[PATCHv2] uml: Fix which_tmpdir failure when /dev/shm is a symlink, and in other edge cases

[Tristan Schmelcher <tschmelcher@google.com>]

From: Tristan Schmelcher <tschmelcher@google.com>

which_tmpdir did the wrong thing if /dev/shm was a symlink (e.g., to /run/shm),
if there were multiple mounts on top of each other, if the mount(s) were
obscured by a later mount, or if /dev/shm was a prefix of another mount point.
This fixes these cases. Applies to 3.9.6.

Signed-off-by: Tristan Schmelcher <tschmelcher@google.com>
---
v1 had a bug where it would misparse the file if it got a truncated path
containing a newline after decoding. (I also mangled the patch.)

--- linux-3.9.6/arch/um/os-Linux/mem.c.orig	2013-06-19 14:50:14.000000000 -0400
+++ linux-3.9.6/arch/um/os-Linux/mem.c	2013-06-19 16:09:39.000000000 -0400
@@ -53,6 +53,25 @@ static void __init find_tempdir(void)
 }
 
 /*
+ * 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;
@@ -61,7 +80,6 @@ static void __init find_tempdir(void)
 static int next(int fd, char *buf, size_t size, char c)
 {
 	ssize_t n;
-	size_t len;
 	char *ptr;
 
 	while ((ptr = strchr(buf, c)) == NULL) {
@@ -74,20 +92,116 @@ static int next(int fd, char *buf, size_
 		buf[n] = '\0';
 	}
 
-	ptr++;
-	len = strlen(ptr);
-	memmove(buf, ptr, len + 1);
+	return pop(fd, buf, size, ptr - buf + 1);
+}
 
-	/*
-	 * Refill the buffer so that if there's a partial string that we care
-	 * about, it will be completed, and we can recognize it.
-	 */
-	n = read(fd, &buf[len], size - len - 1);
-	if (n < 0)
-		return -errno;
+/*
+ * 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.
+ */
+static int decode_path(const char *in, char *out, size_t *ndecoded,
+		       size_t *decodedlen)
+{
+	const char *first_in = in;
+	char *first_out = out;
+	int c;
+	int i;
+	int ret = -EINVAL;
+	while (*in) {
+		switch (*in) {
+		case ' ':
+			*out = '\0';
+			*decodedlen = out - first_out;
+			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');
+			}
+			*out++ = (char) c;
+			break;
 
-	buf[len + n] = '\0';
-	return 1;
+		default:
+			*out++ = *in++;
+			break;
+		}
+	}
+
+out:
+	*ndecoded = in - first_in + 1;
+	return ret;
+}
+
+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_NOT_A_MOUNT_POINT,
+	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;
+	size_t ndecoded;
+	size_t decodedlen;
+
+	enum {
+		MATCH_NONE,
+		MATCH_EXACT,
+		MATCH_PARENT,
+	};
+
+	found = next(fd, buf, bufsize, ' ');
+	if (found != 1)
+		return found;
+
+	match = MATCH_NONE;
+	if (!decode_path(buf, buf, &ndecoded, &decodedlen)) {
+		if (!strcmp(buf, path))
+			match = MATCH_EXACT;
+		else if (!strncmp(buf, path, decodedlen)
+			 && (path[decodedlen] == '/' || !strcmp(buf, "/")))
+			match = MATCH_PARENT;
+	}
+
+	found = pop(fd, buf, bufsize, ndecoded);
+	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_NOT_A_MOUNT_POINT;
+		break;
+	}
+
+	return next(fd, buf, bufsize, '\n');
 }
 
 /* which_tmpdir is called only during early boot */
@@ -106,8 +220,12 @@ static int checked_tmpdir = 0;
  */
 static void which_tmpdir(void)
 {
-	int fd, found;
-	char buf[128] = { '\0' };
+	int fd;
+	int found;
+	int outcome;
+	char *path;
+	char *buf;
+	size_t bufsize;
 
 	if (checked_tmpdir)
 		return;
@@ -116,49 +234,66 @@ static void which_tmpdir(void)
 
 	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);
-		return;
+		goto out1;
 	}
 
+	outcome = OUTCOME_NOT_A_MOUNT_POINT;
 	while (1) {
-		found = next(fd, buf, ARRAY_SIZE(buf), ' ');
-		if (found != 1)
-			break;
-
-		if (!strncmp(buf, "/dev/shm", strlen("/dev/shm")))
-			goto found;
-
-		found = next(fd, buf, ARRAY_SIZE(buf), '\n');
+		found = read_mount(fd, buf, bufsize, path, &outcome);
 		if (found != 1)
 			break;
 	}
 
-err:
-	if (found == 0)
-		printf("nothing mounted on /dev/shm\n");
-	else if (found < 0)
+	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;
 
-out:
-	close(fd);
-
-	return;
-
-found:
-	found = next(fd, buf, ARRAY_SIZE(buf), ' ');
-	if (found != 1)
-		goto err;
+		case OUTCOME_NON_TMPFS_MOUNT:
+			printf("not tmpfs\n");
+			break;
 
-	if (strncmp(buf, "tmpfs", strlen("tmpfs"))) {
-		printf("not tmpfs\n");
-		goto out;
+		default:
+			printf("nothing mounted on /dev/shm\n");
+			break;
+		}
 	}
 
-	printf("OK\n");
-	default_tmpdir = "/dev/shm";
-	goto out;
+	close(fd);
+out1:
+	free(buf);
+out:
+	free(path);
 }
 
 static int __init make_tempfile(const char *template, char **out_tempname,