Can _start be the thumb function? - linux

Question

Help me please with gnu assembler for arm926ejs cpu.
I try to build a simple program(test.S):
.global _start
_start:
mov r0, #2
bx lr
and success build it:
arm-none-linux-gnueabi-as -mthumb -o test.o test.S
arm-none-linux-gnueabi-ld -o test test.o
but when I run the program in the arm target linux environment, I get an error:
./test
Segmentation fault
What am I doing wrong?
Can _start function be the thumb func?
or
It is always arm func?

Answer 1

Can _start be a thumb function (in a Linux user program)?
Yes it can. The steps are not as simple as you may believe.
Please use the .code 16 as described by others. Also look at ARM Script predicate; my answer shows how to detect a thumb binary. The entry symbol must have the traditional _start+1 value or Linux will decide to call your _start in ARM mode.
Also your code is trying to emulate,
int main(void) { return 2; }
The _start symbol must not do this (as per auselen). To do _start to main() in ARM mode you need,
#include <linux/unistd.h>
static inline void exit(int status)
{
asm volatile ("mov r0, %0\n\t"
"mov r7, %1\n\t"
"swi #7\n\t"
: : "r" (status),
"Ir" (__NR_exit)
: "r0", "r7");
}
/* Wrapper for main return code. */
void __attribute__ ((unused)) estart (int argc, char*argv[])
{
int rval = main(argc,argv);
exit(rval);
}
/* Setup arguments for estart [like main()]. */
void __attribute__ ((naked)) _start (void)
{
asm(" sub lr, lr, lr\n" /* Clear the link register. */
" ldr r0, [sp]\n" /* Get argc... */
" add r1, sp, #4\n" /* ... and argv ... */
" b estart\n" /* Let's go! */
);
}
It is good to clear the lr so that stack traces will terminate. You can avoid the argc and argv processing if you want. The start shows how to work with this. The estart is just a wrapper to convert the main() return code to an exit() call.
You need to convert the above assembler to Thumb equivalents. I would suggest using gcc inline assembler. You can convert to pure assembler source if you get inlines to work. However, doing this in 'C' source is probably more practical, unless you are trying to make a very minimal executable.
Helpful gcc arguements are,
-nostartfiles -static -nostdlib -isystem <path to linux user headers>
Add -mthumb and you should have a harness for either mode.

Answer 2

Your problem is you end with
bx lr
and you expect Linux to take over after that. That exact line must be the cause of Segmentation fault.
You can try to create a minimal executable then try to bisect it to see the guts and understand how an executable is expected to behave.
See below for a working example:
.global _start
.thumb_func
_start:
mov r0, #42
mov r7, #1
svc #0
compile with
arm-linux-gnueabihf-as start.s -o start.o && arm-linux-gnueabihf-ld
start.o -o start_test
and dump to see the guts
$ arm-linux-gnueabihf-readelf -a -W start_test
Now you should notice the odd address of _start
ELF Header:
Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
Class: ELF32
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: EXEC (Executable file)
Machine: ARM
Version: 0x1
Entry point address: 0x8055
Start of program headers: 52 (bytes into file)
Start of section headers: 160 (bytes into file)
Flags: 0x5000000, Version5 EABI
Size of this header: 52 (bytes)
Size of program headers: 32 (bytes)
Number of program headers: 1
Size of section headers: 40 (bytes)
Number of section headers: 6
Section header string table index: 3
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .text PROGBITS 00008054 000054 000006 00 AX 0 0 4
[ 2] .ARM.attributes ARM_ATTRIBUTES 00000000 00005a 000014 00 0 0 1
[ 3] .shstrtab STRTAB 00000000 00006e 000031 00 0 0 1
[ 4] .symtab SYMTAB 00000000 000190 0000e0 10 5 6 4
[ 5] .strtab STRTAB 00000000 000270 000058 00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings)
I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
There are no section groups in this file.
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
LOAD 0x000000 0x00008000 0x00008000 0x0005a 0x0005a R E 0x8000
Section to Segment mapping:
Segment Sections...
00 .text
There is no dynamic section in this file.
There are no relocations in this file.
There are no unwind sections in this file.
Symbol table '.symtab' contains 14 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 00000000 0 NOTYPE LOCAL DEFAULT UND
1: 00008054 0 SECTION LOCAL DEFAULT 1
2: 00000000 0 SECTION LOCAL DEFAULT 2
3: 00000000 0 FILE LOCAL DEFAULT ABS start.o
4: 00008054 0 NOTYPE LOCAL DEFAULT 1 $t
5: 00000000 0 FILE LOCAL DEFAULT ABS
6: 0001005a 0 NOTYPE GLOBAL DEFAULT 1 _bss_end__
7: 0001005a 0 NOTYPE GLOBAL DEFAULT 1 __bss_start__
8: 0001005a 0 NOTYPE GLOBAL DEFAULT 1 __bss_end__
9: 00008055 0 FUNC GLOBAL DEFAULT 1 _start
10: 0001005a 0 NOTYPE GLOBAL DEFAULT 1 __bss_start
11: 0001005c 0 NOTYPE GLOBAL DEFAULT 1 __end__
12: 0001005a 0 NOTYPE GLOBAL DEFAULT 1 _edata
13: 0001005c 0 NOTYPE GLOBAL DEFAULT 1 _end
No version information found in this file.
Attribute Section: aeabi
File Attributes
Tag_CPU_arch: v4T
Tag_THUMB_ISA_use: Thumb-1

Answer 3

here answer.
Thanks for all.
http://stuff.mit.edu/afs/sipb/project/egcs/src/egcs/gcc/config/arm/README-interworking
Calls via function pointers should use the BX instruction if the call is made in ARM mode:
.code 32
mov lr, pc
bx rX
This code sequence will not work in Thumb mode however, since the mov instruction will not set the bottom bit of the lr register. Instead a branch-and-link to the _call_via_rX functions should be used instead:
.code 16
bl _call_via_rX
where rX is replaced by the name of the register containing the function address.