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Make the stack pointer the returned mmap pointer. (Linux, 32-bit virtual machine)

I am trying to move the stack pointer to the mmap-ed area to simulate a context switch, but somehow the code below always gives a segmentation error:

C:

struct savectx {
    void *regs[JB_SIZE];
};

struct savectx* initctx=(struct savectx*)malloc(sizeof(savectx));
void *newsp;
if ((newsp=mmap(0,STACK_SIZE,PROT_READ|PROT_WRITE,
    MAP_PRIVATE|MAP_ANONYMOUS,0,0))==MAP_FAILED){
    perror("mmap failed");
}
initctx->regs[4]=newsp;
restorectx(initctx,0);

x86:

 restorectx:
 movl   4(%esp),%ecx        /*Move jump buffer addr to ecx */
 movl   8(%esp),%eax        /*Longjmp return value */
 movl   (JB_SP*4)(%ecx),%esp    /*JB_SP is defined to be 4,*/

The program does not work in the last line of the assembly.

For malloc, I know that I may have to add 0x000f0000 to the pointer, but what about mmap? Or how do we create a stack adjusted to the location of mmapp-ed. (manap page for mmap: http://linux.die.net/man/3/mmap compiled using GCC on ubuntu)

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2 answers

, , mmap (, , ). (mmaped_stack + stack_size) .

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Linux ( UN * X ) :

, ( ), ucontext_t/struct sigcontext ( uc_mcontext ucontext_t). setcontext() - longjmp().

setcontext().

Linux ucontext_t Linux:

  • ucontext_t: Linux,
  • struct sigcontext: Linux, x86,
    ( - x86, ARM, PPC, MIPS,... , , , stackpointer).
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