inc"
+.endif
+MKRMD160?= yes
+.if (${MKRMD160} != "no")
 .include "${.CURDIR}/hash/rmd160/Makefile.inc"
+.endif
+MKSHA1?= yes
+.if (${MKSHA1} != "no")
 .include "${.CURDIR}/hash/sha1/Makefile.inc"
+.endif
+MKSHA2?= yes
+.if (${MKSHA2} != "no")
 .include "${.CURDIR}/hash/sha2/Makefile.inc"
+.endif


--- lib/libc/Makefile   9 Jan 2008 01:33:52 -0000       1.131.4.1
+++ lib/libc/Makefile   30 Apr 2008 09:11:25 -0000

 .include "${.CURDIR}/inet/Makefile.inc"
 .include "${.CURDIR}/isc/Makefile.inc"
 .include "${.CURDIR}/locale/Makefile.inc"
+MKMD?= yes
+.if (${MKMD} != "no")
 .include "${.CURDIR}/md/Makefile.inc"
+.endif
 .include "${.CURDIR}/misc/Makefile.inc"
 .include "${.CURDIR}/net/Makefile.inc"
 .include "${.CURDIR}/nameser/Makefile.inc"

libc size reduces to:

-rwxr-xr-x 1 test test 723780 2008-04-30 12:03 obj/libc.so.12.159

Sections reported by size utility:

   text    data     bss     dec     hex filename
 642476   18456   56968  717900   af44c obj/libc.so.12.159

Misc, Native Language Support (NLS), Regular Expression and Stack Smashing Protection, file size 691884

Indeed, misc object, NLS, regular expression (IEEE Std 1003.2-1992 (“POSIX.2”) regular expressions) and Stack Smashing Protection (SSP) support library are easily removed from the build process with MKMISC=no MKNLS=no MKREGEX=no MKSSP=no and an obvious patch to lib/libc/Makefile:

.include "${.CURDIR}/md/Makefile.inc" +.endif +MKMISC?= yes +.if (${MKMISC} != "no") .include "${.CURDIR}/misc/Makefile.inc" +.endif .include "${.CURDIR}/net/Makefile.inc" .include "${.CURDIR}/nameser/Makefile.inc" +MKNLS?= yes +.if (${MKNLS} != "no") .include "${.CURDIR}/nls/Makefile.inc" +.endif .if (${MACHINE_ARCH} != "alpha") && (${MACHINE_ARCH} != "sparc64") .include "${.CURDIR}/quad/Makefile.inc" .endif +MKREGEX?= yes +.if (${MKREGEX} != "no") .include "${.CURDIR}/regex/Makefile.inc" +.endif .include "${.CURDIR}/resolv/Makefile.inc" ... .include "${.CURDIR}/rpc/Makefile.inc" +.endif +MKSSP?= yes +.if (${MKSSP} != "no") .include "${.CURDIR}/ssp/Makefile.inc" +.endif .include "${.CURDIR}/stdio/Makefile.inc" .include "${.CURDIR}/stdlib/Makefile.inc" .include "${.CURDIR}/string/Makefile.inc"

Boot with sbin/init still works as does bin/sh, but user friendly programs like bin/ps and bin/ls now fail due to missing symbols:

# ls -l lib/libc.so.12.159
/lib/libc.so.12: Undefined PLT symbol "__fgets_chk" (symnum = 156)
# ls lib/libc.so.12.159
lib/libc.so.12.159
# ps aux
/lib/libc.so.12: Undefined PLT symbol "__strcat_chk" (symnum = 207)
# ps
ps: warning: /var/run/dev.db: /lib/libc.so.12: Undefined PLT symbol "_catopen" )

File size now:

-rwxr-xr-x 1 test test 691884 2008-04-30 14:18 obj/libc.so.12.159

Segment sizes reported by size utility:

   text    data     bss     dec     hex filename
 614238   17284   56928  688450   a8142 obj/libc.so.12.159

Ideas

While a few compiler and feature options were a tried out, a number of new ideas were found. Some of these were quickly tried out, but they resulted in the build or smoke/boot test failures.

Compiler options

Thumb instruction set, file size 585440

Compile to 16 bit THUMB instruction set instead of normal 32 bit

• Whole user space needs to be build with -mthumb-interwork
• CPUFLAGS build variable should contain -mthumb and -mthumb-interwork
• If some files (like atomic_init_testset.c) need arm32 code due to embedded ARM assembly or other tool chain issues, the CPUFLAGS build variable can be overridden: Per file build options override
• libc from matt-armv6 branch builds with -mthumb but fails to run with SIGILL: Thumb compilation discussion on port-arm

After a successfull compile with atomic_init_testset.o compiled with -mthumb-interwork only, the file size is:

-rwxr-xr-x 1 mira mira 585440 2008-05-12 11:20 obj/libc.so.12.159

size utility reports:

   text    data     bss     dec     hex filename
 507462   17616   56928  582006   8e176 obj/libc.so.12.159

Feature removals

Database support from lib/libc/db

• sbin/init and possibly others depend on Berkeley DB support
• a custom init and bin/sh propably work without it

Regular memory allocator instead of JEMALLOC

• USE_JEMALLOC: (share/mk/bsd.README) If "no", disables building the "jemalloc" allocator designed for improved performance with threaded applications.

  • seems to require rebuild of user space, but even after that init dies with:

  warning: no /dev/console panic: init died (signal 0, exit 12) Stopped in pid 1.1 (init) at netbsd:cpu_Debugger+0x4: bx r14

Reduce resolver size in lib/libc/resolver

• remove IPv6 support
• remove debug features

Use libhack

• Use size optimized distrib/utils/libhack with/instead of libc

Alternatives

Crunchgen

crunchgen can be used to create statically (at compile time) linked executables, which include only those object files, which are really used by the executable.

• Use crunchgen or related build infra to build a dynamic library from only those object files that are actually required by the applications

References

• The BUILDING and share/mk/bsd.README files in addition to libc specific makefiles and sources contain information on the different build and feature options.
• gcc and ld manual pages
• Shrinking NetBSD - deep final distribution re-linker, discussion on tech-userlevel
• Reducing libc size, discussion on tech-userlevel