pledge utility

[Mikhail]

While reviewing Bob's presentation[1] on pledge and unveil I came across
his idea about pledge utility which will take unveil paths, pledge
promises and execute a program with the restrictions (slide 12).

What do you think about the idea and implementation?

[1] - https://www.openbsd.org/papers/BeckPledgeUnveilBSDCan2018.pdf

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <err.h>

void
usage(void)
{
        fprintf(stderr, "%s: [-p path mode ...] <-P \"promises\"> "
            "<program> [arguments]\n", getprogname());
        exit(1);
}

int
main(int argc, char **argv)
{
        int i, j, k, Pflag = 0, pflag = 0;
        char promises[1024] = { 0 };
        char **paths = NULL;
        char **modes = NULL;

        for (i = 1; i < argc; i++) {
                if (strcmp(argv[i], "-P") == 0) {
                        if (Pflag)
                                usage();
                        Pflag = 1;
                        strlcpy(promises, argv[++i], sizeof(promises));
                        break;
                }
                if (strcmp(argv[i], "-p") == 0) {
                        if (pflag)
                                usage();
                        pflag = 1;
                        i++;
                        j = 0;
                        while (argv[i] != NULL && strcmp(argv[i], "-P") != 0) {
                                paths = reallocarray(paths, j+1,
                                    sizeof(*paths));
                                modes = reallocarray(modes, j+1,
                                    sizeof(*modes));
                                paths[j] = strdup(argv[i++]);
                                /* We've advanced 'i', check if it's not EOL */
                                if (argv[i] == NULL)
                                        usage();
                                modes[j++] = strdup(argv[i++]);
                        }
                        /* If we have no args after -p */
                        if (j == 0)
                                usage();
                        /* Last cycle advanced i to -P, shift it back */
                        i--;
                        continue;
                }
        }

        /* -P is mandatory */
        if (Pflag == 0)
                usage();

        if (pledge("stdio exec unveil", promises) == -1)
                err(1, "pledge");

        if (pflag) {
                for (k = 0; k < j; k++) {
                        if (unveil(paths[k], modes[k]) == -1)
                                err(1, "unveil: %s mode %s", paths[k],
                                    modes[k]);
                }
                unveil(NULL, NULL);
        }

        char *progname = argv[++i];
        char **progargs = &argv[i];
        if (execvp(progname, progargs) == -1)
                err(1, "execvp: %s", progname);

        return (0);
}