mountinfo: run filter after all fields are parsed

Previously, the filter would be run before all of the fields were parsed
(this behaviour also was not documented) -- this resulted in users of
the function accidentally assuming that fields like fsinfo.Root would
actually be filled correctly. It seems that the performance overhead of
parsing a few extra fields is not exorbitant, and optimising this just
leads to incorrect user code.

For a concrete example, this optimisation actually made this runc
change[1] regress a security hardening feature because it relied on
fsinfo.Root being filled correctly.

[1]: opencontainers/runc#2647

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

Author: cyphar

mountinfo/mountinfo_filters.go

@@ -6,9 +6,8 @@ import "strings"
 // used to filter out mountinfo entries we're not interested in,
 // and/or stop further processing if we found what we wanted.
 //
-// It takes a pointer to the Info struct (not fully populated,
-// currently only Mountpoint, FSType, Source, and (on Linux)
-// VFSOptions are filled in), and returns two booleans:
+// It takes a pointer to the Info struct (fully populated with all available
+// fields on the GOOS platform), and returns two booleans:
 //
 // skip: true if the entry should be skipped;
 //

mountinfo/mountinfo_linux.go

@@ -72,7 +72,6 @@ func GetMountsFromReader(r io.Reader, filter FilterFunc) ([]*Info, error) {
 
 		p := &Info{}
 
-		// Fill in the fields that a filter might check
 		p.Mountpoint, err = unescape(fields[4])
 		if err != nil {
 			return nil, fmt.Errorf("Parsing '%s' failed: mount point: %w", fields[4], err)
@@ -87,18 +86,6 @@ func GetMountsFromReader(r io.Reader, filter FilterFunc) ([]*Info, error) {
 		}
 		p.VFSOptions = fields[sepIdx+3]
 
-		// Run a filter early so we can skip parsing/adding entries
-		// the caller is not interested in
-		var skip, stop bool
-		if filter != nil {
-			skip, stop = filter(p)
-			if skip {
-				continue
-			}
-		}
-
-		// Fill in the rest of the fields
-
 		// ignore any numbers parsing errors, as there should not be any
 		p.ID, _ = strconv.Atoi(fields[0])
 		p.Parent, _ = strconv.Atoi(fields[1])
@@ -126,6 +113,15 @@ func GetMountsFromReader(r io.Reader, filter FilterFunc) ([]*Info, error) {
 			p.Optional = strings.Join(fields[6:sepIdx-1], " ")
 		}
 
+		// Run the filter after parsing all of the fields.
+		var skip, stop bool
+		if filter != nil {
+			skip, stop = filter(p)
+			if skip {
+				continue
+			}
+		}
+
 		out = append(out, p)
 		if stop {
 			break