Question

I know that when calling a function in assembly r0 contains the first argument up to r3 being the fourth. I know that when it goes beyond four the stack pointer is used but I’m not too sure on the specifics. Does r0-r3 still hold the first four and the rest goes on the stack? I’m looking at the following piece of assembly trying to understand what it’s doing to call mach_msg (a function with seven arguments). On entry to this code, r0 and r1 contain the two arguments

var_38          = -0x38
var_34          = -0x34
var_30          = -0x30
var_2C          = -0x2C
var_24          = -0x24
var_20          = -0x20
var_18          = -0x18
var_14          = -0x14
var_10          = -0x10
var_C           = -0xC
00001220
00001220                 PUSH            {R7,LR}
00001222                 MOV             R7, SP
00001224                 SUB             SP, SP, #0x30
00001226                 MOV             R2, (_NDR_record_ptr - 0x1232) ; _NDR_record_ptr
0000122E                 ADD             R2, PC ; _NDR_record_ptr
00001230                 LDR             R2, [R2] ; _NDR_record
00001232                 LDR             R3, [R2]
00001234                 LDR             R2, [R2,#4]
00001236                 STR             R2, [SP,#0x38+var_10]
00001238                 MOVS            R2, #0x24 ; '$'
0000123A                 STR             R3, [SP,#0x38+var_14]
0000123C                 MOVS            R3, #0
0000123E                 STRB.W          R1, [SP,#0x38+var_C]
00001242                 MOVS            R1, #0x13
00001244                 STR             R1, [SP,#0x38+var_2C]
00001246                 MOVS            R1, #1
00001248                 STR             R0, [SP,#0x38+var_24]
0000124A                 MOV             R0, 0x1E84EA
00001252                 STR             R3, [SP,#0x38+var_20]
00001254                 STR             R3, [SP,#0x38+var_38]
00001256                 STR             R3, [SP,#0x38+var_34]
00001258                 STR             R0, [SP,#0x38+var_18]
0000125A                 STR             R3, [SP,#0x38+var_30]
0000125C                 ADD             R0, SP, #0x38+var_2C
0000125E                 BLX             _mach_msg
00001262                 ADD             SP, SP, #0x30
00001264                 POP             {R7,PC}

Here are the definitions for the stuff being called and used:

typedef struct {
    unsigned char       mig_vers;
    unsigned char       if_vers;
    unsigned char       reserved1;
    unsigned char       mig_encoding;
    unsigned char       int_rep;
    unsigned char       char_rep;
    unsigned char       float_rep;
    unsigned char       reserved2;
} NDR_record_t;
extern NDR_record_t NDR_record;
extern mach_msg_return_t    mach_msg(
                mach_msg_header_t *msg,
                mach_msg_option_t option,
                mach_msg_size_t send_size,
                mach_msg_size_t rcv_size,
                mach_port_name_t rcv_name,
                mach_msg_timeout_t timeout,
                mach_port_name_t notify);

From what I understand, the stack pointer is being reversed 48 bytes for variables. Is that 48 bytes for the extra 3 arguments or for all of them?

Answer 1

Of the 48 bytes, 12 are for the extra 3 parameters, and the others are for local variables. You can see this in the code where the function passes four parameters in r0 through r3, another in [SP,#0x38+var_38] (which if you do the math resolves to just [sp]), another in [sp,#4], and the last in [sp,#8].