On darwin d_seekoff exists, but is optional and does not seem to
be commonly used by file systems. Use `telldir` instead to obtain
the seek offset.

Signed-off-by: Keno Fischer <keno@juliacomputing.com>
---

Changes since v1:
 * Drop setting d_seekoff in synth_direntry
 * Factor telldir vs d_off logic into v9fs_dent_telldir
 * Error path for telldir failure

 hw/9pfs/9p-synth.c |  2 ++
 hw/9pfs/9p.c       | 36 ++++++++++++++++++++++++++++++++----
 2 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/hw/9pfs/9p-synth.c b/hw/9pfs/9p-synth.c
index eb68b42..a312f8c 100644
--- a/hw/9pfs/9p-synth.c
+++ b/hw/9pfs/9p-synth.c
@@ -221,7 +221,9 @@ static void synth_direntry(V9fsSynthNode *node,
 {
     strcpy(entry->d_name, node->name);
     entry->d_ino = node->attr->inode;
+#ifndef CONFIG_DARWIN
     entry->d_off = off + 1;
+#endif
 }
 
 static struct dirent *synth_get_dentry(V9fsSynthNode *dir,
diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
index 212f569..9751246 100644
--- a/hw/9pfs/9p.c
+++ b/hw/9pfs/9p.c
@@ -1738,6 +1738,25 @@ static int v9fs_xattr_read(V9fsState *s, V9fsPDU *pdu, V9fsFidState *fidp,
     return offset;
 }
 
+/**
+ * Get the seek offset of a dirent. If not available from the structure itself,
+ * obtain it by calling telldir.
+ */
+static int v9fs_dent_telldir(V9fsPDU *pdu, V9fsFidState *fidp,
+                             struct dirent *dent)
+{
+#ifdef CONFIG_DARWIN
+    /*
+     * Darwin has d_seekoff, which appears to function similarly to d_off.
+     * However, it does not appear to be supported on all file systems,
+     * so use telldir for correctness.
+     */
+    return v9fs_co_telldir(pdu, fidp);
+#else
+    return dent->d_off;
+#endif
+}
+
 static int coroutine_fn v9fs_do_readdir_with_stat(V9fsPDU *pdu,
                                                   V9fsFidState *fidp,
                                                   uint32_t max_count)
@@ -1801,7 +1820,11 @@ static int coroutine_fn v9fs_do_readdir_with_stat(V9fsPDU *pdu,
         count += len;
         v9fs_stat_free(&v9stat);
         v9fs_path_free(&path);
-        saved_dir_pos = dent->d_off;
+        saved_dir_pos = v9fs_dent_telldir(pdu, fidp, dent);
+        if (saved_dir_pos < 0) {
+            err = saved_dir_pos;
+            break;
+        }
     }
 
     v9fs_readdir_unlock(&fidp->fs.dir);
@@ -1915,7 +1938,7 @@ static int coroutine_fn v9fs_do_readdir(V9fsPDU *pdu, V9fsFidState *fidp,
     V9fsString name;
     int len, err = 0;
     int32_t count = 0;
-    off_t saved_dir_pos;
+    off_t saved_dir_pos, off;
     struct dirent *dent;
 
     /* save the directory position */
@@ -1951,10 +1974,15 @@ static int coroutine_fn v9fs_do_readdir(V9fsPDU *pdu, V9fsFidState *fidp,
         /* Fill the other fields with dummy values */
         qid.type = 0;
         qid.version = 0;
+        off = v9fs_dent_telldir(pdu, fidp, dent);
+        if (off < 0) {
+            err = off;
+            break;
+        }
 
         /* 11 = 7 + 4 (7 = start offset, 4 = space for storing count) */
         len = pdu_marshal(pdu, 11 + count, "Qqbs",
-                          &qid, dent->d_off,
+                          &qid, off,
                           dent->d_type, &name);
 
         v9fs_readdir_unlock(&fidp->fs.dir);
@@ -1966,7 +1994,7 @@ static int coroutine_fn v9fs_do_readdir(V9fsPDU *pdu, V9fsFidState *fidp,
         }
         count += len;
         v9fs_string_free(&name);
-        saved_dir_pos = dent->d_off;
+        saved_dir_pos = off;
     }
 
     v9fs_readdir_unlock(&fidp->fs.dir);
-- 
2.8.1