_execve.c

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/*      execve() - basic program execution call         Author: Kees J. Bot
 *                                                              21 Jan 1994
 */

#define _MINIX_SOURCE

#define nil 0
#define execve _execve
#define sbrk _sbrk
#include <lib.h>
#include <unistd.h>
#include <string.h>
#include <stddef.h>

int execve(const char *path, char * const *argv, char * const *envp)
{
        char * const *ap;
        char * const *ep;
        char *frame;
        char **vp;
        char *sp;
        size_t argc;
        size_t frame_size;
        size_t string_off;
        size_t n;
        int ov;
        message m;

        /* Assumptions: size_t and char *, it's all the same thing. */

        /* Create a stack image that only needs to be patched up slightly
         * by the kernel to be used for the process to be executed.
         */

        ov= 0;                  /* No overflow yet. */
        frame_size= 0;          /* Size of the new initial stack. */
        string_off= 0;          /* Offset to start of the strings. */
        argc= 0;                /* Argument count. */

        for (ap= argv; *ap != nil; ap++) {
                n = sizeof(*ap) + strlen(*ap) + 1;
                frame_size+= n;
                if (frame_size < n) ov= 1;
                string_off+= sizeof(*ap);
                argc++;
        }

        for (ep= envp; *ep != nil; ep++) {
                n = sizeof(*ep) + strlen(*ep) + 1;
                frame_size+= n;
                if (frame_size < n) ov= 1;
                string_off+= sizeof(*ap);
        }

        /* Add an argument count and two terminating nulls. */
        frame_size+= sizeof(argc) + sizeof(*ap) + sizeof(*ep);
        string_off+= sizeof(argc) + sizeof(*ap) + sizeof(*ep);

        /* Align. */
        frame_size= (frame_size + sizeof(char *) - 1) & ~(sizeof(char *) - 1);

        /* The party is off if there is an overflow. */
        if (ov || frame_size < 3 * sizeof(char *)) {
                errno= E2BIG;
                return -1;
        }

        /* Allocate space for the stack frame. */
        if ((frame = (char *) sbrk(frame_size)) == (char *) -1) {
                errno = E2BIG;
                return -1;
        }

        /* Set arg count, init pointers to vector and string tables. */
        * (size_t *) frame = argc;
        vp = (char **) (frame + sizeof(argc));
        sp = frame + string_off;

        /* Load the argument vector and strings. */
        for (ap= argv; *ap != nil; ap++) {
                *vp++= (char *) (sp - frame);
                n= strlen(*ap) + 1;
                memcpy(sp, *ap, n);
                sp+= n;
        }
        *vp++= nil;

        /* Load the environment vector and strings. */
        for (ep= envp; *ep != nil; ep++) {
                *vp++= (char *) (sp - frame);
                n= strlen(*ep) + 1;
                memcpy(sp, *ep, n);
                sp+= n;
        }
        *vp++= nil;

        /* Padding. */
        while (sp < frame + frame_size) *sp++= 0;

        /* We can finally make the system call. */
        m.m1_i1 = strlen(path) + 1;
        m.m1_i2 = frame_size;
        m.m1_p1 = (char *) path;
        m.m1_p2 = frame;

        /* Clear unused fields */
        m.m1_i3 = 0;
        m.m1_p3 = NULL;

        (void) _syscall(MM, EXEC, &m);

        /* Failure, return the memory used for the frame and exit. */
        (void) sbrk(-frame_size);
        return -1;
}

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