getmntinfo(3): Scale faster, and return sooner

getmntinfo(3) is designed around a relatively static or slow growing set of
current mounts.  It tried to detect a race with somewhat concurrent mount
and re-call getfsstat(2) in that case, looping indefinitely.  It also
allocated space for a single extra mount as slop.

In the case where the user has a large number of mounts and is adding them
at a rapid pace, it fell over.

This patch makes two functional changes:

1. Allocate even more slop.  Double whatever the last getfsstat(2) returned.

2. Abort and return some known results after looping a few times
   (arbitrarily, 3).  If the list is constantly changing, we can't guarantee
   we return a full result to the user at any point anyways.

While here, add very basic functional tests for getmntinfo(3) to the libc
suite.

PR:		221743
Submitted by:	Peter Eriksson <peter AT ifm.liu.se> (earlier version)
Sponsored by:	Dell EMC Isilon

lib/libc/gen/getmntinfo.c

@@ -38,6 +38,9 @@ __FBSDID("$FreeBSD$");
 #include <sys/mount.h>
 #include <stdlib.h>
 
+#define	MAX_TRIES	3
+#define	SCALING_FACTOR	2
+
 /*
  * Return information about mounted filesystems.
  */
@@ -47,20 +50,21 @@ getmntinfo(struct statfs **mntbufp, int mode)
 	static struct statfs *mntbuf;
 	static int mntsize;
 	static long bufsize;
+	unsigned tries = 0;
 
 	if (mntsize <= 0 && (mntsize = getfsstat(0, 0, MNT_NOWAIT)) < 0)
 		return (0);
 	if (bufsize > 0 && (mntsize = getfsstat(mntbuf, bufsize, mode)) < 0)
 		return (0);
-	while (bufsize <= mntsize * sizeof(struct statfs)) {
+	while (tries++ < MAX_TRIES && bufsize <= mntsize * sizeof(*mntbuf)) {
-		if (mntbuf)
+		bufsize = (mntsize * SCALING_FACTOR) * sizeof(*mntbuf);
-			free(mntbuf);
+		if ((mntbuf = reallocf(mntbuf, bufsize)) == NULL)
-		bufsize = (mntsize + 1) * sizeof(struct statfs);
-		if ((mntbuf = malloc(bufsize)) == NULL)
 			return (0);
 		if ((mntsize = getfsstat(mntbuf, bufsize, mode)) < 0)
 			return (0);
 	}
 	*mntbufp = mntbuf;
+	if (mntsize > (bufsize / sizeof(*mntbuf)))
+		return (bufsize / sizeof(*mntbuf));
 	return (mntsize);
 }

lib/libc/tests/gen/Makefile

@@ -8,6 +8,7 @@ ATF_TESTS_C+=		fmtmsg_test
 ATF_TESTS_C+=		fnmatch2_test
 ATF_TESTS_C+=		fpclassify2_test
 ATF_TESTS_C+=		ftw_test
+ATF_TESTS_C+=		getmntinfo_test
 ATF_TESTS_C+=		glob2_test
 ATF_TESTS_C+=		popen_test
 ATF_TESTS_C+=		posix_spawn_test

lib/libc/tests/gen/getmntinfo_test.c

@@ -0,0 +1,86 @@
+/*-
+ * Copyright (c) 2017 Conrad Meyer <cem@FreeBSD.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.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``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 OR CONTRIBUTORS 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.
+ */
+
+/*
+ * Limited test program for getmntinfo(3), a non-standard BSDism.
+ */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include <sys/param.h>
+#include <sys/mount.h>
+#include <sys/ucred.h>
+
+#include <errno.h>
+
+#include <atf-c.h>
+
+static void
+check_mntinfo(struct statfs *mntinfo, int n)
+{
+	int i;
+
+	for (i = 0; i < n; i++) {
+		ATF_REQUIRE_MSG(mntinfo[i].f_version == STATFS_VERSION, "%ju",
+		    (uintmax_t)mntinfo[i].f_version);
+		ATF_REQUIRE(mntinfo[i].f_namemax <= sizeof(mntinfo[0].f_mntonname));
+	}
+}
+
+ATF_TC_WITHOUT_HEAD(getmntinfo_test);
+ATF_TC_BODY(getmntinfo_test, tc)
+{
+	int nmnts;
+	struct statfs *mntinfo;
+
+	/* Test bogus mode */
+	nmnts = getmntinfo(&mntinfo, 199);
+	ATF_REQUIRE_MSG(nmnts == 0 && errno == EINVAL,
+	    "getmntinfo() succeeded; errno=%d", errno);
+
+	/* Valid modes */
+	nmnts = getmntinfo(&mntinfo, MNT_NOWAIT);
+	ATF_REQUIRE_MSG(nmnts != 0, "getmntinfo(MNT_NOWAIT) failed; errno=%d",
+	    errno);
+
+	check_mntinfo(mntinfo, nmnts);
+	memset(mntinfo, 0xdf, sizeof(*mntinfo) * nmnts);
+
+	nmnts = getmntinfo(&mntinfo, MNT_WAIT);
+	ATF_REQUIRE_MSG(nmnts != 0, "getmntinfo(MNT_WAIT) failed; errno=%d",
+	    errno);
+
+	check_mntinfo(mntinfo, nmnts);
+}
+
+ATF_TP_ADD_TCS(tp)
+{
+
+	ATF_TP_ADD_TC(tp, getmntinfo_test);
+
+	return (atf_no_error());
+}