diff --git a/test/util/BUILD b/test/util/BUILD
index 785763d44..07b901ca5 100644
--- a/test/util/BUILD
+++ b/test/util/BUILD
@@ -164,6 +164,7 @@ cc_library(
         "@com_google_absl//absl/container:flat_hash_map",
         "@com_google_absl//absl/strings",
         gtest,
+        "@com_google_absl//absl/types:span",
     ],
 )
 
diff --git a/test/util/mount_util.cc b/test/util/mount_util.cc
index 48640d6a1..f37c1e178 100644
--- a/test/util/mount_util.cc
+++ b/test/util/mount_util.cc
@@ -18,7 +18,9 @@
 #include <unistd.h>
 
 #include "absl/strings/numbers.h"
+#include "absl/strings/str_join.h"
 #include "absl/strings/str_split.h"
+#include "absl/types/span.h"
 
 namespace gvisor {
 namespace testing {
@@ -103,11 +105,11 @@ PosixErrorOr<std::vector<ProcMountInfoEntry>> ProcSelfMountInfoEntriesFrom(
     ProcMountInfoEntry entry;
     std::vector<std::string> fields =
         absl::StrSplit(line, absl::ByChar(' '), absl::AllowEmpty());
-    if (fields.size() < 10 || fields.size() > 11) {
+    if (fields.size() < 10 || fields.size() > 13) {
       return PosixError(
-          EINVAL, absl::StrFormat(
-                      "Unexpected number of tokens, got %d, content: <<%s>>",
-                      fields.size(), content));
+          EINVAL,
+          absl::StrFormat("Unexpected number of tokens, got %d, line: <<%s>>",
+                          fields.size(), line));
     }
 
     ASSIGN_OR_RETURN_ERRNO(entry.id, Atoi<uint64_t>(fields[0]));
@@ -129,12 +131,14 @@ PosixErrorOr<std::vector<ProcMountInfoEntry>> ProcSelfMountInfoEntriesFrom(
     entry.mount_point = fields[4];
     entry.mount_opts = fields[5];
 
-    // The optional field (fields[6]) may or may not be present. We know based
-    // on the total number of tokens.
+    // The optional field (fields[6]) may or may not be present and can have up
+    // to 3 elements. We know based on the total number of tokens.
     int off = -1;
-    if (fields.size() == 11) {
-      entry.optional = fields[6];
-      off = 0;
+    if (fields.size() > 10) {
+      int num_optional_tags = fields.size() - 10;
+      entry.optional = absl::StrJoin(
+          absl::MakeSpan(fields).subspan(6, num_optional_tags), " ");
+      off += num_optional_tags;
     }
     // Field 7 is the optional field terminator char '-'.
     entry.fstype = fields[8 + off];
diff --git a/test/util/mount_util_test.cc b/test/util/mount_util_test.cc
index 2bcb6cc43..1b8a5f03c 100644
--- a/test/util/mount_util_test.cc
+++ b/test/util/mount_util_test.cc
@@ -37,6 +37,13 @@ TEST(ParseMounts, MountInfo) {
       R"proc(22 28 0:20 / /sys rw,relatime shared:7 - sysfs sysfs rw
 23 28 0:21 / /proc rw,relatime shared:14 - proc proc rw
 2007 8844 0:278 / /mnt rw,noexec - tmpfs  rw,mode=123,uid=268601820,gid=5000
+)proc"));
+  EXPECT_EQ(entries.size(), 3);
+
+  entries = ASSERT_NO_ERRNO_AND_VALUE(ProcSelfMountInfoEntriesFrom(
+      R"proc(22 28 0:20 / /sys rw,relatime shared:7 master:20 - sysfs sysfs rw
+23 28 0:21 / /proc rw,relatime shared:14 master:20 propagate_from:1 - proc proc rw
+2007 8844 0:278 / /mnt rw,noexec - tmpfs  rw,mode=123,uid=268601820,gid=5000
 )proc"));
   EXPECT_EQ(entries.size(), 3);
 }
diff --git a/test/util/posix_error.h b/test/util/posix_error.h
index 8f4f97488..111f9386f 100644
--- a/test/util/posix_error.h
+++ b/test/util/posix_error.h
@@ -64,6 +64,7 @@ class ABSL_MUST_USE_RESULT PosixError {
 
  private:
   int errno_ = 0;
+  // std::string is not async-signal-safe. We must use a c string instead.
   char msg_[1024] = {};
 };