Re: [PATCH] Reduce the statically linked startup code [BZ #23323]

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

On 08/08/18 15:48, Florian Weimer wrote:
> This is the patch rebased for glibc 2.29.
> 
> Should we make more far-reaching changes and remove the arguments which are now zero?
> 

there might be code that calls __libc_start_main directly
(e.g. the cgo runtime ?), if removing args breaks something
then i'd keep them (i don't know if it does).

the aarch64 changes look good.

> Subject: [PATCH] Reduce the statically linked startup code [BZ #23323]
> To:libc-alpha@sourceware.org
> 
> It turns out the startup code in csu/elf-unit.c has a perfect pair of

i think you mean csu/elf-init.c

> ROP gadgets (see Marco-Gisbert and Ripoll-Ripoll, “return-to-csu: A
> New Method to Bypass 64-bit Linux ASLR”).  These functions are not
> needed in dynamically-linked binaries because DT_INIT/DT_INIT_ARRAY
> are already processed by the dynamic linker.  However, the dynamic
> linker skipped the main program for some reason.  For maximum
> backwards compatibility, this is not changed, and instead, the main
> map is consulted from __libc_start_main if the init function argument
> is a NULL pointer.
> 
> For statically linked binaries, the old approach based on linker
> symbols is still used because there is nothing else available.
> 
> A new symbol version __libc_start_main@@GLIBC_2.29 is introduced because
> new binaries running on an old libc would not run their ELF
> constructors, leading to difficult-to-debug issues.  This approach
> (multiple symbol versions for the same function symbol) requires a
> recent binutils version (later than 2.25 on some architectures) because
> earlier versions cannot use a symbol alias to define multiple symbol
> versions for the same symbol.
> 
> Tested on aarch64, armhfp, i686, mips64, ppc, ppc64, ppc64el, s390,
> s390x, x86_64, and built with build-many-glibcs.py.  This patch
> depends on the earlier patch which introduces ELF_INITFINI.
> 
> 2018-06-23  Florian Weimer<fweimer@redhat.com>
> 
> 	[BZ #23323]
> 	Reduce statically-linked startup code.
> 	* csu/Makefile (routines): Remove elf-init.
> 	* csu/elf-init.c: Remove file.
> 	* csu/Versions (GLIBC_2.29): Export __libc_start_main from libc.
> 	* csu/libc-start.c: Add note that this is not part of the startup
> 	code for dynamically linked applications.
> 	[SHARED] (call_init): New function, based on call_init from
> 	elf/dl-init.c.
> 	[!SHARED] (call_init, call_fini): New functions, based on
> 	csu/elf-init.c
> 	(LIBC_START_MAIN): Update function comment.  Register call_fini
> 	directly for statically linked applications.  Call call_init
> 	instead of init (in statically linked applications, and if it is
> 	not NULL).
> 	(DO_DEFINE_LIBC_START_MAIN_VERSION)
> 	(DEFINE_LIBC_START_MAIN_VERSION_1)
> 	(DEFINE_LIBC_START_MAIN_VERSION): Define.
> 	* elf/dl-init.c (init_t): Move to <ldsodefs.h>.
> 	(call_init, _dl_init) Adjust.
> 	* sysdeps/generic/ldsodefs.h (dl_init_t): Moved from
> 	elf/dl-init.c.
> 	(DL_CALL_DT_INIT): Adjust.
> 	* sysdeps/hppa/dl-lookupcfg.h (DL_CALL_DT_INIT): Likewise.
> 	* sysdeps/ia64/dl-lookupcfg.h (DL_CALL_DT_INIT): Likewise.
> 	* sysdeps/aarch64/start.S (_start): Pass zero for init and fini.
> 	* sysdeps/alpha/start.S (_start): Likewise.
> 	* sysdeps/arm/start.S (_start): Likewise.  Adjust offset of
> 	pointer to main.
> 	(.L_GOT): Adjust.
> 	* sysdeps/hppa/start.S (.Lp__libc_csu_fini, .Lp__libc_csu_init):
> 	Remove.
> 	(_start): Pass zero for init and fini.
> 	* sysdeps/ia64/start.S (_start): Pass zero for init and fini.
> 	* sysdeps/i386/start.S (_start): Likewise.
> 	* sysdeps/m68k/start.S (_start): Likewise.
> 	* sysdeps/microblaze/start.S (_start): Likewise.
> 	* sysdeps/mips/start.S (ENTRY_POINT): Likewise.
> 	* sysdeps/nios2/start.S (_start): Likewise.
> 	* sydeps/powerpc/powerpc32/start.S (start_addresses): Use zero for
> 	init and fini.
> 	* sysdeps/powerpc/powerpc64/start.S (start_addresses): Likewise.
> 	* sysdeps/unix/sysv/linux/powerpc/libc-start: Use
> 	DEFINE_LIBC_START_MAIN_VERSION.
> 	* sysdeps/riscv/start.S (ENTRY_POINT): Pass zero for init and fini.
> 	* sysdeps/s390/s390-32/start.S (_start): Pass zero for init and
> 	fini.
> 	(.L1, .L2): Remove.
> 	* sysdeps/s390/s390-64/start.S (_start): Pass zero for init and
> 	fini.
> 	* sysdeps/s390/s390-64/start.S (_start): Pass zero for init and
> 	fini.
> 	* sysdeps/sh/start.S (_start): Likewise.
> 	(L_init, L_fini): Remove.
> 	* sysdeps/sparc/sparc32/start.S (_start): Pass zero for init and
> 	fini.
> 	* sysdeps/sparc/sparc64/start.S (_start): Likewise.
> 	* sysdeps/x86_64/start.S (start): Likewise.
> 	* sysdeps/mach/hurd/i386/libc.abilist: Add
> 	__libc_start_main@GLIBC_2.29.
> 	* sysdeps/unix/sysv/linux/aarch64/libc.abilist: Likewise.
> 	* sysdeps/unix/sysv/linux/alpha/libc.abilist: Likewise.
> 	* sysdeps/unix/sysv/linux/arm/libc.abilist: Likewise.
> 	* sysdeps/unix/sysv/linux/hppa/libc.abilist: Likewise.
> 	* sysdeps/unix/sysv/linux/i386/libc.abilist: Likewise.
> 	* sysdeps/unix/sysv/linux/ia64/libc.abilist: Likewise.
> 	* sysdeps/unix/sysv/linux/m68k/coldfire/libc.abilist: Likewise.
> 	* sysdeps/unix/sysv/linux/m68k/m680x0/libc.abilist: Likewise.
> 	* sysdeps/unix/sysv/linux/microblaze/libc.abilist: Likewise.
> 	* sysdeps/unix/sysv/linux/mips/mips32/fpu/libc.abilist: Likewise.
> 	* sysdeps/unix/sysv/linux/mips/mips32/nofpu/libc.abilist:
> 	Likewise.
> 	* sysdeps/unix/sysv/linux/mips/mips64/n32/libc.abilist: Likewise.
> 	* sysdeps/unix/sysv/linux/mips/mips64/n64/libc.abilist: Likewise.
> 	* sysdeps/unix/sysv/linux/nios2/libc.abilist: Likewise.
> 	* sysdeps/unix/sysv/linux/powerpc/powerpc32/fpu/libc.abilist:
> 	Likewise.
> 	* sysdeps/unix/sysv/linux/powerpc/powerpc32/nofpu/libc.abilist:
> 	Likewise.
> 	* sysdeps/unix/sysv/linux/powerpc/powerpc64/libc-le.abilist:
> 	Likewise.
> 	* sysdeps/unix/sysv/linux/powerpc/powerpc64/libc.abilist:
> 	Likewise.
> 	* sysdeps/unix/sysv/linux/riscv/rv64/libc.abilist: Likewise.
> 	* sysdeps/unix/sysv/linux/s390/s390-32/libc.abilist: Likewise.
> 	* sysdeps/unix/sysv/linux/s390/s390-64/libc.abilist: Likewise.
> 	* sysdeps/unix/sysv/linux/sh/libc.abilist: Likewise.
> 	* sysdeps/unix/sysv/linux/sparc/sparc32/libc.abilist: Likewise.
> 	* sysdeps/unix/sysv/linux/sparc/sparc64/libc.abilist: Likewise.
> 	* sysdeps/unix/sysv/linux/x86_64/64/libc.abilist: Likewise.
> 	* sysdeps/unix/sysv/linux/x86_64/x32/libc.abilist: Likewise.