CAP_PWD(3)

📦 version 15.1, in freebsd 14.5, freebsd 15.1

💖 CAP_PWD(3) — bestie edition 💖

okay so bestie, this is CAP_PWD(3) from the Library Functions Manual, and it's literally the library for password database operations in capability mode. no cap, it's giving “let me look up passwd entries without letting the sandbox escape” energy, and we're gonna go through it together. 🫶

✨ NAME ✨

cap_getpwent, cap_getpwnam, cap_getpwuid, cap_getpwent_r, cap_getpwnam_r, cap_getpwuid_r, cap_setpassent, cap_setpwent, cap_endpwent, cap_pwd_limit_cmds, cap_pwd_limit_fields, cap_pwd_limit_users — library for password database operations in capability mode. 💅

📚 LIBRARY 📚

library libcap_pwd — periodt, that's the one. 🎀

🫶 SYNOPSIS 🫶

#include <libcasper.h>
#include <casper/cap_pwd.h>

struct passwd *
cap_getpwent(cap_channel_t *chan);

struct passwd *
cap_getpwnam(cap_channel_t *chan, const char *login);

struct passwd *
cap_getpwuid(cap_channel_t *chan, uid_t uid);

int
cap_getpwent_r(cap_channel_t *chan, struct passwd *pwd, char *buffer,
    size_t bufsize, struct passwd **result);

int
cap_getpwnam_r(cap_channel_t *chan, const char *name,
    struct passwd *pwd, char *buffer, size_t bufsize,
    struct passwd **result);

int
cap_getpwuid_r(cap_channel_t *chan, uid_t uid, struct passwd *pwd,
    char *buffer, size_t bufsize, struct passwd **result);

int
cap_setpassent(cap_channel_t *chan, int stayopen);

void
cap_setpwent(cap_channel_t *chan);

void
cap_endpwent(cap_channel_t *chan);

int
cap_pwd_limit_cmds(cap_channel_t *chan, const char * const *cmds,
    size_t ncmds);

int
cap_pwd_limit_fields(cap_channel_t *chan, const char * const *fields,
    size_t nfields);

int
cap_pwd_limit_users(cap_channel_t *chan, const char * const *names,
    size_t nnames, uid_t *uids, size_t nuids);

🌸 DESCRIPTION 🌸

The functions cap_getpwent(), cap_getpwnam(), cap_getpwuid(), cap_getpwent_r(), cap_getpwnam_r(), cap_getpwuid_r(), cap_setpassent(), cap_setpwent(), and cap_endpwent() are respectively equivalent to getpwent(3), getpwnam(3), getpwuid(3), getpwent_r(3), getpwnam_r(3), getpwuid_r(3), setpassent(3), setpwent(3), and cap_endpwent(3) except that the connection to the system.pwd service needs to be provided. like, that's the whole plot twist. 🦋

cap_pwd_limit_cmds() — this one limits the functions allowed in the service. the cmds variable can be set to getpwent, getpwnam, getpwuid, getpwent_r, getpwnam_r, getpwuid_r, setpassent, setpwent, or endpwent which will allow to use the function associated with the name. the ncmds variable contains the number of cmds provided. lowkey iconic permission locking. 🔥

cap_pwd_limit_fields() — this lets you limit fields returned in the struct passwd structure. the fields variable can be set to pw_name, pw_passwd, pw_uid, pw_gid, pw_change, pw_class, pw_gecos, pw_dir, pw_shell, pw_expire or pw_fields. the field which was set as the limit will be returned, while the rest of the values not set this way will have default values. the nfields variable contains the number of fields provided. it's giving “only the facts I asked for” energy. 🎀

cap_pwd_limit_users() — this one allows to limit access to users. the names variable allows to limit users by name and the uids variable by the user number. the nnames and nuids variables provide numbers of limited names and uids. very main character energy for access control. 👑

🚨 okay but listen: all of these functions are reentrant but not thread-safe. that is, they may be called from separate threads only with different cap_channel_t arguments or with synchronization. do not ignore this bestie, fr fr. 💀

🦋 EXAMPLES 🦋

The following example first opens a capability to casper and then uses this capability to create the system.pwd casper service and uses it to get a user name.

cap_channel_t *capcas, *cappwd;
const char *cmds[] = { "getpwuid" };
const char *fields[] = { "pw_name" };
uid_t uid[] = { 1 };
struct passwd *passwd;

/* Open capability to Casper. */
capcas = cap_init();
if (capcas == NULL)
        err(1, "Unable to contact Casper");

/* Enter capability mode sandbox. */
if (cap_enter() < 0 && errno != ENOSYS)
        err(1, "Unable to enter capability mode");

/* Use Casper capability to create capability to the system.pwd service. */
cappwd = cap_service_open(capcas, "system.pwd");
if (cappwd == NULL)
        err(1, "Unable to open system.pwd service");

/* Close Casper capability, we don't need it anymore. */
cap_close(capcas);

/* Limit service to one single function. */
if (cap_pwd_limit_cmds(cappwd, cmds, nitems(cmds)))
        err(1, "Unable to limit access to system.pwd service");

/* Limit service to one field as we only need name of the user. */
if (cap_pwd_limit_fields(cappwd, fields, nitems(fields)))
        err(1, "Unable to limit access to system.pwd service");

/* Limit service to one uid. */
if (cap_pwd_limit_users(cappwd, NULL, 0, uid, nitems(uid)))
        err(1, "Unable to limit access to system.pwd service");

passwd = cap_getpwuid(cappwd, uid[0]);
if (passwd == NULL)
        err(1, "Unable to get name of user");

printf("UID %d is associated with name %s.\n", uid[0], passwd->pw_name);

cap_close(cappwd);

🌈 SEE ALSO 🌈

👑 HISTORY 👑

The cap_pwd service first appeared in FreeBSD 10.3. slay, vintage but still relevant. ⭐

💜 AUTHORS 💜

The cap_pwd service was implemented by Pawel Jakub Dawidek <pawel@dawidek.net> under sponsorship from the FreeBSD Foundation.

This manual page was written by Mariusz Zaborski <oshogbo@FreeBSD.org>.

xoxo bestie, this page's footer vibe is Debian, December 6, 2023, and CAP_PWD(3) — that's the whole receipt. 💖

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