fly your dream: 07/01/2009

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How to reduce libc size
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Contents
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* 1 Introduction
* 2 Build options
o 2.1 Default -O2, file size 1164025
o 2.2 -O1, file size 1159845
+ 2.2.1 Linker strip, file size 1065624
o 2.3 -Os, file size 1094281
+ 2.3.1 Manual strip, file size 1004180
+ 2.3.2 Linker strip, file size 1000060
* 3 Feature removal
o 3.1 SCCS version strings, file size 953136
o 3.2 Hesiod name service, file size 942468
o 3.3 Yellow Pages (YP), file size 917368
o 3.4 IPv6 support, file size 909272
o 3.5 Stack smashing protection (SSP), file size 894764
o 3.6 Remote procedure call (RPC), file size 806036
o 3.7 Execution profiling control, file size 801720
o 3.8 Citrus I18N, file size 767560
o 3.9 MD2, RMD160, SHA1, SHA2, MD4 and MD5 cryptographic
hash functions, file size 723780
o 3.10 Misc, Native Language Support (NLS), Regular
Expression and Stack Smashing Protection, file size 691884
* 4 Ideas
o 4.1 Compiler options
+ 4.1.1 Thumb instruction set, file size 585440
o 4.2 Feature removals
+ 4.2.1 Database support from lib/libc/db
+ 4.2.2 Regular memory allocator instead of JEMALLOC
+ 4.2.3 Reduce resolver size in lib/libc/resolver
+ 4.2.4 Use libhack
* 5 Alternatives
o 5.1 Crunchgen
* 6 References

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.
o 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

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