uml: Simplify tempdir logic.

Inferring the mount hierarchy correctly from /proc/mounts is hard when MS_MOVE
may have been used, and the previous code did it wrongly. This change simplifies
the logic to only require that /dev/shm be _on_ tmpfs (which can be checked
trivially with statfs) rather than that it be a _mountpoint_ of tmpfs, since
there isn't a compelling reason to be that strict. We also now check for tmpfs
on whatever directory we ultimately use so that the user is better informed.

This change also moves the more standard TMPDIR environment variable check ahead
of the others.

Applies to 3.12.
Signed-off-by: Tristan Schmelcher <tschmelcher@google.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/os-Linux/mem.c

@@ -12,337 +12,117 @@
 #include <string.h>
 #include <sys/stat.h>
 #include <sys/mman.h>
-#include <sys/param.h>
+#include <sys/vfs.h>
+#include <linux/magic.h>
 #include <init.h>
 #include <os.h>
 
-/* Modified by which_tmpdir, which is called during early boot */
-static char *default_tmpdir = "/tmp";
-
-/*
- *  Modified when creating the physical memory file and when checking
- * the tmp filesystem for usability, both happening during early boot.
- */
+/* Set by make_tempfile() during early boot. */
 static char *tempdir = NULL;
 
-static void __init find_tempdir(void)
+/* Check if dir is on tmpfs. Return 0 if yes, -1 if no or error. */
+static int __init check_tmpfs(const char *dir)
 {
-	const char *dirs[] = { "TMP", "TEMP", "TMPDIR", NULL };
-	int i;
-	char *dir = NULL;
-
-	if (tempdir != NULL)
-		/* We've already been called */
-		return;
-	for (i = 0; dirs[i]; i++) {
-		dir = getenv(dirs[i]);
-		if ((dir != NULL) && (*dir != '\0'))
-			break;
-	}
-	if ((dir == NULL) || (*dir == '\0'))
-		dir = default_tmpdir;
+	struct statfs st;
 
-	tempdir = malloc(strlen(dir) + 2);
-	if (tempdir == NULL) {
-		fprintf(stderr, "Failed to malloc tempdir, "
-			"errno = %d\n", errno);
-		return;
-	}
-	strcpy(tempdir, dir);
-	strcat(tempdir, "/");
-}
-
-/*
- * Remove bytes from the front of the buffer and refill it so that if there's a
- * partial string that we care about, it will be completed, and we can recognize
- * it.
- */
-static int pop(int fd, char *buf, size_t size, size_t npop)
-{
-	ssize_t n;
-	size_t len = strlen(&buf[npop]);
-
-	memmove(buf, &buf[npop], len + 1);
-	n = read(fd, &buf[len], size - len - 1);
-	if (n < 0)
-		return -errno;
-
-	buf[len + n] = '\0';
-	return 1;
-}
-
-/*
- * This will return 1, with the first character in buf being the
- * character following the next instance of c in the file.  This will
- * read the file as needed.  If there's an error, -errno is returned;
- * if the end of the file is reached, 0 is returned.
- */
-static int next(int fd, char *buf, size_t size, char c)
-{
-	ssize_t n;
-	char *ptr;
-
-	while ((ptr = strchr(buf, c)) == NULL) {
-		n = read(fd, buf, size - 1);
-		if (n == 0)
-			return 0;
-		else if (n < 0)
-			return -errno;
-
-		buf[n] = '\0';
+	printf("Checking if %s is on tmpfs...", dir);
+	if (statfs(dir, &st) < 0) {
+		printf("%s\n", strerror(errno));
+	} else if (st.f_type != TMPFS_MAGIC) {
+		printf("no\n");
+	} else {
+		printf("OK\n");
+		return 0;
 	}
-
-	return pop(fd, buf, size, ptr - buf + 1);
+	return -1;
 }
 
 /*
- * Decode an octal-escaped and space-terminated path of the form used by
- * /proc/mounts. May be used to decode a path in-place. "out" must be at least
- * as large as the input. The output is always null-terminated. "len" gets the
- * length of the output, excluding the trailing null. Returns 0 if a full path
- * was successfully decoded, otherwise an error.
+ * Choose the tempdir to use. We want something on tmpfs so that our memory is
+ * not subject to the host's vm.dirty_ratio. If a tempdir is specified in the
+ * environment, we use that even if it's not on tmpfs, but we warn the user.
+ * Otherwise, we try common tmpfs locations, and if no tmpfs directory is found
+ * then we fall back to /tmp.
  */
-static int decode_path(const char *in, char *out, size_t *len)
+static char * __init choose_tempdir(void)
 {
-	char *first = out;
-	int c;
+	static const char * const vars[] = {
+		"TMPDIR",
+		"TMP",
+		"TEMP",
+		NULL
+	};
+	static const char fallback_dir[] = "/tmp";
+	static const char * const tmpfs_dirs[] = {
+		"/dev/shm",
+		fallback_dir,
+		NULL
+	};
 	int i;
-	int ret = -EINVAL;
-	while (1) {
-		switch (*in) {
-		case '\0':
-			goto out;
-
-		case ' ':
-			ret = 0;
-			goto out;
-
-		case '\\':
-			in++;
-			c = 0;
-			for (i = 0; i < 3; i++) {
-				if (*in < '0' || *in > '7')
-					goto out;
-				c = (c << 3) | (*in++ - '0');
-			}
-			*(unsigned char *)out++ = (unsigned char) c;
-			break;
-
-		default:
-			*out++ = *in++;
-			break;
+	const char *dir;
+
+	printf("Checking environment variables for a tempdir...");
+	for (i = 0; vars[i]; i++) {
+		dir = getenv(vars[i]);
+		if ((dir != NULL) && (*dir != '\0')) {
+			printf("%s\n", dir);
+			if (check_tmpfs(dir) >= 0)
+				goto done;
+			else
+				goto warn;
 		}
 	}
+	printf("none found\n");
 
-out:
-	*out = '\0';
-	*len = out - first;
-	return ret;
-}
-
-/*
- * Computes the length of s when encoded with three-digit octal escape sequences
- * for the characters in chars.
- */
-static size_t octal_encoded_length(const char *s, const char *chars)
-{
-	size_t len = strlen(s);
-	while ((s = strpbrk(s, chars)) != NULL) {
-		len += 3;
-		s++;
-	}
-
-	return len;
-}
-
-enum {
-	OUTCOME_NOTHING_MOUNTED,
-	OUTCOME_TMPFS_MOUNT,
-	OUTCOME_NON_TMPFS_MOUNT,
-};
-
-/* Read a line of /proc/mounts data looking for a tmpfs mount at "path". */
-static int read_mount(int fd, char *buf, size_t bufsize, const char *path,
-		      int *outcome)
-{
-	int found;
-	int match;
-	char *space;
-	size_t len;
-
-	enum {
-		MATCH_NONE,
-		MATCH_EXACT,
-		MATCH_PARENT,
-	};
-
-	found = next(fd, buf, bufsize, ' ');
-	if (found != 1)
-		return found;
-
-	/*
-	 * If there's no following space in the buffer, then this path is
-	 * truncated, so it can't be the one we're looking for.
-	 */
-	space = strchr(buf, ' ');
-	if (space) {
-		match = MATCH_NONE;
-		if (!decode_path(buf, buf, &len)) {
-			if (!strcmp(buf, path))
-				match = MATCH_EXACT;
-			else if (!strncmp(buf, path, len)
-				 && (path[len] == '/' || !strcmp(buf, "/")))
-				match = MATCH_PARENT;
-		}
-
-		found = pop(fd, buf, bufsize, space - buf + 1);
-		if (found != 1)
-			return found;
-
-		switch (match) {
-		case MATCH_EXACT:
-			if (!strncmp(buf, "tmpfs", strlen("tmpfs")))
-				*outcome = OUTCOME_TMPFS_MOUNT;
-			else
-				*outcome = OUTCOME_NON_TMPFS_MOUNT;
-			break;
-
-		case MATCH_PARENT:
-			/* This mount obscures any previous ones. */
-			*outcome = OUTCOME_NOTHING_MOUNTED;
-			break;
-		}
+	for (i = 0; tmpfs_dirs[i]; i++) {
+		dir = tmpfs_dirs[i];
+		if (check_tmpfs(dir) >= 0)
+			goto done;
 	}
 
-	return next(fd, buf, bufsize, '\n');
+	dir = fallback_dir;
+warn:
+	printf("Warning: tempdir %s is not on tmpfs\n", dir);
+done:
+	/* Make a copy since getenv results may not remain valid forever. */
+	return strdup(dir);
 }
 
-/* which_tmpdir is called only during early boot */
-static int checked_tmpdir = 0;
-
 /*
- * Look for a tmpfs mounted at /dev/shm.  I couldn't find a cleaner
- * way to do this than to parse /proc/mounts.  statfs will return the
- * same filesystem magic number and fs id for both /dev and /dev/shm
- * when they are both tmpfs, so you can't tell if they are different
- * filesystems.  Also, there seems to be no other way of finding the
- * mount point of a filesystem from within it.
- *
- * If a /dev/shm tmpfs entry is found, then we switch to using it.
- * Otherwise, we stay with the default /tmp.
+ * Create an unlinked tempfile in a suitable tempdir. template must be the
+ * basename part of the template with a leading '/'.
  */
-static void which_tmpdir(void)
+static int __init make_tempfile(const char *template)
 {
+	char *tempname;
 	int fd;
-	int found;
-	int outcome;
-	char *path;
-	char *buf;
-	size_t bufsize;
 
-	if (checked_tmpdir)
-		return;
-
-	checked_tmpdir = 1;
-
-	printf("Checking for tmpfs mount on /dev/shm...");
-
-	path = realpath("/dev/shm", NULL);
-	if (!path) {
-		printf("failed to check real path, errno = %d\n", errno);
-		return;
-	}
-	printf("%s...", path);
-
-	/*
-	 * The buffer needs to be able to fit the full octal-escaped path, a
-	 * space, and a trailing null in order to successfully decode it.
-	 */
-	bufsize = octal_encoded_length(path, " \t\n\\") + 2;
-
-	if (bufsize < 128)
-		bufsize = 128;
-
-	buf = malloc(bufsize);
-	if (!buf) {
-		printf("malloc failed, errno = %d\n", errno);
-		goto out;
-	}
-	buf[0] = '\0';
-
-	fd = open("/proc/mounts", O_RDONLY);
-	if (fd < 0) {
-		printf("failed to open /proc/mounts, errno = %d\n", errno);
-		goto out1;
-	}
-
-	outcome = OUTCOME_NOTHING_MOUNTED;
-	while (1) {
-		found = read_mount(fd, buf, bufsize, path, &outcome);
-		if (found != 1)
-			break;
-	}
-
-	if (found < 0) {
-		printf("read returned errno %d\n", -found);
-	} else {
-		switch (outcome) {
-		case OUTCOME_TMPFS_MOUNT:
-			printf("OK\n");
-			default_tmpdir = "/dev/shm";
-			break;
-
-		case OUTCOME_NON_TMPFS_MOUNT:
-			printf("not tmpfs\n");
-			break;
-
-		default:
-			printf("nothing mounted on /dev/shm\n");
-			break;
+	if (tempdir == NULL) {
+		tempdir = choose_tempdir();
+		if (tempdir == NULL) {
+			fprintf(stderr, "Failed to choose tempdir: %s\n",
+				strerror(errno));
+			return -1;
 		}
 	}
 
-	close(fd);
-out1:
-	free(buf);
-out:
-	free(path);
-}
-
-static int __init make_tempfile(const char *template, char **out_tempname,
-				int do_unlink)
-{
-	char *tempname;
-	int fd;
-
-	which_tmpdir();
-	tempname = malloc(MAXPATHLEN);
+	tempname = malloc(strlen(tempdir) + strlen(template) + 1);
 	if (tempname == NULL)
 		return -1;
 
-	find_tempdir();
-	if ((tempdir == NULL) || (strlen(tempdir) >= MAXPATHLEN))
-		goto out;
-
-	if (template[0] != '/')
-		strcpy(tempname, tempdir);
-	else
-		tempname[0] = '\0';
-	strncat(tempname, template, MAXPATHLEN-1-strlen(tempname));
+	strcpy(tempname, tempdir);
+	strcat(tempname, template);
 	fd = mkstemp(tempname);
 	if (fd < 0) {
 		fprintf(stderr, "open - cannot create %s: %s\n", tempname,
 			strerror(errno));
 		goto out;
 	}
-	if (do_unlink && (unlink(tempname) < 0)) {
+	if (unlink(tempname) < 0) {
 		perror("unlink");
 		goto close;
 	}
-	if (out_tempname) {
-		*out_tempname = tempname;
-	} else
-		free(tempname);
+	free(tempname);
 	return fd;
 close:
 	close(fd);
@@ -351,14 +131,14 @@ static int __init make_tempfile(const char *template, char **out_tempname,
 	return -1;
 }
 
-#define TEMPNAME_TEMPLATE "vm_file-XXXXXX"
+#define TEMPNAME_TEMPLATE "/vm_file-XXXXXX"
 
 static int __init create_tmp_file(unsigned long long len)
 {
 	int fd, err;
 	char zero;
 
-	fd = make_tempfile(TEMPNAME_TEMPLATE, NULL, 1);
+	fd = make_tempfile(TEMPNAME_TEMPLATE);
 	if (fd < 0)
 		exit(1);
 
@@ -402,7 +182,6 @@ int __init create_mem_file(unsigned long long len)
 	return fd;
 }
 
-
 void __init check_tmpexec(void)
 {
 	void *addr;
@@ -410,14 +189,13 @@ void __init check_tmpexec(void)
 
 	addr = mmap(NULL, UM_KERN_PAGE_SIZE,
 		    PROT_READ | PROT_WRITE | PROT_EXEC, MAP_PRIVATE, fd, 0);
-	printf("Checking PROT_EXEC mmap in %s...",tempdir);
-	fflush(stdout);
+	printf("Checking PROT_EXEC mmap in %s...", tempdir);
 	if (addr == MAP_FAILED) {
 		err = errno;
-		perror("failed");
+		printf("%s\n", strerror(err));
 		close(fd);
 		if (err == EPERM)
-			printf("%s must be not mounted noexec\n",tempdir);
+			printf("%s must be not mounted noexec\n", tempdir);
 		exit(1);
 	}
 	printf("OK\n");