c,assembly,mmap , mmap2 function write in asm, call in c


mmap2 function write in asm, call in c

Question:

Tag: c,assembly,mmap

I have problem with writing MMAP2 in ASM AT&T and call it in C. I wrote this but didn't know how should it works. I am knowingly that code is not good but I very need help.
Can you tell me how should it looks ?
Thanks for help!

.data

MMAP2 = 192
MUNMAP = 91 
PROT_READ = 0x1 
MAP_ANONYMOUS = 0x20

.bss 
.text


.global moje_mmap
.type moje_map @function 
moje_mmap:

push %ebp           
mov %esp, %ebp          
xor %ebx, %ebx 
mov 8(%ebp), %ecx       
mov $PROT_READ, %edx 
mov $MAP_ANONYMOUS, %esi 
mov $-1, %edi

mov $MMAP2, %eax        
int $0x80
mov %ebp, %esp 
pop %ebp


ret                 

#include <stdlib.h> 
#include <stdio.h>
#include <sys/types.h> 
#include <sys/mman.h>

void* moje_mmap(size_t dlugosc);

int main() {

moje_mmap(30);

return 0; }

Answer:

You're actually returning the value correctly from your assembly function. -22 is a valid return value from mmap2, meaning EINVAL. When you use syscalls directly from assembly, errors are usually returned as the negative version of the error, for example -EINVAL or -22.

Now, as to why you're getting an error, here's an excerpt from the mmap2 man page:

   EINVAL (Various platforms where the page size is not 4096 bytes.)
          offset * 4096 is not a multiple of the system page size.

Looking at your code, you're passing -1 as the offset parameter, but that's valid, so it's not the problem.

The problem is more likely to be with your flags parameter:

   The flags argument determines whether updates to the mapping are
   visible to other processes mapping the same region, and whether
   updates are carried through to the underlying file.  This behavior is
   determined by including exactly one of the following values in flags:

   MAP_SHARED Share this mapping.  Updates to the mapping are visible to
              other processes that map this file, and are carried
              through to the underlying file.  (To precisely control
              when updates are carried through to the underlying file
              requires the use of msync(2).)

   MAP_PRIVATE
              Create a private copy-on-write mapping.  Updates to the
              mapping are not visible to other processes mapping the
              same file, and are not carried through to the underlying
              file.  It is unspecified whether changes made to the file
              after the mmap() call are visible in the mapped region.

As stated here, you must include either MAP_SHARED or MAP_PRIVATE in your flags parameter. Add this, and your program should work.


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