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, 5 insertions, 17 deletions

diff --git a/sysdeps/arm/start.S b/sysdeps/arm/start.S
index 30f69e1b07..9b56bc0cca 100644
--- a/sysdeps/arm/start.S
+++ b/sysdeps/arm/start.S
@@ -94,30 +94,20 @@ _start:
 	adr a4, .L_GOT
 	add sl, sl, a4
 
-	ldr ip, .L_GOT+4	/* __libc_csu_fini */
-	ldr ip, [sl, ip]
+	mov a4, #0		/* Used to be init.  */
+	push { a4 }		/* Used to be fini.  */
 
-	push { ip }		/* Push __libc_csu_fini */
-
-	ldr a4, .L_GOT+8	/* __libc_csu_init */
-	ldr a4, [sl, a4]
-
-	ldr a1, .L_GOT+12	/* main */
+	ldr a1, .L_GOT+4	/* main */
 	ldr a1, [sl, a1]
 
 	/* __libc_start_main (main, argc, argv, init, fini, rtld_fini, stack_end) */
 	/* Let the libc call main and exit with its return code.  */
 	bl __libc_start_main(PLT)
 #else
-	/* Fetch address of __libc_csu_fini */
-	ldr ip, =__libc_csu_fini
-
-	/* Push __libc_csu_fini */
-	push { ip }
 
-	/* Set up the other arguments in registers */
+	mov a4, #0		/* Used to init.  */
+	push { a4 }		/* Used to fini.  */
 	ldr a1, =main
-	ldr a4, =__libc_csu_init
 
 	/* __libc_start_main (main, argc, argv, init, fini, rtld_fini, stack_end) */
 	/* Let the libc call main and exit with its return code.  */
@@ -131,8 +121,6 @@ _start:
 	.align 2
 .L_GOT:
 	.word _GLOBAL_OFFSET_TABLE_ - .L_GOT
-	.word __libc_csu_fini(GOT)
-	.word __libc_csu_init(GOT)
 	.word main(GOT)
 #endif