cap/cap_functions.c

This is cap/cap_functions.c, an example to accompany the book, The Linux Programming Interface.

This file is not printed in the book; it is a supplementary file for Chapter 39.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU Lesser General Public License, version 3.

In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.

 

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Function list (Bold in this list means a function is not static)

/* cap_functions.c

   Useful functions for working with capabilities.
*/

#include <sys/prctl.h>
#include <linux/securebits.h>
#include "cap_functions.h"
/* Change the 'setting' of the specified 'capability' in the capability set
   specified by 'flag'.

   'flag' is one of CAP_PERMITTED, CAP_EFFECTIVE, or CAP_INHERITABLE.
   'setting' is one of CAP_SET or CAP_CLEAR.

   Returns: 0 on success or -1 on error. */

int
modifyCapSetting(cap_flag_t flag, int capability, int setting)
{
    /* Retrieve caller's current capabilities */

    cap_t caps = cap_get_proc();
    if (caps == NULL)
        return -1;

    /* Change setting of 'capability' in the 'flag' capability set in 'caps'.
       The third argument, 1, is the number of items in the array 'capList'. */

    cap_value_t capList[1];
    capList[0] = capability;
    if (cap_set_flag(caps, flag, 1, capList, setting) == -1) {
        cap_free(caps);
        return -1;
    }

    /* Push modified capability sets back to kernel, to change
       caller's capabilities */

    if (cap_set_proc(caps) == -1) {
        cap_free(caps);
        return -1;
    }

    /* Free the structure that was allocated by cap_get_proc() */

    if (cap_free(caps) == -1)
        return -1;

    return 0;
}
/* Display a securebits mask in either short or long form, depending on
   the value of 'verbose'. */

void
printSecbits(int secbits, bool verbose, FILE *fp)
{
    struct secbitInfo {
        int   flag;
        char *name;
        char  letter;
    };

    struct secbitInfo secbitInfoList[] = {
        {SECBIT_NO_CAP_AMBIENT_RAISE,   "NO_CAP_AMBIENT_RAISE",   'a'},
        {SECBIT_NO_CAP_AMBIENT_RAISE_LOCKED,
                                   "NO_CAP_AMBIENT_RAISE_LOCKED", 'A'},
        {SECBIT_KEEP_CAPS,              "KEEP_CAPS",              'k'},
        {SECBIT_KEEP_CAPS_LOCKED,       "KEEP_CAPS_LOCKED",       'K'},
        {SECBIT_NOROOT,                 "NOROOT",                 'r'},
        {SECBIT_NOROOT_LOCKED,          "NOROOT_LOCKED",          'R'},
        {SECBIT_NO_SETUID_FIXUP,        "NO_SETUID_FIXUP",        's'},
        {SECBIT_NO_SETUID_FIXUP_LOCKED, "NO_SETUID_FIXUP_LOCKED", 'S'},
        {0, NULL, '\0'}
    };

    int printed = 0;

    if (verbose) {
        fprintf(fp, "[");
        for (struct secbitInfo *p = secbitInfoList; p->flag != 0; p++) {
            if (secbits & p->flag) {
                if (printed > 0)
                    fprintf(fp, ", ");
                fprintf(fp, "%s", p->name);
                printed++;
            }
        }
        fprintf(fp, "]");
    } else {
        for (struct secbitInfo *p = secbitInfoList; p->flag != 0; p++) {
            if (secbits & p->flag)
                fputc(p->letter, fp);
            else
                fputc('-', fp);
        }
    }
}

 

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