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>

1 files changed, 75 insertions, 297 deletions

diff --git a/arch/um/os-Linux/mem.c b/arch/um/os-Linux/mem.c
index 3c4af77e51a2..897e9ad0c108 100644
--- a/arch/um/os-Linux/mem.c
+++ b/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)
-{
+        for (i = 0; tmpfs_dirs[i]; i++) {
+                dir = tmpfs_dirs[i];
+                if (check_tmpfs(dir) >= 0)
+                        goto done;