support alternate backends for the passwd and group dbs

when we fail to find the entry in the commonly accepted files,  we
query a server over a Unix domain socket on /var/run/nscd/socket.
the protocol used here is compatible with glibc's nscd protocol on
most systems (all that use 32-bit numbers for all the protocol fields,
which appears to be everything but Alpha).

src/passwd/getgr_a.c

 #include <pthread.h>
+#include <byteswap.h>
+#include <string.h>
+#include <unistd.h>
 #include "pwf.h"
+#include "nscd.h"
+
+static char *itoa(char *p, uint32_t x)
+{
+	// number of digits in a uint32_t + NUL
+	p += 11;
+	*--p = 0;
+	do {
+		*--p = '0' + x % 10;
+		x /= 10;
+	} while (x);
+	return p;
+}
 
 int __getgr_a(const char *name, gid_t gid, struct group *gr, char **buf, size_t *size, char ***mem, size_t *nmem, struct group **res)
 {
@@ -10,7 +26,6 @@ int __getgr_a(const char *name, gid_t gid, struct group *gr, char **buf, size_t
 	*res = 0;
 
 	pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
-
 	f = fopen("/etc/group", "rbe");
 	if (!f) {
 		rv = errno;
@@ -25,6 +40,129 @@ int __getgr_a(const char *name, gid_t gid, struct group *gr, char **buf, size_t
 	}
 	fclose(f);
 
+	if (!*res && (rv == 0 || rv == ENOENT || rv == ENOTDIR)) {
+		int32_t req = name ? GETGRBYNAME : GETGRBYGID;
+		int32_t i;
+		const char *key;
+		int32_t groupbuf[GR_LEN] = {0};
+		size_t len = 0;
+		size_t grlist_len = 0;
+		char gidbuf[11] = {0};
+		int swap = 0;
+		char *ptr;
+
+		if (name) {
+			key = name;
+		} else {
+			if (gid < 0 || gid > UINT32_MAX) {
+				rv = 0;
+				goto done;
+			}
+			key = itoa(gidbuf, gid);
+		}
+
+		f = __nscd_query(req, key, groupbuf, sizeof groupbuf, &swap);
+		if (!f) { rv = errno; goto done; }
+		if (f == (FILE*)-1) { rv = 0; goto done; }
+
+		if (!groupbuf[GRFOUND]) { rv = 0; goto cleanup_f; }
+
+		if (!groupbuf[GRNAMELEN] || !groupbuf[GRPASSWDLEN]) {
+			rv = EIO;
+			goto cleanup_f;
+		}
+
+		if (groupbuf[GRNAMELEN] > SIZE_MAX - groupbuf[GRPASSWDLEN]) {
+			rv = ENOMEM;
+			goto cleanup_f;
+		}
+		len = groupbuf[GRNAMELEN] + groupbuf[GRPASSWDLEN];
+
+		for (i = 0; i < groupbuf[GRMEMCNT]; i++) {
+			uint32_t name_len;
+			if (fread(&name_len, sizeof name_len, 1, f) < 1) {
+				rv = ferror(f) ? errno : EIO;
+				goto cleanup_f;
+			}
+			if (swap) {
+				name_len = bswap_32(name_len);
+			}
+			if (name_len > SIZE_MAX - grlist_len
+			|| name_len > SIZE_MAX - len) {
+				rv = ENOMEM;
+				goto cleanup_f;
+			}
+			len += name_len;
+			grlist_len += name_len;
+		}
+
+		if (len > *size || !*buf) {
+			char *tmp = realloc(*buf, len);
+			if (!tmp) {
+				rv = errno;
+				goto cleanup_f;
+			}
+			*buf = tmp;
+			*size = len;
+		}
+
+		if (!fread(*buf, len, 1, f)) {
+			rv = ferror(f) ? errno : EIO;
+			goto cleanup_f;
+		}
+
+		if (groupbuf[GRMEMCNT] + 1 > *nmem) {
+			if (groupbuf[GRMEMCNT] + 1 > SIZE_MAX/sizeof(char*)) {
+				rv = ENOMEM;
+				goto cleanup_f;
+			}
+			char **tmp = realloc(*mem, (groupbuf[GRMEMCNT]+1)*sizeof(char*));
+			if (!tmp) {
+				rv = errno;
+				goto cleanup_f;
+			}
+			*mem = tmp;
+			*nmem = groupbuf[GRMEMCNT] + 1;
+		}
+
+		if (groupbuf[GRMEMCNT]) {
+			mem[0][0] = *buf + groupbuf[GRNAMELEN] + groupbuf[GRPASSWDLEN];
+			for (ptr = mem[0][0], i = 0; ptr != mem[0][0]+grlist_len; ptr++)
+				if (!*ptr) mem[0][++i] = ptr+1;
+			mem[0][i] = 0;
+
+			if (i != groupbuf[GRMEMCNT]) {
+				rv = EIO;
+				goto cleanup_f;
+			}
+		} else {
+			mem[0][0] = 0;
+		}
+
+		gr->gr_name = *buf;
+		gr->gr_passwd = gr->gr_name + groupbuf[GRNAMELEN];
+		gr->gr_gid = groupbuf[GRGID];
+		gr->gr_mem = *mem;
+
+		if (gr->gr_passwd[-1]
+		|| gr->gr_passwd[groupbuf[GRPASSWDLEN]-1]) {
+			rv = EIO;
+			goto cleanup_f;
+		}
+
+		if (name && strcmp(name, gr->gr_name)
+		|| !name && gid != gr->gr_gid) {
+			rv = EIO;
+			goto cleanup_f;
+		}
+
+		*res = gr;
+
+cleanup_f:
+		fclose(f);
+		goto done;
+	}
+
 done:
 	pthread_setcancelstate(cs, 0);
 	if (rv) errno = rv;

src/passwd/getpw_a.c

-#include "pwf.h"
 #include <pthread.h>
+#include <byteswap.h>
+#include <string.h>
+#include <unistd.h>
+#include "pwf.h"
+#include "nscd.h"
+
+static char *itoa(char *p, uint32_t x)
+{
+	// number of digits in a uint32_t + NUL
+	p += 11;
+	*--p = 0;
+	do {
+		*--p = '0' + x % 10;
+		x /= 10;
+	} while (x);
+	return p;
+}
 
 int __getpw_a(const char *name, uid_t uid, struct passwd *pw, char **buf, size_t *size, struct passwd **res)
 {
@@ -24,6 +40,102 @@ int __getpw_a(const char *name, uid_t uid, struct passwd *pw, char **buf, size_t
 	}
 	fclose(f);
 
+	if (!*res && (rv == 0 || rv == ENOENT || rv == ENOTDIR)) {
+		int32_t req = name ? GETPWBYNAME : GETPWBYUID;
+		const char *key;
+		int32_t passwdbuf[PW_LEN] = {0};
+		size_t len = 0;
+		char uidbuf[11] = {0};
+
+		if (name) {
+			key = name;
+		} else {
+			/* uid outside of this range can't be queried with the
+			 * nscd interface, but might happen if uid_t ever
+			 * happens to be a larger type (this is not true as of
+			 * now)
+			 */
+			if(uid < 0 || uid > UINT32_MAX) {
+				rv = 0;
+				goto done;
+			}
+			key = itoa(uidbuf, uid);
+		}
+
+		f = __nscd_query(req, key, passwdbuf, sizeof passwdbuf, (int[]){0});
+		if (!f) { rv = errno; goto done; }
+		if (f == (FILE*)-1) { rv = 0; goto done; }
+
+		if(!passwdbuf[PWFOUND]) { rv = 0; goto cleanup_f; }
+
+		/* A zero length response from nscd is invalid. We ignore
+		 * invalid responses and just report an error, rather than
+		 * trying to do something with them.
+		 */
+		if (!passwdbuf[PWNAMELEN] || !passwdbuf[PWPASSWDLEN]
+		|| !passwdbuf[PWGECOSLEN] || !passwdbuf[PWDIRLEN]
+		|| !passwdbuf[PWSHELLLEN]) {
+			rv = EIO;
+			goto cleanup_f;
+		}
+
+		if ((passwdbuf[PWNAMELEN]|passwdbuf[PWPASSWDLEN]
+		     |passwdbuf[PWGECOSLEN]|passwdbuf[PWDIRLEN]
+		     |passwdbuf[PWSHELLLEN]) >= SIZE_MAX/8) {
+			rv = ENOMEM;
+			goto cleanup_f;
+		}
+
+		len = passwdbuf[PWNAMELEN] + passwdbuf[PWPASSWDLEN]
+		    + passwdbuf[PWGECOSLEN] + passwdbuf[PWDIRLEN]
+		    + passwdbuf[PWSHELLLEN];
+
+		if (len > *size || !*buf) {
+			char *tmp = realloc(*buf, len);
+			if (!tmp) {
+				rv = errno;
+				goto cleanup_f;
+			}
+			*buf = tmp;
+			*size = len;
+		}
+
+		if (!fread(*buf, len, 1, f)) {
+			rv = ferror(f) ? errno : EIO;
+			goto cleanup_f;
+		}
+
+		pw->pw_name = *buf;
+		pw->pw_passwd = pw->pw_name + passwdbuf[PWNAMELEN];
+		pw->pw_gecos = pw->pw_passwd + passwdbuf[PWPASSWDLEN];
+		pw->pw_dir = pw->pw_gecos + passwdbuf[PWGECOSLEN];
+		pw->pw_shell = pw->pw_dir + passwdbuf[PWDIRLEN];
+		pw->pw_uid = passwdbuf[PWUID];
+		pw->pw_gid = passwdbuf[PWGID];
+
+		/* Don't assume that nscd made sure to null terminate strings.
+		 * It's supposed to, but malicious nscd should be ignored
+		 * rather than causing a crash.
+		 */
+		if (pw->pw_passwd[-1] || pw->pw_gecos[-1] || pw->pw_dir[-1]
+		|| pw->pw_shell[passwdbuf[PWSHELLLEN]-1]) {
+			rv = EIO;
+			goto cleanup_f;
+		}
+
+		if (name && strcmp(name, pw->pw_name)
+		|| !name && uid != pw->pw_uid) {
+			rv = EIO;
+			goto cleanup_f;
+		}
+
+
+		*res = pw;
+cleanup_f:
+		fclose(f);
+		goto done;
+	}
+
 done:
 	pthread_setcancelstate(cs, 0);
 	if (rv) errno = rv;

src/passwd/nscd.h

+#ifndef NSCD_H
+#define NSCD_H
+
+#include <stdint.h>
+
+#define NSCDVERSION 2
+#define GETPWBYNAME 0
+#define GETPWBYUID 1
+#define GETGRBYNAME 2
+#define GETGRBYGID 3
+
+#define REQVERSION 0
+#define REQTYPE 1
+#define REQKEYLEN 2
+#define REQ_LEN 3
+
+#define PWVERSION 0
+#define PWFOUND 1
+#define PWNAMELEN 2
+#define PWPASSWDLEN 3
+#define PWUID 4
+#define PWGID 5
+#define PWGECOSLEN 6
+#define PWDIRLEN 7
+#define PWSHELLLEN 8
+#define PW_LEN 9
+
+#define GRVERSION 0
+#define GRFOUND 1
+#define GRNAMELEN 2
+#define GRPASSWDLEN 3
+#define GRGID 4
+#define GRMEMCNT 5
+#define GR_LEN 6
+
+FILE *__nscd_query(int32_t req, const char *key, int32_t *buf, size_t len, int *swap);
+
+#endif

src/passwd/nscd_query.c

+#include <sys/socket.h>
+#include <byteswap.h>
+#include <unistd.h>
+#include <stdio.h>
+#include <string.h>
+#include <errno.h>
+#include "nscd.h"
+
+static const struct {
+	short sun_family;
+	char sun_path[21];
+} addr = {
+	AF_UNIX,
+	"/var/run/nscd/socket"
+};
+
+FILE *__nscd_query(int32_t req, const char *key, int32_t *buf, size_t len, int *swap)
+{
+	size_t i;
+	int fd;
+	FILE *f = 0;
+	int32_t req_buf[REQ_LEN] = {
+		NSCDVERSION,
+		req,
+		strlen(key)+1
+	};
+	struct msghdr msg = {
+		.msg_iov = (struct iovec[]){
+			{&req_buf, sizeof(req_buf)},
+			{(char*)key, strlen(key)+1}
+		},
+		.msg_iovlen = 2
+	};
+
+	if (strlen(key) > INT32_MAX - 1) {
+		return (FILE*)-1;
+	}
+
+	*swap = 0;
+retry:
+
+	fd = socket(PF_UNIX, SOCK_STREAM | SOCK_CLOEXEC, 0);
+	if (fd < 0) return NULL;
+
+	if (connect(fd, (struct sockaddr*)&addr, sizeof(addr)) < 0) {
+		/* If there isn't a running nscd we return -1 to indicate that
+		 * that is precisely what happened
+		 */
+		if (errno == EACCES || errno == ECONNREFUSED || errno == ENOENT) {
+			close(fd);
+			return (FILE *)-1;
+		}
+		goto error;
+	}
+
+	if (sendmsg(fd, &msg, MSG_NOSIGNAL) < 0)
+		goto error;
+
+	if(!(f = fdopen(fd, "r"))) goto error;
+
+	if (!fread(buf, len, 1, f)) {
+		/* If the VERSION entry mismatches nscd will disconnect. The
+		 * most likely cause is that the endianness mismatched. So, we
+		 * byteswap and try once more. (if we already swapped, just
+		 * fail out)
+		 */
+		if (ferror(f)) goto error;
+		if (!*swap) {
+			fclose(f);
+			for (i = 0; i < sizeof(req_buf)/sizeof(req_buf[0]); i++) {
+				req_buf[i] = bswap_32(req_buf[i]);
+			}
+			*swap = 1;
+			goto retry;
+		} else {
+			errno = EIO;
+			goto error;
+		}
+	}
+
+	if (*swap) {
+		for (i = 0; i < len/sizeof(buf[0]); i++) {
+			buf[i] = bswap_32(buf[i]);
+		}
+	}
+
+	/* The first entry in every nscd response is the version number. This
+	 * really shouldn't happen, and is evidence of some form of malformed
+	 * response.
+	 */
+	if(buf[0] != NSCDVERSION) {
+		errno = EIO;
+		goto error;
+	}
+
+	return f;
+error:
+	if (f) fclose(f); else close(fd);
+	return 0;
+}