NSS: Don't use printf(3) on user provided strings.

This also fixes several corner cases and crashers.

It's not prudent to pass user input to (even admin) input as a
format string to printf, and various distros now check for this.
This can cause accessing memory incorrectly, and various also
various libc abort()'s.

In addition various assumptions were made about full_name_format
that aren't necessarily the case if the user uses a more complex
format.

Use safe-printf.c implementation for formatting full_name_format.

Adapt the NSS resolver so it doesn't barf on formatted strings that
are shorter than expected given a full_name_format.

Tests added and updated appropriately.

6 files changed, 97 insertions, 209 deletions

diff --git a/src/responder/nss/nsssrv_cmd.c b/src/responder/nss/nsssrv_cmd.c
index 715313f89..0b02bf6e8 100644
--- a/src/responder/nss/nsssrv_cmd.c
+++ b/src/responder/nss/nsssrv_cmd.c
@@ -298,20 +298,14 @@ static int fill_pwent(struct sss_packet *packet,
     uint32_t uid;
     uint32_t gid;
     size_t rsize, rp, blen;
-    size_t dom_len = 0;
-    int delim = 0;
-    int i, ret, num, t;
+    int fq_len = 0;
+    int i, ret, num;
     bool add_domain = (!IS_SUBDOMAIN(dom) && dom->fqnames);
     const char *domain = dom->name;
     bool packet_initialized = false;
     int ncret;
     TALLOC_CTX *tmp_ctx = NULL;
 
-    if (add_domain) {
-        delim = 1;
-        dom_len = sss_fqdom_len(dom->names, dom);
-    }
-
     to_sized_string(&pwfield, nctx->pwfield);
 
     rp = 2*sizeof(uint32_t);
@@ -381,7 +375,17 @@ static int fill_pwent(struct sss_packet *packet,
 
         rsize = 2 * sizeof(uint32_t) + name.len + gecos.len +
                                        homedir.len + shell.len + pwfield.len;
-        if (add_domain) rsize += delim + dom_len;
+
+        if (add_domain) {
+            fq_len = sss_fqname(NULL, 0, dom->names, dom, name.str);
+            if (fq_len >= 0) {
+                fq_len += 1;
+                rsize -= name.len;
+                rsize += fq_len;
+            } else {
+                fq_len = 0;
+            }
+        }
 
         ret = sss_packet_grow(packet, rsize);
         if (ret != EOK) {
@@ -394,25 +398,8 @@ static int fill_pwent(struct sss_packet *packet,
         SAFEALIGN_SET_UINT32(&body[rp], gid, &rp);
 
         if (add_domain) {
-            ret = sss_fqname((char *) &body[rp], name.len + delim + dom_len,
-                             dom->names, dom, name.str);
-            if (ret >= (name.len + delim + dom_len)) {
-                /* need more space, got creative with the print format ? */
-                t = ret - (name.len + delim + dom_len) + 1;
-                ret = sss_packet_grow(packet, t);
-                if (ret != EOK) {
-                    num = 0;
-                    goto done;
-                }
-                delim += t;
-                sss_packet_get_body(packet, &body, &blen);
-
-                /* retry */
-                ret = sss_fqname((char *) &body[rp], name.len + delim + dom_len,
-                                 dom->names, dom, name.str);
-            }
-
-            if (ret != name.len + delim + dom_len - 1) {
+            ret = sss_fqname((char *) &body[rp], fq_len, dom->names, dom, name.str);
+            if (ret < 0 || ret != fq_len - 1) {
                 DEBUG(1, ("Failed to generate a fully qualified name for user "
                           "[%s] in [%s]! Skipping user.\n", name.str, domain));
                 continue;
@@ -2281,8 +2268,7 @@ static int fill_members(struct sss_packet *packet,
     struct sized_string name;
     TALLOC_CTX *tmp_ctx = NULL;
 
-    size_t delim = 0;
-    size_t dom_len = 0;
+    int nlen = 0;
 
     uint8_t *body;
     size_t blen;
@@ -2319,9 +2305,6 @@ static int fill_members(struct sss_packet *packet,
             }
         }
 
-        delim = 0;
-        dom_len = 0;
-
         ret = parse_member(tmp_ctx, dom, tmpstr, &member_dom, &name, &add_domain);
         if (ret != EOK) {
             DEBUG(SSSDBG_MINOR_FAILURE,
@@ -2330,44 +2313,33 @@ static int fill_members(struct sss_packet *packet,
         }
 
         if (add_domain) {
-            delim = 1;
-            dom_len = sss_fqdom_len(member_dom->names, member_dom);
+            nlen = sss_fqname(NULL, 0, dom->names, dom, name.str);
+            if (nlen >= 0) {
+                nlen += 1;
+            } else {
+                /* Other failures caught below */
+                nlen = 0;
+            }
+        } else {
+            nlen = name.len;
         }
 
-        ret = sss_packet_grow(packet, name.len + delim + dom_len);
+        ret = sss_packet_grow(packet, nlen);
         if (ret != EOK) {
             goto done;
         }
         sss_packet_get_body(packet, &body, &blen);
 
         if (add_domain) {
-            ret = sss_fqname((char *)&body[rzero + rsize],
-                             name.len + delim + dom_len,
+            ret = sss_fqname((char *)&body[rzero + rsize], nlen,
                              member_dom->names, member_dom, name.str);
-            if (ret >= (name.len + delim + dom_len)) {
-                /* need more space,
-                 * got creative with the print format ? */
-                int t = ret - (name.len + delim + dom_len) + 1;
-                ret = sss_packet_grow(packet, t);
-                if (ret != EOK) {
-                    goto done;
-                }
-                sss_packet_get_body(packet, &body, &blen);
-                delim += t;
-
-                /* retry */
-                ret = sss_fqname((char *)&body[rzero + rsize],
-                                name.len + delim + dom_len,
-                                member_dom->names, member_dom, name.str);
-            }
-
-            if (ret != name.len + delim + dom_len - 1) {
+            if (ret < 0 || ret != nlen - 1) {
                 DEBUG(SSSDBG_OP_FAILURE, ("Failed to generate a fully qualified name"
                                           " for member [%s@%s] of group [%s]!"
                                           " Skipping\n", name.str, domain,
                                           (char *)&body[rzero+STRS_ROFFSET]));
                 /* reclaim space */
-                ret = sss_packet_shrink(packet, name.len + delim + dom_len);
+                ret = sss_packet_shrink(packet, nlen);
                 if (ret != EOK) {
                     goto done;
                 }
@@ -2378,7 +2350,7 @@ static int fill_members(struct sss_packet *packet,
             memcpy(&body[rzero + rsize], name.str, name.len);
         }
 
-        rsize += name.len + delim + dom_len;
+        rsize += nlen;
         memnum++;
     }
 
@@ -2409,8 +2381,7 @@ static int fill_grent(struct sss_packet *packet,
     struct sized_string name;
     struct sized_string pwfield;
     struct sized_string fullname;
-    size_t delim = 0;
-    size_t dom_len = 0;
+    int fq_len = 0;
     int i = 0;
     int ret, num, memnum;
     size_t rzero, rsize;
@@ -2418,11 +2389,6 @@ static int fill_grent(struct sss_packet *packet,
     const char *domain = dom->name;
     TALLOC_CTX *tmp_ctx = NULL;
 
-    if (add_domain) {
-        delim = 1;
-        dom_len = sss_fqdom_len(dom->names, dom);
-    }
-
     to_sized_string(&pwfield, nctx->pwfield);
 
     num = 0;
@@ -2483,7 +2449,18 @@ static int fill_grent(struct sss_packet *packet,
 
         /* fill in gid and name and set pointer for number of members */
         rsize = STRS_ROFFSET + name.len + pwfield.len; /* name\0x\0 */
-        if (add_domain) rsize += delim + dom_len;
+
+        if (add_domain) {
+            fq_len = sss_fqname(NULL, 0, dom->names, dom, name.str);
+            if (fq_len >= 0) {
+                fq_len += 1;
+                rsize -= name.len;
+                rsize += fq_len;
+            } else {
+                /* Other failures caught below */
+                fq_len = 0;
+            }
+        }
 
         ret = sss_packet_grow(packet, rsize);
         if (ret != EOK) {
@@ -2500,28 +2477,9 @@ static int fill_grent(struct sss_packet *packet,
 
         /*  8-X: sequence of strings (name, passwd, mem..) */
         if (add_domain) {
-            ret = sss_fqname((char *)&body[rzero+STRS_ROFFSET],
-                             name.len + delim + dom_len,
+            ret = sss_fqname((char *)&body[rzero+STRS_ROFFSET], fq_len,
                              dom->names, dom, name.str);
-            if (ret >= (name.len + delim + dom_len)) {
-                /* need more space, got creative with the print format ? */
-                int t = ret - (name.len + delim + dom_len) + 1;
-                ret = sss_packet_grow(packet, t);
-                if (ret != EOK) {
-                    num = 0;
-                    goto done;
-                }
-                sss_packet_get_body(packet, &body, &blen);
-                rsize += t;
-                delim += t;
-
-                /* retry */
-                ret = sss_fqname((char *)&body[rzero+STRS_ROFFSET],
-                                 name.len + delim + dom_len,
-                                 dom->names, dom, name.str);
-            }
-
-            if (ret != name.len + delim + dom_len - 1) {
+            if (ret < 0 || ret != fq_len - 1) {
                 DEBUG(1, ("Failed to generate a fully qualified name for"
                           " group [%s] in [%s]! Skipping\n", name.str, domain));
                 /* reclaim space */
diff --git a/src/tests/cmocka/test_fqnames.c b/src/tests/cmocka/test_fqnames.c
index e0735083a..90f60c5be 100644
--- a/src/tests/cmocka/test_fqnames.c
+++ b/src/tests/cmocka/test_fqnames.c
@@ -76,7 +76,6 @@ void test_default(void **state)
     char *fqdn;
     const int fqdn_size = 255;
     char fqdn_s[fqdn_size];
-    size_t domsize;
 
     if (test_ctx == NULL) {
         DEBUG(SSSDBG_CRIT_FAILURE, ("Type mismatch\n"));
@@ -87,7 +86,6 @@ void test_default(void **state)
                                    "(?P<name>[^@]+)@?(?P<domain>[^@]*$)",
                                    "%1$s@%2$s", &test_ctx->nctx);
     assert_int_equal(ret, EOK);
-    assert_int_equal(test_ctx->nctx->fq_flags, FQ_FMT_NAME | FQ_FMT_DOMAIN);
 
     fqdn = sss_tc_fqname(test_ctx, test_ctx->nctx, test_ctx->dom, NAME);
     assert_non_null(fqdn);
@@ -98,9 +96,6 @@ void test_default(void **state)
     assert_int_equal(ret + 1, sizeof(NAME"@"DOMNAME));
     assert_string_equal(fqdn_s, NAME"@"DOMNAME);
 
-    domsize = sss_fqdom_len(test_ctx->nctx, test_ctx->dom);
-    assert_int_equal(domsize, sizeof(DOMNAME)-1);
-
     talloc_free(test_ctx->nctx);
 }
 
@@ -113,7 +108,6 @@ void test_all(void **state)
     char *fqdn;
     const int fqdn_size = 255;
     char fqdn_s[fqdn_size];
-    size_t domsize;
 
     if (test_ctx == NULL) {
         DEBUG(SSSDBG_CRIT_FAILURE, ("Type mismatch\n"));
@@ -124,8 +118,6 @@ void test_all(void **state)
                                    "(?P<name>[^@]+)@?(?P<domain>[^@]*$)",
                                    "%1$s@%2$s@%3$s", &test_ctx->nctx);
     assert_int_equal(ret, EOK);
-    assert_int_equal(test_ctx->nctx->fq_flags,
-                     FQ_FMT_NAME | FQ_FMT_DOMAIN | FQ_FMT_FLAT_NAME);
 
     fqdn = sss_tc_fqname(test_ctx, test_ctx->nctx, test_ctx->dom, NAME);
     assert_non_null(fqdn);
@@ -136,9 +128,6 @@ void test_all(void **state)
     assert_int_equal(ret + 1, sizeof(NAME"@"DOMNAME"@"FLATNAME));
     assert_string_equal(fqdn_s, NAME"@"DOMNAME"@"FLATNAME);
 
-    domsize = sss_fqdom_len(test_ctx->nctx, test_ctx->dom);
-    assert_int_equal(domsize, sizeof(DOMNAME)-1 + sizeof(FLATNAME)-1);
-
     talloc_free(test_ctx->nctx);
 }
 
@@ -151,7 +140,6 @@ void test_flat(void **state)
     char *fqdn;
     const int fqdn_size = 255;
     char fqdn_s[fqdn_size];
-    size_t domsize;
 
     if (test_ctx == NULL) {
         DEBUG(SSSDBG_CRIT_FAILURE, ("Type mismatch\n"));
@@ -162,7 +150,6 @@ void test_flat(void **state)
                                    "(?P<name>[^@]+)@?(?P<domain>[^@]*$)",
                                    "%1$s@%3$s", &test_ctx->nctx);
     assert_int_equal(ret, EOK);
-    assert_int_equal(test_ctx->nctx->fq_flags, FQ_FMT_NAME | FQ_FMT_FLAT_NAME);
 
     fqdn = sss_tc_fqname(test_ctx, test_ctx->nctx, test_ctx->dom, NAME);
     assert_non_null(fqdn);
@@ -173,9 +160,6 @@ void test_flat(void **state)
     assert_int_equal(ret + 1, sizeof(NAME"@"FLATNAME));
     assert_string_equal(fqdn_s, NAME"@"FLATNAME);
 
-    domsize = sss_fqdom_len(test_ctx->nctx, test_ctx->dom);
-    assert_int_equal(domsize, sizeof(FLATNAME)-1);
-
     talloc_free(test_ctx->nctx);
 }
 
@@ -188,7 +172,6 @@ void test_flat_fallback(void **state)
     char *fqdn;
     const int fqdn_size = 255;
     char fqdn_s[fqdn_size];
-    size_t domsize;
 
     if (test_ctx == NULL) {
         DEBUG(SSSDBG_CRIT_FAILURE, ("Type mismatch\n"));
@@ -199,7 +182,6 @@ void test_flat_fallback(void **state)
                                    "(?P<name>[^@]+)@?(?P<domain>[^@]*$)",
                                    "%1$s@%3$s", &test_ctx->nctx);
     assert_int_equal(ret, EOK);
-    assert_int_equal(test_ctx->nctx->fq_flags, FQ_FMT_NAME | FQ_FMT_FLAT_NAME);
 
     test_ctx->dom->flat_name = NULL;
 
@@ -215,9 +197,6 @@ void test_flat_fallback(void **state)
     assert_int_equal(ret + 1, sizeof(NAME"@"DOMNAME));
     assert_string_equal(fqdn_s, NAME"@"DOMNAME);
 
-    domsize = sss_fqdom_len(test_ctx->nctx, test_ctx->dom);
-    assert_int_equal(domsize, sizeof(DOMNAME)-1);
-
     talloc_free(test_ctx->nctx);
 }
 
diff --git a/src/tests/cmocka/test_nss_srv.c b/src/tests/cmocka/test_nss_srv.c
index d1a4d4bd8..4db108a79 100644
--- a/src/tests/cmocka/test_nss_srv.c
+++ b/src/tests/cmocka/test_nss_srv.c
@@ -540,8 +540,8 @@ void test_nss_getpwnam_fqdn(void **state)
  * Check that FQDN processing is able to handle arbitrarily sized
  * delimeter
  */
-static int test_nss_getpwnam_check_resize_fqdn(uint32_t status,
-                                               uint8_t *body, size_t blen)
+static int test_nss_getpwnam_check_fancy_fqdn(uint32_t status,
+                                              uint8_t *body, size_t blen)
 {
     struct passwd pwd;
     errno_t ret;
@@ -555,29 +555,28 @@ static int test_nss_getpwnam_check_resize_fqdn(uint32_t status,
 
     assert_int_equal(pwd.pw_uid, 125);
     assert_int_equal(pwd.pw_gid, 458);
-    assert_string_equal(pwd.pw_name, "testuser_fqdn_resize@@@@@"TEST_DOM_NAME);
+    assert_string_equal(pwd.pw_name, "testuser_fqdn_fancy@@@@@"TEST_DOM_NAME);
     assert_string_equal(pwd.pw_shell, "/bin/sh");
     return EOK;
 }
 
-void test_nss_getpwnam_fqdn_resize(void **state)
+void test_nss_getpwnam_fqdn_fancy(void **state)
 {
     errno_t ret;
 
     /* Prime the cache with a valid user */
     ret = sysdb_add_user(nss_test_ctx->tctx->dom,
-                         "testuser_fqdn_resize", 125, 458, "test user",
+                         "testuser_fqdn_fancy", 125, 458, "test user",
                          "/home/testuser", "/bin/sh", NULL,
                          NULL, 300, 0);
     assert_int_equal(ret, EOK);
 
-    mock_input_user_or_group("testuser_fqdn_resize@"TEST_DOM_NAME);
+    mock_input_user_or_group("testuser_fqdn_fancy@"TEST_DOM_NAME);
     will_return(__wrap_sss_packet_get_cmd, SSS_NSS_GETPWNAM);
     mock_fill_user();
-    will_return(__wrap_sss_packet_get_body, WRAP_CALL_REAL);
 
     /* Query for that user, call a callback when command finishes */
-    set_cmd_cb(test_nss_getpwnam_check_resize_fqdn);
+    set_cmd_cb(test_nss_getpwnam_check_fancy_fqdn);
     nss_test_ctx->cctx->rctx->domains[0].fqnames = true;
     ret = sss_cmd_execute(nss_test_ctx->cctx, SSS_NSS_GETPWNAM,
                           nss_test_ctx->nss_cmds);
@@ -1298,7 +1297,7 @@ void nss_subdom_test_setup(void **state)
     nss_test_ctx->subdom = subdomain;
 }
 
-void nss_fqdn_resize_test_setup(void **state)
+void nss_fqdn_fancy_test_setup(void **state)
 {
     struct sss_test_conf_param params[] = {
         { "enumerate", "false" },
@@ -1339,8 +1338,8 @@ int main(int argc, const char *argv[])
                                  nss_test_setup, nss_test_teardown),
         unit_test_setup_teardown(test_nss_getpwnam_fqdn,
                                  nss_fqdn_test_setup, nss_test_teardown),
-        unit_test_setup_teardown(test_nss_getpwnam_fqdn_resize,
-                                 nss_fqdn_resize_test_setup, nss_test_teardown),
+        unit_test_setup_teardown(test_nss_getpwnam_fqdn_fancy,
+                                 nss_fqdn_fancy_test_setup, nss_test_teardown),
         unit_test_setup_teardown(test_nss_getgrnam_no_members,
                                  nss_test_setup, nss_test_teardown),
         unit_test_setup_teardown(test_nss_getgrnam_members,
diff --git a/src/tests/cmocka/test_utils.c b/src/tests/cmocka/test_utils.c
index eeef9ee0c..13a67f756 100644
--- a/src/tests/cmocka/test_utils.c
+++ b/src/tests/cmocka/test_utils.c
@@ -531,8 +531,7 @@ void test_sss_names_init(void **state)
     assert_int_equal(ret, EOK);
     assert_non_null(names_ctx);
     assert_string_equal(names_ctx->re_pattern, GLOBAL_RE_EXPRESSION);
-    assert_string_equal(names_ctx->fq_fmt, GLOBAL_FULL_NAME_FORMAT"%3$s");
-    assert_int_equal(names_ctx->fq_flags, FQ_FMT_NAME|FQ_FMT_DOMAIN);
+    assert_string_equal(names_ctx->fq_fmt, GLOBAL_FULL_NAME_FORMAT);
 
     talloc_free(names_ctx);
 
@@ -541,8 +540,7 @@ void test_sss_names_init(void **state)
     assert_int_equal(ret, EOK);
     assert_non_null(names_ctx);
     assert_string_equal(names_ctx->re_pattern, DOMAIN_RE_EXPRESSION);
-    assert_string_equal(names_ctx->fq_fmt, DOMAIN_FULL_NAME_FORMAT"%2$s");
-    assert_int_equal(names_ctx->fq_flags, FQ_FMT_NAME|FQ_FMT_FLAT_NAME);
+    assert_string_equal(names_ctx->fq_fmt, DOMAIN_FULL_NAME_FORMAT);
 
     talloc_free(names_ctx);
 }
diff --git a/src/util/usertools.c b/src/util/usertools.c
index cd57d0c57..c77aa7ce0 100644
--- a/src/util/usertools.c
+++ b/src/util/usertools.c
@@ -26,6 +26,7 @@
 
 #include "confdb/confdb.h"
 #include "util/util.h"
+#include "util/safe-format-string.h"
 #include "responder/common/responder.h"
 
 #ifdef HAVE_LIBPCRE_LESSER_THAN_7
@@ -138,18 +139,7 @@ done:
 
 static errno_t sss_fqnames_init(struct sss_names_ctx *nctx, const char *fq_fmt)
 {
-    struct pattern_desc {
-        const char *pattern;
-        const char *desc;
-        int flag;
-    };
-
-    struct pattern_desc fqname_patterns[] = {
-        { "%1$s", "user name", FQ_FMT_NAME },
-        { "%2$s", "domain name", FQ_FMT_DOMAIN },
-        { "%3$s", "domain flat name", FQ_FMT_FLAT_NAME },
-        { NULL, NULL, 0 }
-    };
+    char *fq;
 
     nctx->fq_fmt = talloc_strdup(nctx, fq_fmt);
     if (nctx->fq_fmt == NULL) {
@@ -158,34 +148,20 @@ static errno_t sss_fqnames_init(struct sss_names_ctx *nctx, const char *fq_fmt)
 
     DEBUG(SSSDBG_CONF_SETTINGS, ("Using fq format [%s].\n", nctx->fq_fmt));
 
-    /* Fail if the name specifier is missing and warn if the domain
-     * specifier is missing
-     */
-    if (strstr(fq_fmt, fqname_patterns[0].pattern) == NULL) {
+    /* Fail if the name specifier is missing, or if the format is
+     * invalid */
+    fq = sss_tc_fqname2 (nctx, nctx, "unused.example.com", "unused", "the-test-user");
+    if (fq == NULL) {
+        DEBUG(SSSDBG_OP_FAILURE,
+              ("The fq format is invalid [%s]", nctx->fq_fmt));
+        return EINVAL;
+    } else if (strstr (fq, "the-test-user") == NULL) {
         DEBUG(SSSDBG_OP_FAILURE,
               ("Username pattern not found in [%s]\n", nctx->fq_fmt));
         return ENOENT;
     }
-    nctx->fq_flags = FQ_FMT_NAME;
-
-    for (int i = 1; fqname_patterns[i].pattern; i++) {
-        char *s;
-        s = strstr(fq_fmt, fqname_patterns[i].pattern);
-        if (s == NULL) {
-            /* Append the format specifier */
-            nctx->fq_fmt = talloc_strdup_append(nctx->fq_fmt,
-                                                fqname_patterns[i].pattern);
-            if (nctx->fq_fmt == NULL) {
-                return ENOMEM;
-            }
-            continue;
-        }
-
-        DEBUG(SSSDBG_CONF_SETTINGS,
-              ("Found the pattern for %s\n", fqname_patterns[i].desc));
-        nctx->fq_flags |= fqname_patterns[i].flag;
-    }
 
+    talloc_free (fq);
     return EOK;
 }
 
@@ -568,19 +544,13 @@ sss_get_cased_name_list(TALLOC_CTX *mem_ctx, const char * const *orig,
 }
 
 static inline const char *
-safe_fq_str(struct sss_names_ctx *nctx, uint8_t part, const char *str)
-{
-    return nctx->fq_flags & part ? str : "";
-}
-
-static inline const char *
-safe_flat_name(struct sss_names_ctx *nctx, struct sss_domain_info *domain)
+calc_flat_name(struct sss_domain_info *domain)
 {
     const char *s;
 
-    s = safe_fq_str(nctx, FQ_FMT_FLAT_NAME, domain->flat_name);
+    s = domain->flat_name;
     if (s == NULL) {
-        DEBUG(SSSDBG_CRIT_FAILURE, ("Flat name requested but domain has no"
+        DEBUG(SSSDBG_MINOR_FAILURE, ("Flat name requested but domain has no"
               "flat name set, falling back to domain name\n"));
         s = domain->name;
     }
@@ -588,28 +558,24 @@ safe_flat_name(struct sss_names_ctx *nctx, struct sss_domain_info *domain)
     return s;
 }
 
-static inline size_t
-fq_part_len(struct sss_names_ctx *nctx, struct sss_domain_info *dom,
-            uint8_t part, const char *str)
-{
-    const char *s = str;
-
-    if (part == FQ_FMT_FLAT_NAME) {
-        s = safe_flat_name(nctx, dom);
-    }
-    return nctx->fq_flags & part ? strlen(s) : 0;
-}
-
 char *
 sss_tc_fqname(TALLOC_CTX *mem_ctx, struct sss_names_ctx *nctx,
               struct sss_domain_info *domain, const char *name)
 {
     if (domain == NULL || nctx == NULL) return NULL;
 
-    return talloc_asprintf(mem_ctx, nctx->fq_fmt,
-                           safe_fq_str(nctx, FQ_FMT_NAME, name),
-                           safe_fq_str(nctx, FQ_FMT_DOMAIN, domain->name),
-                           safe_flat_name(nctx, domain));
+    return sss_tc_fqname2 (mem_ctx, nctx, domain->name,
+                           calc_flat_name (domain), name);
+}
+
+static void
+safe_talloc_callback (void *data,
+                      const char *piece,
+                      size_t len)
+{
+    char **output = data;
+    if (*output != NULL)
+        *output = talloc_strndup_append(*output, piece, len);
 }
 
 char *
@@ -617,12 +583,17 @@ sss_tc_fqname2(TALLOC_CTX *mem_ctx, struct sss_names_ctx *nctx,
                const char *domain_name, const char *flat_dom_name,
                const char *name)
 {
+    const char *args[] = { name, domain_name, flat_dom_name, NULL };
+    char *output;
+
     if (nctx == NULL) return NULL;
 
-    return talloc_asprintf(mem_ctx, nctx->fq_fmt,
-                           safe_fq_str(nctx, FQ_FMT_NAME, name),
-                           safe_fq_str(nctx, FQ_FMT_DOMAIN, domain_name),
-                           safe_fq_str(nctx, FQ_FMT_FLAT_NAME, flat_dom_name));
+    output = talloc_strdup(mem_ctx, "");
+    if (safe_format_string_cb(safe_talloc_callback, &output, nctx->fq_fmt, args, 3) < 0)
+        output = NULL;
+    else if (output == NULL)
+        errno = ENOMEM;
+    return output;
 }
 
 int
@@ -631,19 +602,8 @@ sss_fqname(char *str, size_t size, struct sss_names_ctx *nctx,
 {
     if (domain == NULL || nctx == NULL) return -EINVAL;
 
-    return snprintf(str, size, nctx->fq_fmt,
-                    safe_fq_str(nctx, FQ_FMT_NAME, name),
-                    safe_fq_str(nctx, FQ_FMT_DOMAIN, domain->name),
-                    safe_flat_name(nctx, domain));
-}
-
-size_t
-sss_fqdom_len(struct sss_names_ctx *nctx,
-              struct sss_domain_info *domain)
-{
-    size_t len = fq_part_len(nctx, domain, FQ_FMT_DOMAIN, domain->name);
-    len += fq_part_len(nctx, domain, FQ_FMT_FLAT_NAME, domain->flat_name);
-    return len;
+    return safe_format_string(str, size, nctx->fq_fmt,
+                              name, domain->name, calc_flat_name (domain), NULL);
 }
 
 char *
diff --git a/src/util/util.h b/src/util/util.h
index 101270571..4a9dbb75f 100644
--- a/src/util/util.h
+++ b/src/util/util.h
@@ -324,14 +324,9 @@ char *get_username_from_uid(TALLOC_CTX *mem_ctx, uid_t uid);
 
 char *get_uppercase_realm(TALLOC_CTX *memctx, const char *name);
 
-#define FQ_FMT_NAME         0x01
-#define FQ_FMT_DOMAIN       0x02
-#define FQ_FMT_FLAT_NAME    0x04
-
 struct sss_names_ctx {
     char *re_pattern;
     char *fq_fmt;
-    uint8_t fq_flags;
 
     pcre *re;
 };
@@ -384,15 +379,14 @@ sss_tc_fqname2(TALLOC_CTX *mem_ctx, struct sss_names_ctx *nctx,
 /* Return fully-qualified name formatted according to the fq_fmt. The buffer in "str" is
  * "size" bytes long. Returns the number of bytes written on success or a negative
  * value of failure.
+ *
+ * Pass a zero size to calculate the length that would be needed by the fully-qualified
+ * name.
  */
 int
 sss_fqname(char *str, size_t size, struct sss_names_ctx *nctx,
            struct sss_domain_info *domain, const char *name);
 
-size_t
-sss_fqdom_len(struct sss_names_ctx *nctx,
-              struct sss_domain_info *domain);
-
 /* Subdomains use fully qualified names in the cache while primary domains use
  * just the name. Return either of these for a specified domain or subdomain
  */