This is cap/check_password_caps.c (Listing 39-1, page 808), an example from the book, The Linux Programming Interface.

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

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Download cap/check_password_caps.c

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/* check_password_caps.c

   This program provides an example of the use of capabilities to create a
   program that performs a task that requires privileges, but operates without
   the full power of 'root'. The program reads a username and password and
   checks if they are valid by authenticating against the (shadow) password

   The program executable file must be installed with the CAP_DAC_READ_SEARCH
   permitted capability, as follows:

        $ sudo setcap "cap_dac_read_search=p" check_password_caps

   This program is Linux-specific.

   See also check_password.c.
#define _BSD_SOURCE             /* Get getpass() declaration from <unistd.h> */
#define _XOPEN_SOURCE           /* Get crypt() declaration from <unistd.h> */
#include <sys/capability.h>
#include <unistd.h>
#include <limits.h>
#include <pwd.h>
#include <shadow.h>
#include "tlpi_hdr.h"
/* Change setting of capability in caller's effective capabilities */

static int
modifyCap(int capability, int setting)
    cap_t caps;
    cap_value_t capList[1];

    /* Retrieve caller's current capabilities */

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

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

    capList[0] = capability;
    if (cap_set_flag(caps, CAP_EFFECTIVE, 1, capList, setting) == -1) {
        return -1;

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

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

    /* Free the structure that was allocated by libcap */

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

    return 0;
static int              /* Raise capability in caller's effective set */
raiseCap(int capability)
    return modifyCap(capability, CAP_SET);
/* An analogous dropCap() (unneeded in this program), could be
   defined as: modifyCap(capability, CAP_CLEAR); */

static int              /* Drop all capabilities from all sets */
    cap_t empty;
    int s;

    empty = cap_init();
    if (empty == NULL)
        return -1;

    s = cap_set_proc(empty);

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

    return s;
main(int argc, char *argv[])
    char *username, *password, *encrypted, *p;
    struct passwd *pwd;
    struct spwd *spwd;
    Boolean authOk;
    size_t len;
    long lnmax;

    /* Determine size of buffer required for a username, and allocate it */

    lnmax = sysconf(_SC_LOGIN_NAME_MAX);
    if (lnmax == -1)                        /* If limit is indeterminate */
        lnmax = 256;                        /* make a guess */

    username = malloc(lnmax);
    if (username == NULL)

    printf("Username: ");
    if (fgets(username, lnmax, stdin) == NULL)
        exit(EXIT_FAILURE);                 /* Exit on EOF */

    len = strlen(username);
    if (username[len - 1] == '\n')
        username[len - 1] = '\0';           /* Remove trailing '\n' */

    /* Look up password record for username */

    pwd = getpwnam(username);
    if (pwd == NULL)
        fatal("couldn't get password record");

    /* Only raise CAP_DAC_READ_SEARCH for as long as we need it */

    if (raiseCap(CAP_DAC_READ_SEARCH) == -1)
        fatal("raiseCap() failed");

    /* Look up shadow password record for username */

    spwd = getspnam(username);
    if (spwd == NULL && errno == EACCES)
        fatal("no permission to read shadow password file");

    /* At this point, we won't need any more capabilities,
       so drop all capabilities from all sets */

    if (dropAllCaps() == -1)
        fatal("dropAllCaps() failed");

    if (spwd != NULL)           /* If there is a shadow password record */
        pwd->pw_passwd = spwd->sp_pwdp;     /* Use the shadow password */

    password = getpass("Password: ");

    /* Encrypt password and erase cleartext version immediately */

    encrypted = crypt(password, pwd->pw_passwd);
    for (p = password; *p != '\0'; )
        *p++ = '\0';

    if (encrypted == NULL)

    authOk = strcmp(encrypted, pwd->pw_passwd) == 0;
    if (!authOk) {
        printf("Incorrect password\n");

    printf("Successfully authenticated: UID=%ld\n", (long) pwd->pw_uid);

    /* Now do authenticated work... */



Download cap/check_password_caps.c

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