mountinfo.mountedByMountinfo: normalize path

kolyshkin · kolyshkin · commit 682f2bc29980 · 2020-09-14T15:26:03.000-07:00
Currently, the path provided to mountinfo.Mounted() must be
 - an absolute path;
 - with all symlinks resolved;
 - cleaned.

Otherwise, the function may return a false negative (in case this is
a bind mount and the kernel is too old to have openat2) -- i.e. it
will return "not mounted" while the path is in fact mounted.

This commit removes the above limitation by doing a path normalization.
This should slow things down but it's not affecting any of the fast
paths, so the slowdown will only be seen in the "bind mount and older
kernel" case (which is already pretty slow since we have to parse
mountinfo).

v2: as in mountedByOpenat2 and mountedByStat, treat ENOENT as "not
mounted".

Signed-off-by: Kir Kolyshkin &lt;kolyshkin@gmail.com&gt;
diff --git a/mountinfo/doc.go b/mountinfo/doc.go
@@ -14,9 +14,8 @@
 // parse filters while reading mountinfo. A filter can skip some entries, or stop
 // processing the rest of the file once the needed information is found.
 //
-// For functions that have path as an argument (such as Mounted or various filters),
-// the argument must be
-//  - an absolute path;
+// For mountinfo filters that accept path as an argument, the path must be:
+//  - absolute;
 //  - having all symlinks resolved;
 //  - being cleaned.
 //
diff --git a/mountinfo/mounted_unix.go b/mountinfo/mounted_unix.go
@@ -3,6 +3,8 @@
 package mountinfo
 
 import (
+	"errors"
+	"fmt"
 	"os"
 	"path/filepath"
 
@@ -33,7 +35,28 @@ func mountedByStat(path string) (bool, error) {
 	return false, nil
 }
 
+func normalizePath(path string) (realPath string, err error) {
+	if realPath, err = filepath.Abs(path); err != nil {
+		return "", fmt.Errorf("unable to get absolute path for %q: %w", path, err)
+	}
+	if realPath, err = filepath.EvalSymlinks(realPath); err != nil {
+		return "", fmt.Errorf("failed to canonicalise path for %q: %w", path, err)
+	}
+	if _, err := os.Stat(realPath); err != nil {
+		return "", fmt.Errorf("failed to stat target of %q: %w", path, err)
+	}
+	return realPath, nil
+}
+
 func mountedByMountinfo(path string) (bool, error) {
+	path, err := normalizePath(path)
+	if err != nil {
+		if errors.Is(err, unix.ENOENT) {
+			// treat ENOENT as "not mounted"
+			return false, nil
+		}
+		return false, err
+	}
 	entries, err := GetMounts(SingleEntryFilter(path))
 	if err != nil {
 		return false, err
