Fix TOCTOU race conditions reported by CodeQL and Coverity

CodeQL and Coverity both complained about:

 * lib/libshare/os/linux/smb.c
 * tests/zfs-tests/cmd/mmapwrite.c
 	* twice
 * tests/zfs-tests/tests/functional/tmpfile/tmpfile_002_pos.c
 * tests/zfs-tests/tests/functional/tmpfile/tmpfile_stat_mode.c
	* coverity had a second complaint that CodeQL did not have
 * tests/zfs-tests/cmd/suid_write_to_file.c
	* Coverity had two complaints and CodeQL had one complaint, both
	  differed. The CodeQL complaint is about the main point of the
	  test, so it is not fixable without a hack involving `fork()`.

The issues reported by CodeQL are fixed, with the exception of the last
one, which is deemed to be a false positive that is too much trouble to
wrokaround. The issues reported by Coverity were only fixed if CodeQL
complained about them.

There were issues reported by Coverity in a number of other files that
were not reported by CodeQL, but fixing the CodeQL complaints is
considered a priority since we want to integrate it into a github
workflow, so the remaining Coverity complaints are left for future work.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14098

lib/libshare/os/linux/smb.c

@@ -90,21 +90,32 @@ smb_retrieve_shares(void)
 
 	/* Go through the directory, looking for shares */
 	while ((directory = readdir(shares_dir))) {
+		int fd;
+
 		if (directory->d_name[0] == '.')
 			continue;
 
 		snprintf(file_path, sizeof (file_path),
 		    "%s/%s", SHARE_DIR, directory->d_name);
 
-		if (stat(file_path, &eStat) == -1) {
+		if ((fd = open(file_path, O_RDONLY | O_CLOEXEC)) == -1) {
 			rc = SA_SYSTEM_ERR;
 			goto out;
 		}
 
-		if (!S_ISREG(eStat.st_mode))
-			continue;
+		if (stat(file_path, &eStat) == -1) {
+			close(fd);
+			rc = SA_SYSTEM_ERR;
+			goto out;
+		}
 
-		if ((share_file_fp = fopen(file_path, "re")) == NULL) {
+		if (!S_ISREG(eStat.st_mode)) {
+			close(fd);
+			continue;
+		}
+
+		if ((share_file_fp = fdopen(fd, "r")) == NULL) {
+			close(fd);
 			rc = SA_SYSTEM_ERR;
 			goto out;
 		}

tests/zfs-tests/cmd/mmapwrite.c

@@ -88,29 +88,21 @@ map_writer(void *filename)
 	int ret = 0;
 	char *buf = NULL;
 	int page_size = getpagesize();
-	int op_errno = 0;
 	char *file_path = filename;
 
 	while (1) {
-		ret = access(file_path, F_OK);
-		if (ret) {
-			op_errno = errno;
-			if (op_errno == ENOENT) {
+		fd = open(file_path, O_RDWR);
+		if (fd == -1) {
+			if (errno == ENOENT) {
 				fd = open(file_path, O_RDWR | O_CREAT, 0777);
 				if (fd == -1) {
 					err(1, "open file failed");
 				}
-
 				ret = ftruncate(fd, page_size);
 				if (ret == -1) {
 					err(1, "truncate file failed");
 				}
 			} else {
-				err(1, "access file failed!");
-			}
-		} else {
-			fd = open(file_path, O_RDWR, 0777);
-			if (fd == -1) {
 				err(1, "open file failed");
 			}
 		}

tests/zfs-tests/tests/functional/tmpfile/tmpfile_002_pos.c

@@ -46,7 +46,6 @@ main(void)
 	int i, fd;
 	char spath[1024], dpath[1024];
 	const char *penv[] = {"TESTDIR", "TESTFILE0"};
-	struct stat sbuf;
 
 	(void) fprintf(stdout, "Verify O_TMPFILE file can be linked.\n");
 
@@ -73,12 +72,10 @@ main(void)
 	run("export");
 	run("import");
 
-	if (stat(dpath, &sbuf) < 0) {
-		perror("stat");
-		unlink(dpath);
+	if (unlink(dpath) == -1) {
+		perror("unlink");
 		exit(5);
 	}
-	unlink(dpath);
 
 	return (0);
 }

tests/zfs-tests/tests/functional/tmpfile/tmpfile_stat_mode.c

@@ -48,7 +48,7 @@
 static void
 test_stat_mode(mode_t mask)
 {
-	struct stat st, fst;
+	struct stat fst;
 	int i, fd;
 	char spath[1024], dpath[1024];
 	const char *penv[] = {"TESTDIR", "TESTFILE0"};
@@ -68,7 +68,7 @@ test_stat_mode(mode_t mask)
 		err(2, "open(%s)", penv[0]);
 
 	if (fstat(fd, &fst) == -1)
-		err(3, "open");
+		err(3, "fstat(%s)", penv[0]);
 
 	snprintf(spath, sizeof (spath), "/proc/self/fd/%d", fd);
 	snprintf(dpath, sizeof (dpath), "%s/%s", penv[0], penv[1]);
@@ -78,19 +78,22 @@ test_stat_mode(mode_t mask)
 		err(4, "linkat");
 	close(fd);
 
-	if (stat(dpath, &st) == -1)
-		err(5, "stat");
-	unlink(dpath);
-
-	/* Verify fstat(2) result */
+	/* Verify fstat(2) result at old path */
 	mode = fst.st_mode & 0777;
 	if (mode != masked)
-		errx(6, "fstat(2) %o != %o\n", mode, masked);
+		errx(5, "fstat(2) %o != %o\n", mode, masked);
 
-	/* Verify stat(2) result */
-	mode = st.st_mode & 0777;
+	fd = open(dpath, O_RDWR);
+	if (fd == -1)
+		err(6, "open(%s)", dpath);
+
+	if (fstat(fd, &fst) == -1)
+		err(7, "fstat(%s)", dpath);
+
+	/* Verify fstat(2) result at new path */
+	mode = fst.st_mode & 0777;
 	if (mode != masked)
-		errx(7, "stat(2) %o != %o\n", mode, masked);
+		errx(8, "fstat(2) %o != %o\n", mode, masked);
 }
 
 int