I have some simple code that finds the difference between two assembly labels:
#include <stdio.h>
static void foo(void){
__asm__ __volatile__("_foo_start:");
printf("Hello, world.\n");
__asm__ __volatile__("_foo_end:");
}
int main(void){
extern const char foo_start[], foo_end[];
printf("foo_start: %p, foo_end: %p\n", foo_start, foo_end);
printf("Difference = 0x%tx.\n", foo_end - foo_start);
foo();
return 0;
}
Now, this code works perfectly on 64-bit processors, just like you would expect it to. However, on 32-bit processors, the address of foo_start is the same as foo_end.
I'm sure it has to do with 32 to 64 bit. On i386, it results in 0x0, and x86_64 results in 0x7. On ARMv7 (32 bit), it results in 0x0, while on ARM64, it results in 0xC. (the 64-bit results are correct, I checked them with a disassembler)
I'm using Clang+LLVM to compile.
I'm wondering if it has to do with non-lazy pointers. In the assembly output of both 32-bit processor archs mentioned above, they have something like this at the end:
L_foo_end$non_lazy_ptr:
.indirect_symbol _foo_end
.long 0
L_foo_start$non_lazy_ptr:
.indirect_symbol _foo_start
.long 0
However, this is not present in the assembly output of both x86_64 and ARM64. I messed with removing the non-lazy pointers and addressing the labels directly yesterday, but to no avail. Any ideas on why this happens?
EDIT:
It appears that when compiled for 32 bit processors, foo_start[] and foo_end[] point to main. I....I'm so confused.
I didn't check on real code but suspect you are a victim of instruction reordering. As long as you do not define proper memory barriers, the compiler ist free to move your code within the function around as it sees fit since there is no interdependency between labels and printf() call.
Try adding ::: "memory" to your asm statements which should nail them where you wrote them.
I finally found the solution (or, alternative, I suppose). Apparently, the && operator can be used to get the address of C labels, removing the need for me to use inline assembly at all. I don't think it's in the C standard, but it looks like Clang supports it, and I've heard GCC does too.
#include <stdio.h>
int main(void){
foo_start:
printf("Hello, world.\n");
foo_end:
printf("Foo has ended.");
void* foo_start_ptr = &&foo_start;
void* foo_end_ptr = &&foo_end;
printf("foo_start: %p, foo_end: %p\n", foo_start_ptr, foo_end_ptr);
printf("Difference: 0x%tx\n", (long)foo_end_ptr - (long)foo_start_ptr);
return 0;
}
Now, this only works if the labels are in the same function, but for what I intend to use this for, it's perfect. No more ASM, and it doesn't leave a symbol behind. It appears to work just how I need it to. (Not tested on ARM64)
Related
I am trying to decompile an executable for the 68000 processor into C code, replacing the original subroutines with C functions one by one.
The problem I faced is that I don't know how to make gcc use the calling convention that matches the one used in the original program. I need the parameters on the stack to be packed, not aligned.
Let's say we have the following function
int fun(char arg1, short arg2, int arg3) {
return arg1 + arg2 + arg3;
}
If we compile it with
gcc -m68000 -Os -fomit-frame-pointer -S source.c
we get the following output
fun:
move.b 7(%sp),%d0
ext.w %d0
move.w 10(%sp),%a0
lea (%a0,%d0.w),%a0
move.l %a0,%d0
add.l 12(%sp),%d0
rts
As we can see, the compiler assumed that parameters have addresses 7(%sp), 10(%sp) and 12(%sp):
but to work with the original program they need to have addresses 4(%sp), 5(%sp) and 7(%sp):
One possible solution is to write the function in the following way (the processor is big-endian):
int fun(int bytes4to7, int bytes8to11) {
char arg1 = bytes4to7>>24;
short arg2 = (bytes4to7>>8)&0xffff;
int arg3 = ((bytes4to7&0xff)<<24) | (bytes8to11>>8);
return arg1 + arg2 + arg3;
}
However, the code looks messy, and I was wondering: is there a way to both keep the code clean and achieve the desired result?
UPD: I made a mistake. The offsets I'm looking for are actually 5(%sp), 6(%sp) and 8(%sp) (the char-s should be aligned with the short-s, but the short-s and the int-s are still packed):
Hopefully, this doesn't change the essence of the question.
UPD 2: It turns out that the 68000 C Compiler by Sierra Systems gives the described offsets (as in UPD, with 2-byte alignment).
However, the question is about tweaking calling conventions in gcc (or perhaps another modern compiler).
Here's a way with a packed struct. I compiled it on an x86 with -m32 and got the desired offsets in the disassembly, so I think it should still work for an mc68000:
typedef struct {
char arg1;
short arg2;
int arg3;
} __attribute__((__packed__)) fun_t;
int
fun(fun_t fun)
{
return fun.arg1 + fun.arg2 + fun.arg3;
}
But, I think there's probably a still cleaner way. It would require knowing more about the other code that generates such a calling sequence. Do you have the source code for it?
Does the other code have to remain in asm? With the source, you could adjust the offsets in the asm code to be compatible with modern C ABI calling conventions.
I've been programming in C since 1981 and spent years doing mc68000 C and assembler code (for apps, kernel, device drivers), so I'm somewhat familiar with the problem space.
It's not a gcc 'fault', it is 68k architecture that requires stack to be always aligned on 2 bytes.
So there is simply no way to break 2-byte alignment on the hardware stack.
but to work with the original program they need to have addresses
4(%sp), 5(%sp) and 7(%sp):
Accessing word or long values off the ODD memory address will immediately trigger alignment exception on 68000.
To get integral parameters passed using 2 byte alignment instead of 4 byte alignment, you can change the default int size to be 16 bit by -mshort. You need to replace all int in your code by long (if you want them to be 32 bit wide). The crude way to do that is to also pass -Dint=long to your compiler. Obviously, you will break ABI compatibility to object files compiled with -mno-short (which appears to be the default for gcc).
This is basically to perform swap for the buffers while transferring a message buffer. This statement left me puzzled (because of my unfamiliarity with the embedded assembly code in c). This is a power pc instruction
#define ASMSWAP32(dest_addr,data) __asm__ volatile ("stwbrx %0, 0, %1" : : "r" (data), "r" (dest_addr))
Besides being unsafe because of a bug, this macro is also less efficient than what the compiler will generate for you.
stwbrx = store word byte-reversed. The x stands for indexed.
You don't need inline asm for this in GNU C, where you can use __builtin_bswap32 and let the compiler emit this instruction for you.
void swapstore_asm(int a, int *p) {
ASMSWAP32(p, a);
}
void swapstore_c(int a, int *p) {
*p = __builtin_bswap32(a);
}
Compiled with gcc4.8.5 -O3 -mregnames, we get identical code from both functions (Godbolt compiler explorer):
swapstore:
stwbrx %r3, 0, %r4
blr
swapstore_c:
stwbrx %r3,0,%r4
blr
But with a more complicated address (storing to p[off], where off is an integer function arg), the compiler knows how to use both register inputs, while your macro forces the compiler to have the address in a single register:
void swapstore_offset(int a, int *p, int off) {
= __builtin_bswap32(a);
}
swapstore_offset:
slwi %r5,%r5,2 # *4 = sizeof(int)
stwbrx %r3,%r4,%r5 # use an indexed addressing mode, with both registers non-zero
blr
swapstore_offset_asm:
slwi %r5,%r5,2
add %r4,%r4,%r5 # extra instruction forced by using the macro
stwbrx %r3, 0, %r4
blr
BTW, if you're having trouble understanding GNU C inline asm templates, looking at the compiler's asm output can be a useful way to see what gets substituted in. See How to remove "noise" from GCC/clang assembly output? for more about reading compiler asm output.
Also note that this macro is buggy: it's missing a "memory" clobber for the store. And yes, you still need that with asm volatile. The compiler doesn't assume that *dest_addr is modified unless you tell it, so it could hoist a non-volatile load of *dest_addr ahead of this insn, or more likely to be a real problem, sink a store after it. (e.g. if you zeroed a buffer before storing to it with this, the compiler might actually zero after this instruction.)
Instead of a "memory" clobber (and also leaving out volatile), you could tell the compiler which memory location you modify with a =m" (*dest_addr) operand, either as a dummy operand or with a constraint on the addressing mode so you could use it as reg+reg. (IDK PPC well enough to know what "=m" usually expands to.)
In most cases this bug won't bite you, but it's still a bug. Upgrading your compiler version or using link-time optimization could maybe make your program buggy with no source-level changes.
This kind of thing is why https://gcc.gnu.org/wiki/DontUseInlineAsm
See also https://stackoverflow.com/tags/inline-assembly/info.
#define ASMSWAP32(dest_addr,data) ...
This part should be clear
__asm__ volatile ( ... : : "r" (data), "r" (dest_addr))
This is the actual inline assembly:
Two values are passed to the assmbly code; no value is returned from the assembly code (this is the colons after the actual assembly code).
Both parameters are passed in registers ("r"). The expression %0 will be replaced by the register that contains the value of data while the expression %1 will be replaced by the register that contains the value of dest_addr (which will be a pointer in this case).
The volatile here means that the assembly code has to be executed at this point and cannot be moved to somewhere else.
So if you use the following code in the C source:
ASMSWAP(&a, b);
... the following assembler code will be generated:
# write the address of a to register 5 (for example)
...
# write the value of b to register 6
...
stwbrx 6, 0, 5
So the first argument of the stwbrx instruction is the value of b and the last argument is the address of a.
stwbrx x, 0, y
This instruction writes the value in register x to the address stored in register y; however it writes the value in "reverse endian" (on a big-endian CPU it writes the value "little endian".
The following code:
uint32 a;
ASMSWAP32(&a, 0x12345678);
... should therefore result in a = 0x78563412.
Well, this is obviously a beginner's question, but this is my first attempt at making an operating system in C (Actually, I'm almost entirely new to C.. I'm used to asm) so, why exactly is this not valid? As far as I know, a pointer in C is just a uint16_t used to point to a certain area in memory, right (or a uint32_t and that's why it's not working)?
I've made the following kernel ("I've already made a bootloader and all in assembly to load the resulting KERNEL.BIN file):
kernel.c
void printf(char *str)
{
__asm__(
"mov si, %0\n"
"pusha\n"
"mov ah, 0x0E\n"
".repeat:\n"
"lodsb\n"
"cmp al, 0\n"
"je .done\n"
"int 0x10\n"
"jmp .repeat\n"
".done:\n"
"popa\n"
:
: "r" (str)
);
return;
}
int main()
{
char *msg = "Hello, world!";
printf(msg);
__asm__("jmp $");
return 0;
}
I've used the following command to compile it kernel.c:
gcc kernel.c -ffreestanding -m32 -std=c99 -g -O0 -masm=intel -o kernel.bin
which returns the following error:
kernel.c:3: Error: operand type mismatch for 'mov'
Why exactly might be the cause of this error?
As Michael Petch already explained, you use inline assembly only for the absolute minimum of code that cannot be done in C. For the rest there is inline assembly, but you have to be extremely careful to set the constraints and clobber list right.
Let always GCC do the job of passing the values in the right register and just specify in which register the values should be.
For your problem you probably want to do something like this
#include <stdint.h>
void print( const char *str )
{
for ( ; *str; str++) {
__asm__ __volatile__("int $0x10" : : "a" ((int16_t)((0x0E << 8) + *str)), "b" ((int16_t)0) : );
}
}
EDIT: Your assembly has the problem that you try to pass a pointer in a 16 bit register. This cannot work for 32 bit code, as 32 bit is also the pointer size.
If you in case want to generate 16 bit real-mode code, there is the -m16 option. But that does not make GCC a true 16 bit compiler, it has its limitations. Essentially it issues a .code16gcc directive in the code.
You can't simply use 16bit assembly instructions on 32-bit pointers and expect it to work. si is the lower 16bit of the esi register (which is 32bit).
gcc -m32 and -m16 both use 32-bit pointers. -m16 just uses address-size and operand-size prefixes to do mostly the same thing as normal -m32 mode, but running in real mode.
If you try to use 16bit addressing in a 32bit application you'll drop the high part of your pointers, and simply go to a different place.
Just try to read a book on intel 32bit addressing modes, and protected mode, and you'll see that many things are different on that mode.
(and if you try to switch to 64bit mode, you'll see that everything changes again)
A bootloader is something different as normally, cpu reset forces the cpu to begin in 16bit real mode. This is completely different from 32bit protected mode, which is one of the first things the operating system does. Bootloaders work in 16bit mode, and there, pointers are 16bit wide (well, not, 20bits wide, when the proper segment register is appended to the address)
How can I print out the current value at the stack pointer in C in Linux (Debian and Ubuntu)?
I tried google but found no results.
One trick, which is not portable or really even guaranteed to work, is to simple print out the address of a local as a pointer.
void print_stack_pointer() {
void* p = NULL;
printf("%p", (void*)&p);
}
This will essentially print out the address of p which is a good approximation of the current stack pointer
There is no portable way to do that.
In GNU C, this may work for target ISAs that have a register named SP, including x86 where gcc recognizes "SP" as short for ESP or RSP.
// broken with clang, but usually works with GCC
register void *sp asm ("sp");
printf("%p", sp);
This usage of local register variables is now deprecated by GCC:
The only supported use for this feature is to specify registers for input and output operands when calling Extended asm
Defining a register variable does not reserve the register. Other than when invoking the Extended asm, the contents of the specified register are not guaranteed. For this reason, the following uses are explicitly not supported. If they appear to work, it is only happenstance, and may stop working as intended due to (seemingly) unrelated changes in surrounding code, or even minor changes in the optimization of a future version of gcc. ...
It's also broken in practice with clang where sp is treated like any other uninitialized variable.
In addition to duedl0r's answer with specifically GCC you could use __builtin_frame_address(0) which is GCC specific (but not x86 specific).
This should also work on Clang (but there are some bugs about it).
Taking the address of a local (as JaredPar answered) is also a solution.
Notice that AFAIK the C standard does not require any call stack in theory.
Remember Appel's paper: garbage collection can be faster than stack allocation; A very weird C implementation could use such a technique! But AFAIK it has never been used for C.
One could dream of a other techniques. And you could have split stacks (at least on recent GCC), in which case the very notion of stack pointer has much less sense (because then the stack is not contiguous, and could be made of many segments of a few call frames each).
On Linuxyou can use the proc pseudo-filesystem to print the stack pointer.
Have a look here, at the /proc/your-pid/stat pseudo-file, at the fields 28, 29.
startstack %lu
The address of the start (i.e., bottom) of the
stack.
kstkesp %lu
The current value of ESP (stack pointer), as found
in the kernel stack page for the process.
You just have to parse these two values!
You can also use an extended assembler instruction, for example:
#include <stdint.h>
uint64_t getsp( void )
{
uint64_t sp;
asm( "mov %%rsp, %0" : "=rm" ( sp ));
return sp;
}
For a 32 bit system, 64 has to be replaced with 32, and rsp with esp.
You have that info in the file /proc/<your-process-id>/maps, in the same line as the string [stack] appears(so it is independent of the compiler or machine). The only downside of this approach is that for that file to be read it is needed to be root.
Try lldb or gdb. For example we can set backtrace format in lldb.
settings set frame-format "frame #${frame.index}: ${ansi.fg.yellow}${frame.pc}: {pc:${frame.pc},fp:${frame.fp},sp:${frame.sp}} ${ansi.normal}{ ${module.file.basename}{\`${function.name-with-args}{${frame.no-debug}${function.pc-offset}}}}{ at ${ansi.fg.cyan}${line.file.basename}${ansi.normal}:${ansi.fg.yellow}${line.number}${ansi.normal}{:${ansi.fg.yellow}${line.column}${ansi.normal}}}{${function.is-optimized} [opt]}{${frame.is-artificial} [artificial]}\n"
So we can print the bp , sp in debug such as
frame #10: 0x208895c4: pc:0x208895c4,fp:0x01f7d458,sp:0x01f7d414 UIKit`-[UIApplication _handleDelegateCallbacksWithOptions:isSuspended:restoreState:] + 376
Look more at https://lldb.llvm.org/use/formatting.html
You can use setjmp. The exact details are implementation dependent, look in the header file.
#include <setjmp.h>
jmp_buf jmp;
setjmp(jmp);
printf("%08x\n", jmp[0].j_esp);
This is also handy when executing unknown code. You can check the sp before and after and do a longjmp to clean up.
If you are using msvc you can use the provided function _AddressOfReturnAddress()
It'll return the address of the return address, which is guaranteed to be the value of RSP at a functions' entry. Once you return from that function, the RSP value will be increased by 8 since the return address is pop'ed off.
Using that information, you can write a simple function that return the current address of the stack pointer like this:
uintptr_t GetStackPointer() {
return (uintptr_t)_AddressOfReturnAddress() + 0x8;
}
int main(int argc, const char argv[]) {
uintptr_t rsp = GetStackPointer();
printf("Stack pointer: %p\n", rsp);
}
Showcase
You may use the following:
uint32_t msp_value = __get_MSP(); // Read Main Stack pointer
By the same way if you want to get the PSP value:
uint32_t psp_value = __get_PSP(); // Read Process Stack pointer
If you want to use assembly language, you can also use MSP and PSP process:
MRS R0, MSP // Read Main Stack pointer to R0
MRS R0, PSP // Read Process Stack pointer to R0
Given this code:
#include <stdio.h>
int main(int argc, char **argv)
{
int x = 1;
printf("Hello x = %d\n", x);
}
I'd like to access and manipulate the variable x in inline assembly. Ideally, I want to change its value using inline assembly. GNU assembler, and using the AT&T syntax.
In GNU C inline asm, with x86 AT&T syntax:
(But https://gcc.gnu.org/wiki/DontUseInlineAsm if you can avoid it).
// this example doesn't really need volatile: the result is the same every time
asm volatile("movl $0, %[some]"
: [some] "=r" (x)
);
after this, x contains 0.
Note that you should generally avoid mov as the first or last instruction of an asm statement. Don't copy from %[some] to a hard-coded register like %%eax, just use %[some] as a register, letting the compiler do register allocation.
See https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html and https://stackoverflow.com/tags/inline-assembly/info for more docs and guides.
Not all compilers support GNU syntax.
For example, for MSVC you do this:
__asm mov x, 0 and x will have the value of 0 after this statement.
Please specify the compiler you would want to use.
Also note, doing this will restrict your program to compile with only a specific compiler-assembler combination, and will be targeted only towards a particular architecture.
In most cases, you'll get as good or better results from using pure C and intrinsics, not inline asm.
asm("mov $0, %1":"=r" (x):"r" (x):"cc"); -- this may get you on the right track. Specify register use as much as possible for performance and efficiency. However, as Aniket points out, highly architecture dependent and requires gcc.