Fix corner cases of getgroups and setgroups syscall decoders

* uid.c (print_groups): New function.
(SYS_FUNC(setgroups), SYS_FUNC(getgroups)): Use it.
Print first syscall argument using %u format.
* tests/getgroups.c: New file.
* tests/getgroups.test: New test.
* tests/getgroups32.c: New file.
* tests/getgroups32.test: New test.
* tests/setgroups.c: New file.
* tests/setgroups.test: New test.
* tests/setgroups32.c: New file.
* tests/setgroups32.test: New test.
* tests/.gitignore: Add getgroups, getgroups32, setgroups,
and setgroups32.
* tests/Makefile.am (check_PROGRAMS): Likewise.
(DECODER_TESTS): Add getgroups.test, getgroups32.test,
setgroups.test, and setgroups32.test.

11 files changed, 454 insertions, 89 deletions

diff --git a/tests/.gitignore b/tests/.gitignore
index d6621e053..af1de088e 100644
--- a/tests/.gitignore
+++ b/tests/.gitignore
@@ -59,6 +59,8 @@ ftruncate64
 getcwd
 getdents
 getdents64
+getgroups
+getgroups32
 getrandom
 getrusage
 getxxid
@@ -148,6 +150,8 @@ setfsuid
 setfsuid32
 setgid
 setgid32
+setgroups
+setgroups32
 sethostname
 setregid
 setregid32
diff --git a/tests/Makefile.am b/tests/Makefile.am
index f051fc96c..6234711e5 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -111,6 +111,8 @@ check_PROGRAMS = \
 	getcwd \
 	getdents \
 	getdents64 \
+	getgroups \
+	getgroups32 \
 	getrandom \
 	getrusage \
 	getxxid \
@@ -198,6 +200,8 @@ check_PROGRAMS = \
 	setfsuid32 \
 	setgid \
 	setgid32 \
+	setgroups \
+	setgroups32 \
 	sethostname \
 	setregid \
 	setregid32 \
@@ -340,6 +344,8 @@ DECODER_TESTS = \
 	getcwd.test \
 	getdents.test \
 	getdents64.test \
+	getgroups.test \
+	getgroups32.test \
 	getrandom.test \
 	getrusage.test \
 	getxxid.test \
@@ -421,6 +427,8 @@ DECODER_TESTS = \
 	setfsuid32.test \
 	setgid.test \
 	setgid32.test \
+	setgroups.test \
+	setgroups32.test \
 	sethostname.test \
 	setregid.test \
 	setregid32.test \
diff --git a/tests/getgroups.c b/tests/getgroups.c
new file mode 100644
index 000000000..e853b25c5
--- /dev/null
+++ b/tests/getgroups.c
@@ -0,0 +1,161 @@
+/*
+ * Check decoding of getgroups/getgroups32 syscalls.
+ *
+ * Copyright (c) 2016 Dmitry V. Levin <ldv@altlinux.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. The name of the author may not be used to endorse or promote products
+ *    derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifdef __NR_getgroups32
+
+# define SYSCALL_NR	__NR_getgroups32
+# define SYSCALL_NAME	"getgroups32"
+# define GID_TYPE	unsigned int
+
+#else
+
+# include "tests.h"
+# include <sys/syscall.h>
+
+# ifdef __NR_getgroups
+
+#  define SYSCALL_NR	__NR_getgroups
+#  define SYSCALL_NAME	"getgroups"
+#  if defined __NR_getgroups32 && __NR_getgroups != __NR_getgroups32
+#   define GID_TYPE	unsigned short
+#  else
+#   define GID_TYPE	unsigned int
+#  endif
+
+# endif
+
+#endif
+
+#ifdef GID_TYPE
+
+# include <errno.h>
+# include <stdio.h>
+# include <unistd.h>
+
+static const char *
+errno2str(void)
+{
+	switch (errno) {
+#define CASE(x) case x: return #x
+		CASE(EINVAL);
+		CASE(EFAULT);
+		default: perror_msg_and_fail(SYSCALL_NAME);
+	}
+}
+
+#define MAX_STRLEN 32
+static long ngroups;
+
+static void
+get_groups(const long size, GID_TYPE *const g)
+{
+	long i = syscall(SYSCALL_NR, size, g);
+	if (i != ngroups)
+		perror_msg_and_fail("%s(%#lx, %p)", SYSCALL_NAME, size, g);
+
+	printf("%s(%u, [", SYSCALL_NAME, (unsigned) size);
+	for (i = 0; i < ngroups; ++i) {
+		if (i)
+			printf(", ");
+		if (i >= MAX_STRLEN) {
+			printf("...");
+			break;
+		}
+		printf("%u", (unsigned int) g[i]);
+	}
+	printf("]) = %ld\n", ngroups);
+}
+
+int
+main(void)
+{
+	long rc;
+
+	/* check how the first argument is decoded */
+	ngroups = syscall(SYSCALL_NR, 0, 0);
+	printf("%s(0, NULL) = %ld\n", SYSCALL_NAME, ngroups);
+	if (ngroups < 0)
+		perror_msg_and_fail(SYSCALL_NAME);
+
+	rc = syscall(SYSCALL_NR, (long) 0xffffffff00000000ULL, 0);
+	printf("%s(0, NULL) = %ld\n", SYSCALL_NAME, rc);
+
+	rc = syscall(SYSCALL_NR, -1U, 0);
+	printf("%s(%u, NULL) = %ld %s (%m)\n",
+	       SYSCALL_NAME, -1U, rc, errno2str());
+
+	rc = syscall(SYSCALL_NR, -1L, 0);
+	printf("%s(%u, NULL) = %ld %s (%m)\n",
+	       SYSCALL_NAME, -1U, rc, errno2str());
+
+	const unsigned int ngroups_max = sysconf(_SC_NGROUPS_MAX);
+
+	rc = syscall(SYSCALL_NR, ngroups_max, 0);
+	if (rc < 0)
+		printf("%s(%u, NULL) = %ld %s (%m)\n",
+		       SYSCALL_NAME, ngroups_max, rc, errno2str());
+	else
+		printf("%s(%u, NULL) = %ld\n",
+		       SYSCALL_NAME, ngroups_max, rc);
+
+	rc = syscall(SYSCALL_NR, (long) 0xffffffff00000000ULL | ngroups_max, 0);
+	if (rc < 0)
+		printf("%s(%u, NULL) = %ld %s (%m)\n",
+		       SYSCALL_NAME, ngroups_max, rc, errno2str());
+	else
+		printf("%s(%u, NULL) = %ld\n",
+		       SYSCALL_NAME, ngroups_max, rc);
+
+	/* check how the second argument is decoded */
+	GID_TYPE *const g1 =
+		tail_alloc(ngroups ? sizeof(*g1) * ngroups : 1);
+	GID_TYPE *const g2 = tail_alloc(sizeof(*g2) * (ngroups + 1));
+	void *efault = g2 + ngroups + 1;
+	(void) tail_alloc(1);
+
+	get_groups(ngroups, g1);
+	get_groups(ngroups + 1, g1);
+	get_groups(ngroups + 1, g2);
+
+	if (ngroups) {
+		rc = syscall(SYSCALL_NR, ngroups, efault);
+		printf("%s(%u, %p) = %ld %s (%m)\n",
+		       SYSCALL_NAME, (unsigned) ngroups, efault,
+		       rc, errno2str());
+	}
+
+	puts("+++ exited with 0 +++");
+	return 0;
+}
+
+#else
+
+SKIP_MAIN_UNDEFINED("__NR_getgroups")
+
+#endif
diff --git a/tests/getgroups.test b/tests/getgroups.test
new file mode 100755
index 000000000..2eb3056d9
--- /dev/null
+++ b/tests/getgroups.test
@@ -0,0 +1,6 @@
+#!/bin/sh
+
+# Check getgroups syscall decoding.
+
+. "${srcdir=.}/init.sh"
+run_strace_match_diff -a17
diff --git a/tests/getgroups32.c b/tests/getgroups32.c
new file mode 100644
index 000000000..2576548b6
--- /dev/null
+++ b/tests/getgroups32.c
@@ -0,0 +1,12 @@
+#include "tests.h"
+#include <sys/syscall.h>
+
+#ifdef __NR_getgroups32
+
+# include "getgroups.c"
+
+#else
+
+SKIP_MAIN_UNDEFINED("__NR_getgroups32")
+
+#endif
diff --git a/tests/getgroups32.test b/tests/getgroups32.test
new file mode 100755
index 000000000..12a8bcc0d
--- /dev/null
+++ b/tests/getgroups32.test
@@ -0,0 +1,6 @@
+#!/bin/sh
+
+# Check getgroups32 syscall decoding.
+
+. "${srcdir=.}/init.sh"
+run_strace_match_diff -a19
diff --git a/tests/setgroups.c b/tests/setgroups.c
new file mode 100644
index 000000000..ca3584853
--- /dev/null
+++ b/tests/setgroups.c
@@ -0,0 +1,186 @@
+/*
+ * Check decoding of setgroups/setgroups32 syscalls.
+ *
+ * Copyright (c) 2016 Dmitry V. Levin <ldv@altlinux.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. The name of the author may not be used to endorse or promote products
+ *    derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifdef __NR_setgroups32
+
+# define SYSCALL_NR	__NR_setgroups32
+# define SYSCALL_NAME	"setgroups32"
+# define GID_TYPE	unsigned int
+
+#else
+
+# include "tests.h"
+# include <sys/syscall.h>
+
+# ifdef __NR_setgroups
+
+#  define SYSCALL_NR	__NR_setgroups
+#  define SYSCALL_NAME	"setgroups"
+#  if defined __NR_setgroups32 && __NR_setgroups != __NR_setgroups32
+#   define GID_TYPE	unsigned short
+#  else
+#   define GID_TYPE	unsigned int
+#  endif
+
+# endif
+
+#endif
+
+#ifdef GID_TYPE
+
+# include <errno.h>
+# include <stdio.h>
+# include <unistd.h>
+
+static const char *
+errno2str(void)
+{
+	switch (errno) {
+#define CASE(x) case x: return #x
+		CASE(EPERM);
+		CASE(EINVAL);
+		CASE(ENOMEM);
+		CASE(EFAULT);
+		default: perror_msg_and_fail(SYSCALL_NAME);
+	}
+}
+
+int
+main(void)
+{
+	/* check how the first argument is decoded */
+	if (syscall(SYSCALL_NR, 0, 0))
+		printf("%s(0, NULL) = -1 %s (%m)\n", SYSCALL_NAME, errno2str());
+	else
+		printf("%s(0, NULL) = 0\n", SYSCALL_NAME);
+
+	if (syscall(SYSCALL_NR, (long) 0xffffffff00000000ULL, 0))
+		printf("%s(0, NULL) = -1 %s (%m)\n",
+		       SYSCALL_NAME, errno2str());
+	else
+		printf("%s(0, NULL) = 0\n", SYSCALL_NAME);
+
+	syscall(SYSCALL_NR, 1, 0);
+	printf("%s(1, NULL) = -1 %s (%m)\n", SYSCALL_NAME, errno2str());
+
+	syscall(SYSCALL_NR, (long) 0xffffffff00000001ULL, 0);
+	printf("%s(1, NULL) = -1 %s (%m)\n", SYSCALL_NAME, errno2str());
+
+	syscall(SYSCALL_NR, -1U, 0);
+	printf("%s(%u, NULL) = -1 %s (%m)\n", SYSCALL_NAME, -1U, errno2str());
+
+	syscall(SYSCALL_NR, -1L, 0);
+	printf("%s(%u, NULL) = -1 %s (%m)\n", SYSCALL_NAME, -1U, errno2str());
+
+	/* check how the second argument is decoded */
+	const GID_TYPE *const g1 = tail_alloc(sizeof(*g1));
+	GID_TYPE *const g2 = tail_alloc(sizeof(*g2) * 2);
+	GID_TYPE *const g3 = tail_alloc(sizeof(*g3) * 3);
+	(void) tail_alloc(1);
+
+	if (syscall(SYSCALL_NR, 0, g1 + 1))
+		printf("%s(0, []) = -1 %s (%m)\n",
+		       SYSCALL_NAME, errno2str());
+	else
+		printf("%s(0, []) = 0\n", SYSCALL_NAME);
+
+	if (syscall(SYSCALL_NR, 1, g1))
+		printf("%s(1, [%u]) = -1 %s (%m)\n",
+		       SYSCALL_NAME, (unsigned) *g1, errno2str());
+	else
+		printf("%s(1, [%u]) = 0\n",
+		       SYSCALL_NAME, (unsigned) *g1);
+
+	syscall(SYSCALL_NR, 1, g1 + 1);
+	printf("%s(1, %p) = -1 %s (%m)\n",
+	       SYSCALL_NAME, g1 + 1, errno2str());
+
+	syscall(SYSCALL_NR, 1, -1L);
+	printf("%s(1, %#lx) = -1 %s (%m)\n", SYSCALL_NAME, -1L, errno2str());
+
+	syscall(SYSCALL_NR, 2, g1);
+	printf("%s(2, [%u, %p]) = -1 %s (%m)\n",
+	       SYSCALL_NAME, (unsigned) *g1, g1 + 1, errno2str());
+
+	g2[0] = -2;
+	g2[1] = -3;
+	if (syscall(SYSCALL_NR, 2, g2))
+		printf("%s(2, [%u, %u]) = -1 %s (%m)\n", SYSCALL_NAME,
+		       (unsigned) g2[0], (unsigned) g2[1], errno2str());
+	else
+		printf("%s(2, [%u, %u]) = 0\n", SYSCALL_NAME,
+		       (unsigned) g2[0], (unsigned) g2[1]);
+
+	syscall(SYSCALL_NR, 3, g2);
+	printf("%s(3, [%u, %u, %p]) = -1 %s (%m)\n", SYSCALL_NAME,
+	       (unsigned) g2[0], (unsigned) g2[1], g2 + 2, errno2str());
+
+	g3[0] = 0;
+	g3[1] = 1;
+	if (syscall(SYSCALL_NR, 3, g3))
+		printf("%s(3, [%u, %u, ...]) = -1 %s (%m)\n", SYSCALL_NAME,
+		       (unsigned) g3[0], (unsigned) g3[1], errno2str());
+	else
+		printf("%s(3, [%u, %u]) = 0\n", SYSCALL_NAME,
+		       (unsigned) g3[0], (unsigned) g3[1]);
+
+	syscall(SYSCALL_NR, 4, g3);
+	printf("%s(4, [%u, %u, ...]) = -1 %s (%m)\n", SYSCALL_NAME,
+	       (unsigned) g3[0], (unsigned) g3[1], errno2str());
+
+	long rc = sysconf(_SC_NGROUPS_MAX);
+	const unsigned ngroups_max = rc;
+
+	if ((unsigned long) rc == ngroups_max && (int) ngroups_max > 0) {
+		syscall(SYSCALL_NR, ngroups_max, g3);
+		printf("%s(%u, [%u, %u, ...]) = -1 %s (%m)\n", SYSCALL_NAME,
+		       ngroups_max, (unsigned) g3[0], (unsigned) g3[1],
+		       errno2str());
+
+		const unsigned long size =
+			(unsigned long) 0xffffffff00000000ULL | ngroups_max;
+		syscall(SYSCALL_NR, size, g3);
+		printf("%s(%u, [%u, %u, ...]) = -1 %s (%m)\n", SYSCALL_NAME,
+		       ngroups_max, (unsigned) g3[0], (unsigned) g3[1],
+		       errno2str());
+
+		syscall(SYSCALL_NR, ngroups_max + 1, g3);
+		printf("%s(%u, %p) = -1 %s (%m)\n", SYSCALL_NAME,
+		       ngroups_max + 1, g3, errno2str());
+	}
+
+	puts("+++ exited with 0 +++");
+	return 0;
+}
+
+#else
+
+SKIP_MAIN_UNDEFINED("__NR_setgroups")
+
+#endif
diff --git a/tests/setgroups.test b/tests/setgroups.test
new file mode 100755
index 000000000..0dcc8f756
--- /dev/null
+++ b/tests/setgroups.test
@@ -0,0 +1,6 @@
+#!/bin/sh
+
+# Check setgroups syscall decoding.
+
+. "${srcdir=.}/init.sh"
+run_strace_match_diff -s2 -a17
diff --git a/tests/setgroups32.c b/tests/setgroups32.c
new file mode 100644
index 000000000..bd0a1fdec
--- /dev/null
+++ b/tests/setgroups32.c
@@ -0,0 +1,12 @@
+#include "tests.h"
+#include <sys/syscall.h>
+
+#ifdef __NR_setgroups32
+
+# include "setgroups.c"
+
+#else
+
+SKIP_MAIN_UNDEFINED("__NR_setgroups32")
+
+#endif
diff --git a/tests/setgroups32.test b/tests/setgroups32.test
new file mode 100755
index 000000000..ee4750d1b
--- /dev/null
+++ b/tests/setgroups32.test
@@ -0,0 +1,6 @@
+#!/bin/sh
+
+# Check setgroups32 syscall decoding.
+
+. "${srcdir=.}/init.sh"
+run_strace_match_diff -s2 -a19
diff --git a/uid.c b/uid.c
index 7cf545fa2..6952e8e2d 100644
--- a/uid.c
+++ b/uid.c
@@ -3,7 +3,7 @@
  * Copyright (c) 1993 Branko Lankester <branko@hacktic.nl>
  * Copyright (c) 1993-1996 Rick Sladkey <jrs@world.std.com>
  * Copyright (c) 1996-1999 Wichert Akkerman <wichert@cistron.nl>
- * Copyright (c) 2003-2015 Dmitry V. Levin <ldv@altlinux.org>
+ * Copyright (c) 2003-2016 Dmitry V. Levin <ldv@altlinux.org>
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -153,111 +153,69 @@ printuid(const char *text, const unsigned int uid)
 		tprintf("%s%u", text, (uid_t) uid);
 }
 
-SYS_FUNC(setgroups)
+static void
+print_groups(struct tcb *tcp, const unsigned int len, const unsigned long addr)
 {
-	unsigned long cur, abbrev_end;
-	uid_t gid;
-	int failed = 0;
-	const unsigned long len = tcp->u_arg[0];
-	const unsigned long start = tcp->u_arg[1];
-	const unsigned long size = len * sizeof(gid);
-	const unsigned long end = start + size;
-
-	tprintf("%lu, ", len);
+	static unsigned long ngroups_max;
+	if (!ngroups_max)
+		ngroups_max = sysconf(_SC_NGROUPS_MAX);
+
+	const unsigned int size = len * sizeof(uid_t);
+	const unsigned long end = addr + size;
+	if (!addr || !verbose(tcp) || size / sizeof(uid_t) != len
+	    || len > ngroups_max || end < addr) {
+		printaddr(addr);
+		return;
+	}
+
 	if (len == 0) {
 		tprints("[]");
-		return RVAL_DECODED;
-	}
-	if (!start || !verbose(tcp) ||
-	    size / sizeof(gid) != len || end < start) {
-		printaddr(start);
-		return RVAL_DECODED;
+		return;
 	}
-	if (abbrev(tcp)) {
-		abbrev_end = start + max_strlen * sizeof(gid);
-		if (abbrev_end < start)
-			abbrev_end = end;
-	} else {
-		abbrev_end = end;
-	}
-	tprints("[");
-	for (cur = start; cur < end; cur += sizeof(gid)) {
-		if (cur > start)
+
+	const unsigned long abbrev_end =
+		(abbrev(tcp) && max_strlen < len) ?
+			addr + max_strlen * sizeof(uid_t) : end;
+
+	unsigned long cur;
+	for (cur = addr; cur < end; cur += sizeof(uid_t)) {
+		if (cur != addr)
 			tprints(", ");
+
+		uid_t gid;
+		if (umove_or_printaddr(tcp, cur, &gid))
+			break;
+
+		if (cur == addr)
+			tprints("[");
+
 		if (cur >= abbrev_end) {
 			tprints("...");
+			cur = end;
 			break;
 		}
-		if (umoven(tcp, cur, sizeof(gid), &gid) < 0) {
-			tprints("?");
-			failed = 1;
-			break;
-		}
+
 		tprintf("%u", (unsigned int) gid);
 	}
-	tprints("]");
-	if (failed) {
-		tprints(" ");
-		printaddr(start);
-	}
+	if (cur != addr)
+		tprints("]");
+}
+
+SYS_FUNC(setgroups)
+{
+	const unsigned int len = tcp->u_arg[0];
 
+	tprintf("%u, ", len);
+	print_groups(tcp, len, tcp->u_arg[1]);
 	return RVAL_DECODED;
 }
 
 SYS_FUNC(getgroups)
 {
-	if (entering(tcp)) {
-		tprintf("%lu, ", tcp->u_arg[0]);
-	} else {
-		unsigned long cur, abbrev_end;
-		uid_t gid;
-		int failed = 0;
-		const unsigned long len = tcp->u_rval;
-		const unsigned long size = len * sizeof(gid);
-		const unsigned long start = tcp->u_arg[1];
-		const unsigned long end = start + size;
-
-		if (!start) {
-			printaddr(start);
-			return 0;
-		}
-		if (len == 0) {
-			tprints("[]");
-			return 0;
-		}
-		if (!verbose(tcp) || syserror(tcp) ||
-		    size / sizeof(gid) != len || end < start) {
-			printaddr(start);
-			return 0;
-		}
-		if (abbrev(tcp)) {
-			abbrev_end = start + max_strlen * sizeof(gid);
-			if (abbrev_end < start)
-				abbrev_end = end;
-		} else {
-			abbrev_end = end;
-		}
-		tprints("[");
-		for (cur = start; cur < end; cur += sizeof(gid)) {
-			if (cur > start)
-				tprints(", ");
-			if (cur >= abbrev_end) {
-				tprints("...");
-				break;
-			}
-			if (umoven(tcp, cur, sizeof(gid), &gid) < 0) {
-				tprints("?");
-				failed = 1;
-				break;
-			}
-			tprintf("%u", (unsigned int) gid);
-		}
-		tprints("]");
-		if (failed) {
-			tprints(" ");
-			printaddr(start);
-		}
-	}
+	if (entering(tcp))
+		tprintf("%u, ", (unsigned int) tcp->u_arg[0]);
+	else
+		print_groups(tcp, tcp->u_rval, tcp->u_arg[1]);
 	return 0;
 }