Re: "ls -l": Avoid unnecessary getxattr() overhead

[Jim Meyering]

Pádraig Brady wrote:
> On 02/09/2012 11:32 AM, Jim Meyering wrote:
>> Technically, we could probably exempt all files on that device, but
>> ...
>
> So you avoid symlinks as they can point outside the device.
> Unfortunately so can bind mounts, so you probably have to key on device?

Thanks.  That confirms my impression that my patch was not worthwhile:
It was buggy, too.

I suppose we could use a little hash table, whose entries are
<dev_t device_number, bool getfilecon_required> pairs.

Or even a single static dev_t selinux_challenged_device,
that if equal to f->stat.st_dev, we can skip the *getfilecon call.
That should be good enough for most uses.

Here's that simpler patch:

diff --git a/src/ls.c b/src/ls.c
index f5cd37a..cb9f834 100644
--- a/src/ls.c
+++ b/src/ls.c
@@ -2777,6 +2777,45 @@ clear_files (void)
   file_size_width = 0;
 }

+static bool
+errno_unsupported (int err)
+{
+  return err == ENOTSUP || err == ENODATA || err == EOPNOTSUPP;
+}
+
+/* st_dev of the most recently processed device for which
+   we've found that getfilecon or lgetfilecon fails with
+   e.g., ENOTSUP or EOPNOTSUPP.  */
+static dev_t selinux_challenged_device;
+
+static int
+getfilecon_cache (char const *file, struct fileinfo *f)
+{
+  if (f->stat.st_dev == selinux_challenged_device)
+    {
+      errno = ENOTSUP;
+      return -1;
+    }
+  int r = getfilecon (file, &f->scontext);
+  if (r < 0 && errno_unsupported (errno))
+    selinux_challenged_device = f->stat.st_dev;
+  return r;
+}
+
+static int
+lgetfilecon_cache (char const *file, struct fileinfo *f)
+{
+  if (f->stat.st_dev == selinux_challenged_device)
+    {
+      errno = ENOTSUP;
+      return -1;
+    }
+  int r = lgetfilecon (file, &f->scontext);
+  if (r < 0 && errno_unsupported (errno))
+    selinux_challenged_device = f->stat.st_dev;
+  return r;
+}
+
 /* Add a file to the current table of files.
    Verify that the file exists, and print an error message if it does not.
    Return the number of blocks that the file occupies.  */
@@ -2919,8 +2958,8 @@ gobble_file (char const *name, enum filetype type, ino_t 
inode,
           bool have_selinux = false;
           bool have_acl = false;
           int attr_len = (do_deref
-                          ?  getfilecon (absolute_name, &f->scontext)
-                          : lgetfilecon (absolute_name, &f->scontext));
+                          ? getfilecon_cache (absolute_name, f)
+                          : lgetfilecon_cache (absolute_name, f));
           err = (attr_len < 0);

           if (err == 0)