How to disassemble a memory range with GDB?

gdb disassembly

I'm trying to disassemble a program to see a syscall assembly instruction (the INT instruction, I believe) and the handler with GDB and have written a little program (see below) for it that opens and closes a file.

I was able to follow the call to fopen with GDB until it executed a call.

When I tried to tell GDB "disassemble 0x...." (address of call) it responded with 'No function contains specified address.'

Is it possible to force GDB to disassemble (or display it in assembler as good as possible) that memory address? If so, how?

#include <stdio.h>
#include <stdlib.h>

int main() {
    FILE* f;
    f = fopen("main.c", "r");
    if (!f) { 
      perror("open");
      return -1;
    }
    fclose(f);
    return 0;
}

fopen() is not a system call, it's a call to the C standard library. And why do you think a system call must be made via an INT instruction?

I may be wrong, but we were taught that fopen calls ultimately result in a system call to the kernel to open the file and return a file descriptor?

Patrick: Yes, but does not need to do that directy. Normally it calls libc function which then enters kernel. But calling kernel may be done not only with int (this is slow) but with syscall/sysenter depending on processor architecture...

kexik - thank you for the information. I saw that Wikipedia mentions this in its system call article (en.wikipedia.org/w/…). Apparently Linux started using the special calls in 2.5 kernels. Another thing learned about my Operating System's architecture.

Michael Snyder

Yeah, disassemble is not the best command to use here. The command you want is "x/i" (examine as instructions):

(gdb) x/i 0xdeadbeef

THANKS! Adding this text to help others find this hint: this is the instruction to be used to disassemble binary blob, disassemble ROM, examine instruction in a binary image file etc. Write a small C program to fread() the binary blob into a buffer. Then do 'x /i' on the buffer.

@user188012 if you want to disassemble a binary blob, an easier way to do it is to use a standalone disassembler like ndisasm or similar.

You can use: x/i $pc to get the instruction for the pc which is the address of current instruction

You can use: "(gdb) x/i 0xaddress" to print of instructions e.x. "(gdb) x/10i 0xaddress" to print 10 instructions

Is the address 0xdeadbeef of any significance here? I mean to ask that is it any special address?

timrau

Do you only want to disassemble your actual main? If so try this:

(gdb) info line main 
(gdb) disas STARTADDRESS ENDADDRESS

Like so:

USER@MACHINE /cygdrive/c/prog/dsa
$ gcc-3.exe -g main.c

USER@MACHINE /cygdrive/c/prog/dsa
$ gdb a.exe
GNU gdb 6.8.0.20080328-cvs (cygwin-special)
...
(gdb) info line main
Line 3 of "main.c" starts at address 0x401050 <main> and ends at 0x401075 <main+
(gdb) disas 0x401050 0x401075
Dump of assembler code from 0x401050 to 0x401075:
0x00401050 <main+0>:    push   %ebp
0x00401051 <main+1>:    mov    %esp,%ebp
0x00401053 <main+3>:    sub    $0x18,%esp
0x00401056 <main+6>:    and    $0xfffffff0,%esp
0x00401059 <main+9>:    mov    $0x0,%eax
0x0040105e <main+14>:   add    $0xf,%eax
0x00401061 <main+17>:   add    $0xf,%eax
0x00401064 <main+20>:   shr    $0x4,%eax
0x00401067 <main+23>:   shl    $0x4,%eax
0x0040106a <main+26>:   mov    %eax,-0xc(%ebp)
0x0040106d <main+29>:   mov    -0xc(%ebp),%eax
0x00401070 <main+32>:   call   0x4010c4 <_alloca>
End of assembler dump.

I don't see your system interrupt call however. (its been a while since I last tried to make a system call in assembly. INT 21h though, last I recall

ok, then I'll try to look for INT 21h in the future. Thanks for that hint. But what I wanted to try is to follow the call sequence originating in fopen() (don't see it in your code...) 'down' until I can see the INT command.

Managed it - The way to go is to use both your answer and Falaina's. I had to compile it statically with gcc --static main.c and then use gdb/objdump to go deep down into the C library. Ultimaltively, it resulted in a call to __open_nocancel, which did an INT 0x80. Thanks to both of you

Note: the disas 0x401050 0x401075 syntax at least in version gdb 7.7 won't work. You have rather write it like disas 0x401050,0x401075. Also probably you might want to add the prefix «/m» to show a source code around: disas \m 0x401050,0x401075

@Patrick, Although this was quite some time ago, it's worth noting that INT 0x80 is just how Linux does it. That is to say, Linux's syscall handler is registered at interrupt 128. Other operating systems may vary -- which they do.

Nubok

This isn't the direct answer to your question, but since you seem to just want to disassemble the binary, perhaps you could just use objdump:

objdump -d program

This should give you its dissassembly. You can add -S if you want it source-annotated.

⁺¹ for -S, I didn't knew it could include the source code.

佚

佚名

You can force gcc to output directly to assembly code by adding the -S switch

gcc -S hello.c

Joel

fopen() is a C library function and so you won't see any syscall instructions in your code, just a regular function call. At some point, it does call open(2), but it does that via a trampoline. There is simply a jump to the VDSO page, which is provided by the kernel to every process. The VDSO then provides code to make the system call. On modern processors, the SYSCALL or SYSENTER instructions will be used, but you can also use INT 80h on x86 processors.

Al.

If all that you want is to see the disassembly with the INTC call, use objdump -d as someone mentioned but use the -static option when compiling. Otherwise the fopen function is not compiled into the elf and is linked at runtime.

Lakshman Kumar

gdb disassemble has a /m to include source code alongside the instructions. This is equivalent of objdump -S, with the extra benefit of confining to just the one function (or address-range) of interest.

Milhous

You don't have to use gdb. GCC will do it.

 gcc -S foo.c

This will create foo.s which is the assembly.

gcc -m32 -c -g -Wa,-a,-ad foo.c > foo.lst

The above version will create a listing file that has both the C and the assembly generated by it. GCC FAQ

YU Chen Shih

full example for disassembling a memory range to C

/opt/gcc-arm-none-eabi-9-2019-q4-major/bin/arm-none-eabi-gdb

(gdb)file /root/ncs/zephyr/samples/hello_world/build_nrf9160dk_nrf9160ns/zephyr/zephyr.elf
(gdb) directory /root/ncs/zephyr/samples/hello_world/src
#here you want 1
(gdb) info line* 0x000328C0
#here you want 2, -0x04 ~ +0x04 is your range size
(gdb) disassemble /m 0x000328C0-0x04, 0x000328C0+0x04
#here with binary code
(gdb) disassemble /r 0x000328C0-0x04, 0x000328C0+0x04
(gdb) info thread
(gdb) interpreter-exec mi -thread-info

How to disassemble a memory range with GDB?

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