Revert "xattrs: Use /proc/self/fd path in glnx_fd_set_all_xattrs"

We can't use fsetxattr() to get the extended attributes of a symlink via
the magic symlink in /proc/self/fd, because opening with O_NOFOLLOW
would try to set the extended attributes on the magic symlink
/proc/self/fd/N, and opening without O_NOFOLLOW would dereference the
symlink.

This reverts commit 128578be4d.

Helps: https://github.com/flatpak/flatpak/issues/6818
Signed-off-by: Simon McVittie <smcv@collabora.com>

glnx-xattrs.c

@@ -338,12 +338,25 @@ glnx_dfd_name_set_all_xattrs (int            dfd,
 gboolean
 glnx_fd_set_all_xattrs (int            fd,
                         GVariant      *xattrs,
-                        GCancellable  *cancellable,
+                        G_GNUC_UNUSED GCancellable *cancellable,
                         GError       **error)
 {
-  char buf[PATH_MAX];
-  snprintf (buf, sizeof (buf), "/proc/self/fd/%d", fd);
-  return set_all_xattrs_for_path (buf, xattrs, cancellable, error);
+  const guint n = g_variant_n_children (xattrs);
+  for (guint i = 0; i < n; i++)
+    {
+      const guint8* name;
+      g_autoptr(GVariant) value = NULL;
+      g_variant_get_child (xattrs, i, "(^&ay@ay)",
+                           &name, &value);
+
+      gsize value_len;
+      const guint8* value_data = g_variant_get_fixed_array (value, &value_len, 1);
+
+      if (TEMP_FAILURE_RETRY (fsetxattr (fd, (char*)name, (char*)value_data, value_len, 0)) < 0)
+        return glnx_throw_errno_prefix (error, "Setting xattrs: fsetxattr(%s)", name);
+    }
+
+  return TRUE;
 }
 
 /**