virtiofsd: passthrough_ll: add fallback for racy ops

We have two operations that cannot be done race-free on a symlink in
certain cases: utimes and link.

Add racy fallback for these if the race-free method doesn't work.  We do
our best to avoid races even in this case:

  - get absolute path by reading /proc/self/fd/NN symlink

  - lookup parent directory: after this we are safe against renames in
    ancestors

  - lookup name in parent directory, and verify that we got to the original
    inode,  if not retry the whole thing

Both utimes(2) and link(2) hold i_lock on the inode across the operation,
so a racing rename/delete by this fuse instance is not possible, only from
other entities changing the filesystem.

If the "norace" option is given, then disable the racy fallbacks.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

tools/virtiofsd/helper.c

@@ -142,7 +142,10 @@ void fuse_cmdline_help(void)
            "    --daemonize                run in background\n"
            "    -o max_idle_threads        the maximum number of idle worker "
            "threads\n"
-           "                               allowed (default: 10)\n");
+           "                               allowed (default: 10)\n"
+           "    -o norace                  disable racy fallback\n"
+           "                               default: false\n"
+          );
 }
 
 static int fuse_helper_opt_proc(void *data, const char *arg, int key,

tools/virtiofsd/passthrough_ll.c

@@ -98,6 +98,7 @@ enum {
 struct lo_data {
     pthread_mutex_t mutex;
     int debug;
+    int norace;
     int writeback;
     int flock;
     int xattr;
@@ -124,10 +125,15 @@ static const struct fuse_opt lo_opts[] = {
     { "cache=never", offsetof(struct lo_data, cache), CACHE_NEVER },
     { "cache=auto", offsetof(struct lo_data, cache), CACHE_NORMAL },
     { "cache=always", offsetof(struct lo_data, cache), CACHE_ALWAYS },
-
+    { "norace", offsetof(struct lo_data, norace), 1 },
     FUSE_OPT_END
 };
 
+static void unref_inode(struct lo_data *lo, struct lo_inode *inode, uint64_t n);
+
+static struct lo_inode *lo_find(struct lo_data *lo, struct stat *st);
+
+
 static struct lo_data *lo_data(fuse_req_t req)
 {
     return (struct lo_data *)fuse_req_userdata(req);
@@ -347,23 +353,127 @@ static void lo_getattr(fuse_req_t req, fuse_ino_t ino,
     fuse_reply_attr(req, &buf, lo->timeout);
 }
 
-static int utimensat_empty_nofollow(struct lo_inode *inode,
+static int lo_parent_and_name(struct lo_data *lo, struct lo_inode *inode,
-                                    const struct timespec *tv)
+                              char path[PATH_MAX], struct lo_inode **parent)
+{
+    char procname[64];
+    char *last;
+    struct stat stat;
+    struct lo_inode *p;
+    int retries = 2;
+    int res;
+
+retry:
+    sprintf(procname, "/proc/self/fd/%i", inode->fd);
+
+    res = readlink(procname, path, PATH_MAX);
+    if (res < 0) {
+        fuse_log(FUSE_LOG_WARNING, "%s: readlink failed: %m\n", __func__);
+        goto fail_noretry;
+    }
+
+    if (res >= PATH_MAX) {
+        fuse_log(FUSE_LOG_WARNING, "%s: readlink overflowed\n", __func__);
+        goto fail_noretry;
+    }
+    path[res] = '\0';
+
+    last = strrchr(path, '/');
+    if (last == NULL) {
+        /* Shouldn't happen */
+        fuse_log(
+            FUSE_LOG_WARNING,
+            "%s: INTERNAL ERROR: bad path read from proc\n", __func__);
+        goto fail_noretry;
+    }
+    if (last == path) {
+        p = &lo->root;
+        pthread_mutex_lock(&lo->mutex);
+        p->refcount++;
+        pthread_mutex_unlock(&lo->mutex);
+    } else {
+        *last = '\0';
+        res = fstatat(AT_FDCWD, last == path ? "/" : path, &stat, 0);
+        if (res == -1) {
+            if (!retries) {
+                fuse_log(FUSE_LOG_WARNING,
+                         "%s: failed to stat parent: %m\n", __func__);
+            }
+            goto fail;
+        }
+        p = lo_find(lo, &stat);
+        if (p == NULL) {
+            if (!retries) {
+                fuse_log(FUSE_LOG_WARNING,
+                         "%s: failed to find parent\n", __func__);
+            }
+            goto fail;
+        }
+    }
+    last++;
+    res = fstatat(p->fd, last, &stat, AT_SYMLINK_NOFOLLOW);
+    if (res == -1) {
+        if (!retries) {
+            fuse_log(FUSE_LOG_WARNING,
+                     "%s: failed to stat last\n", __func__);
+        }
+        goto fail_unref;
+    }
+    if (stat.st_dev != inode->dev || stat.st_ino != inode->ino) {
+        if (!retries) {
+            fuse_log(FUSE_LOG_WARNING,
+                     "%s: failed to match last\n", __func__);
+        }
+        goto fail_unref;
+    }
+    *parent = p;
+    memmove(path, last, strlen(last) + 1);
+
+    return 0;
+
+fail_unref:
+    unref_inode(lo, p, 1);
+fail:
+    if (retries) {
+        retries--;
+        goto retry;
+    }
+fail_noretry:
+    errno = EIO;
+    return -1;
+}
+
+static int utimensat_empty(struct lo_data *lo, struct lo_inode *inode,
+                           const struct timespec *tv)
 {
     int res;
-    char procname[64];
+    struct lo_inode *parent;
+    char path[PATH_MAX];
 
     if (inode->is_symlink) {
-        res = utimensat(inode->fd, "", tv, AT_EMPTY_PATH | AT_SYMLINK_NOFOLLOW);
+        res = utimensat(inode->fd, "", tv, AT_EMPTY_PATH);
         if (res == -1 && errno == EINVAL) {
             /* Sorry, no race free way to set times on symlink. */
-            errno = EPERM;
+            if (lo->norace) {
+                errno = EPERM;
+            } else {
+                goto fallback;
+            }
         }
         return res;
     }
-    sprintf(procname, "/proc/self/fd/%i", inode->fd);
+    sprintf(path, "/proc/self/fd/%i", inode->fd);
 
-    return utimensat(AT_FDCWD, procname, tv, 0);
+    return utimensat(AT_FDCWD, path, tv, 0);
+
+fallback:
+    res = lo_parent_and_name(lo, inode, path, &parent);
+    if (res != -1) {
+        res = utimensat(parent->fd, path, tv, AT_SYMLINK_NOFOLLOW);
+        unref_inode(lo, parent, 1);
+    }
+
+    return res;
 }
 
 static int lo_fi_fd(fuse_req_t req, struct fuse_file_info *fi)
@@ -387,6 +497,7 @@ static void lo_setattr(fuse_req_t req, fuse_ino_t ino, struct stat *attr,
 {
     int saverr;
     char procname[64];
+    struct lo_data *lo = lo_data(req);
     struct lo_inode *inode;
     int ifd;
     int res;
@@ -459,7 +570,7 @@ static void lo_setattr(fuse_req_t req, fuse_ino_t ino, struct stat *attr,
         if (fi) {
             res = futimens(fd, tv);
         } else {
-            res = utimensat_empty_nofollow(inode, tv);
+            res = utimensat_empty(lo, inode, tv);
         }
         if (res == -1) {
             goto out_err;
@@ -709,24 +820,38 @@ static void lo_symlink(fuse_req_t req, const char *link, fuse_ino_t parent,
     lo_mknod_symlink(req, parent, name, S_IFLNK, 0, link);
 }
 
-static int linkat_empty_nofollow(struct lo_inode *inode, int dfd,
+static int linkat_empty_nofollow(struct lo_data *lo, struct lo_inode *inode,
-                                 const char *name)
+                                 int dfd, const char *name)
 {
     int res;
-    char procname[64];
+    struct lo_inode *parent;
+    char path[PATH_MAX];
 
     if (inode->is_symlink) {
         res = linkat(inode->fd, "", dfd, name, AT_EMPTY_PATH);
         if (res == -1 && (errno == ENOENT || errno == EINVAL)) {
             /* Sorry, no race free way to hard-link a symlink. */
-            errno = EPERM;
+            if (lo->norace) {
+                errno = EPERM;
+            } else {
+                goto fallback;
+            }
         }
         return res;
     }
 
-    sprintf(procname, "/proc/self/fd/%i", inode->fd);
+    sprintf(path, "/proc/self/fd/%i", inode->fd);
 
-    return linkat(AT_FDCWD, procname, dfd, name, AT_SYMLINK_FOLLOW);
+    return linkat(AT_FDCWD, path, dfd, name, AT_SYMLINK_FOLLOW);
+
+fallback:
+    res = lo_parent_and_name(lo, inode, path, &parent);
+    if (res != -1) {
+        res = linkat(parent->fd, path, dfd, name, 0);
+        unref_inode(lo, parent, 1);
+    }
+
+    return res;
 }
 
 static void lo_link(fuse_req_t req, fuse_ino_t ino, fuse_ino_t parent,
@@ -748,7 +873,7 @@ static void lo_link(fuse_req_t req, fuse_ino_t ino, fuse_ino_t parent,
     e.attr_timeout = lo->timeout;
     e.entry_timeout = lo->timeout;
 
-    res = linkat_empty_nofollow(inode, lo_fd(req, parent), name);
+    res = linkat_empty_nofollow(lo, inode, lo_fd(req, parent), name);
     if (res == -1) {
         goto out_err;
     }