I am trying to define and call a function at run-time with c language in arm cpu(cortex a72). in order to do that, i implemented a code like below:
#include <stdio.h>
#include <sys/mman.h>
char* ibuf;
int pbuf = 0;
#define ADD_BYTE(val) do{ibuf[pbuf] = val; pbuf++;} while(0)
void (*routine)(void);
void MakeRoutineSimpleFunc(void)
{
//nop
ADD_BYTE(0x00);
ADD_BYTE(0xf0);
ADD_BYTE(0x20);
ADD_BYTE(0xe3);
//bx lr
ADD_BYTE(0x1e);
ADD_BYTE(0xff);
ADD_BYTE(0x2f);
ADD_BYTE(0xe1);
}
int main(void)
{
ibuf = (char*)mmap(NULL, 4 * 1024, PROT_READ | PROT_WRITE | PROT_EXEC, MAP_POPULATE | MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
MakeRoutineSimpleFunc();
routine = (void(*)())(ibuf);
routine();
}
as you can see, in above code at first i allocate an executable memory region and assign address of that to ibuf, then i put some simple instruction in ibuf (a "nop" and "bx lr" that means return in arm) and then i try to call this function through a function pointer.
but when i want to call function through function pointer i got an "segmentation fault" error. BTW when i try to run the app with GDB debugger program run successfully without any error.
there is anything that i missed in above code that cause "segmentation fault"?
i want to add, when i add above instructions (a "nop" and "bx lr" that means return in arm) at compile-time to a function like below, function work without any error.
void f2(void)
{
__asm__ volatile (".byte 0x00, 0xf0, 0x20, 0xe3");
__asm__ volatile (".byte 0x1e, 0xff, 0x2f, 0xe1");
}
EDIT1: in order to check validity of run-time function, i have removed f2 prolog and epilogue with ghidra disassembler, so assembly code of f2 is like this:
**************************************************************
FUNCTION
**************************************************************
undefined FUN_0000083c()
undefined r0:1 <RETURN>
undefined4 Stack[-0x4]:4 local_4
FUN_0000083c XREF[1]: FUN_00000868:000008a4(c)
0000083c 00 f0 20 e3 nop
00000840 00 f0 20 e3 nop
00000844 00 f0 20 e3 nop
00000848 00 f0 20 e3 nop
0000084c 00 f0 20 e3 nop
00000850 00 f0 20 e3 nop
00000854 1e ff 2f e1 bx lr
00000858 00 f0 20 e3 nop
0000085c 00 f0 20 e3 nop
00000860 00 f0 20 e3 nop
00000864 00 f0 20 e3 nop
and also it work again without problem.
EDIT2: something that i want to add that may be helpful to solve the problem, as i saw in the assembler, compiler call "routine" function with "blx r3" instruction while it call other functions with "bl 'symbol name'". as i know blx can change processor state from ARM to Thumb or vise versa. could this point cause the problem?
EDIT3: disassemble of main function is something like below:
**************************************************************
FUNCTION
**************************************************************
int __stdcall main(void)
int r0:4 <RETURN>
undefined4 Stack[-0xc]:4 local_c XREF[1]: 00010d44(W)
undefined4 Stack[-0x10]:4 local_10 XREF[1]: 00010d4c(W)
main XREF[4]: Entry Point(*),
_start:00010394(*), 000103a8(*),
.debug_frame::000000a0(*)
00010d34 00 48 2d e9 stmdb sp!,{ r11 lr }
00010d38 04 b0 8d e2 add r11,sp,#0x4
00010d3c 08 d0 4d e2 sub sp,sp,#0x8
00010d40 00 30 a0 e3 mov r3,#0x0
00010d44 04 30 8d e5 str r3,[sp,#local_c]
00010d48 00 30 e0 e3 mvn r3,#0x0
00010d4c 00 30 8d e5 str r3,[sp,#0x0]=>local_10
00010d50 22 30 a0 e3 mov r3,#0x22
00010d54 07 20 a0 e3 mov r2,#0x7
00010d58 01 1a a0 e3 mov r1,#0x1000
00010d5c 00 00 a0 e3 mov r0,#0x0
00010d60 7d fd ff eb bl mmap
00010d64 00 20 a0 e1 cpy r2,r0
00010d68 50 30 9f e5 ldr r3,[->ibuf]
00010d6c 00 20 83 e5 str r2,[r3,#0x0]=>ibuf
00010d70 48 30 9f e5 ldr r3,[->ibuf]
00010d74 00 30 93 e5 ldr r3,[r3,#0x0]=>ibuf
00010d78 03 10 a0 e1 cpy r1,r3
00010d7c 40 00 9f e5 ldr r0=>s_ibuf:_%x_00010e40,[PTR_s_ibuf:_%x_00010d
00010d80 69 fd ff eb bl printf
00010d84 ae fe ff eb bl MakeRoutineSimpleFunc
00010d88 30 30 9f e5 ldr r3,[->ibuf]
00010d8c 00 30 93 e5 ldr r3,[r3,#0x0]=>ibuf
00010d90 03 20 a0 e1 cpy r2,r3
00010d94 2c 30 9f e5 ldr r3,[->routine]
00010d98 00 20 83 e5 str r2,[r3,#0x0]=>routine
00010d9c 24 30 9f e5 ldr r3,[->routine]
00010da0 00 30 93 e5 ldr r3,[r3,#0x0]=>routine
00010da4 33 ff 2f e1 blx r3
00010da8 1c 00 9f e5 ldr r0=>DAT_00010e4c,
00010dac 61 fd ff eb bl puts
00010db0 00 30 a0 e3 mov r3,#0x0
00010db4 03 00 a0 e1 cpy r0,r3
00010db8 04 d0 4b e2 sub sp,r11,#0x4
00010dbc 00 88 bd e8 ldmia sp!,{ r11 pc }
as you can see, routine called with "blx r3" instruction at address "00010da4". and also i printed address of ibuf, it is was "0xb6ff8000".
I think, you can enter the opcodes directly in a string "binary-code" and execute the code using ((void*)STRING)(). However, you may want to read also about how gcc implements trampolines, because this is how gcc generates code that creates code on the stack and jumps the execution there.