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Date: Tue, 17 Feb 2015 01:41:00 +0000
From: David Guillen Fandos <david@...idgf.es>
To: musl@...ts.openwall.com
Subject: Executable crashes at __libc_start_main
Hello!
I'm creating an app which is an ARM ELF (linux) which runs in very small
machines (routers). Using buildroot to create my toolchain I can choose
between uClibc and musl. Using uclibc my binary crashes at loading, so I
switched to musl and tried. It fails too.
The problem seems to be at __libc_start_main, in this part:
uintptr_t a = (uintptr_t)&__init_array_start;
for (; a<(uintptr_t)&__init_array_end; a+=sizeof(void(*)()))
(*(void (**)())a)();
I checked a little bit (dumping the map file) and I get:
.init_array 0x0000000000016230 0x4
0x0000000000016230 PROVIDE
(__init_array_start, .)
*(SORT(.init_array.*))
*(.init_array)
.init_array 0x0000000000016230 0x4
/XXX/arm-buildroot-linux-musleabi/4.8.3/crtbeginT.o
0x0000000000016234 PROVIDE
(__init_array_end, .)
.fini_array 0x0000000000016234 0x4
0x0000000000016234
Which tells me there is only one function pointer there. Now dumping the
binary:
00016230 <__frame_dummy_init_array_entry>:
16230: 00008210 andeq r8, r0, r0, lsl r2
Disassembly of section .fini_array:
Which is pointer 0x8210 which points to function:
00008210 <frame_dummy>:
8210: e92d4008 push {r3, lr}
8214: e59f3034 ldr r3, [pc, #52] ; 8250
<frame_dummy+0x40>
8218: e3530000 cmp r3, #0
...
So far so good. The binary runs OK on a ARM machine running Debian, but
when I run this program on this other machine it crashes. The CPU is:
ARMv6-compatible processor rev 7 (v6l)
CPU implementer : 0x41
CPU architecture: 6TEJ
CPU variant : 0x0
CPU part : 0xb76
CPU revision : 7
Finally I got a core dump and the program crashes here:
88c8: e1550007 cmp r5, r7
88cc: 2a000003 bcs 88e0 <__libc_start_main+0x1b0>
88d0: e4953004 ldr r3, [r5], #4
88d4: e1a0e00f mov lr, pc
88d8: e12fff13 bx r3
88dc: eafffff9 b 88c8 <__libc_start_main+0x198>
In the 88d8 instruction to be more exact. Seems that R3 is holding the
value 0xc8000082!!! Where is that 0xC8 at the beginning comming from?
The PC reported by the core dump is 0xc8000080 which I guess it's just
the vlaue of R3 aligned to 4 byte boundary. R5 points to the right
place, it's just the value loaded by the load. Could it be that
something corrupts my ELF? Could it be the OS being really dumb at
loading the ELF? It's a pretty old kernel, 2.6.21.
Thanks a lot!
David
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