Reduce the statically linked startup code [BZ #23323]

It turns out the startup code in csu/elf-init.c has a perfect pair of
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_2.34 is introduced because
new binaries running on an old libc would not run their ELF
constructors, leading to difficult-to-debug issues.

1 files changed, 2 insertions, 7 deletions

diff --git a/sysdeps/sh/start.S b/sysdeps/sh/start.S
index ab29f194f7..606ee59222 100644
--- a/sysdeps/sh/start.S
+++ b/sysdeps/sh/start.S
@@ -70,12 +70,11 @@ _start:
 
 	/* Push the last arguments to main() onto the stack */
 	mov.l r4,@-r15
-	mov.l L_fini,r0
-	mov.l r0,@-r15
+	mov.l r14,@-r15		/* Used to be fini.  */
 
 	/* Set up the other arguments for main() that go in registers */
 	mov.l L_main,r4
-	mov.l L_init,r7
+	mov #0,r7		/* Used to be init.  */
 
 	/* __libc_start_main (main, argc, argv, init, fini, rtld_fini) */
 
@@ -90,10 +89,6 @@ _start:
 	.align	2
 L_main:
 	.long	main
-L_init:
-	.long	__libc_csu_init
-L_fini:
-	.long	__libc_csu_fini
 L_libc_start_main:
 	.long	__libc_start_main
 L_abort: