mountinfo: linux: use /proc/thread-self/mountinfo

In a Go program with many threads, which goroutine is running on the
thread-group leader thread is not something programs can controls. Thus,
even if you use runtime.LockOSThread() for threads that have different
mount namespaces, it's possible that /proc/self will refer to the
"wrong" thread.

The solution is to simply use /proc/thread-self, which will always
provide the correct result for the calling thread. For pre-3.17 kernels
we use /proc/self/task/<tid> as a fallback.

The usage of /proc/self/mountinfo caused isuses for a patch to runc
which creates a thread to create id-mapped mounts (which requires
joining the container mount namespace).

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

mountinfo/mountinfo_linux.go

@@ -5,15 +5,19 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"runtime"
 	"strconv"
 	"strings"
+	"sync"
+
+	"golang.org/x/sys/unix"
 )
 
 // GetMountsFromReader retrieves a list of mounts from the
 // reader provided, with an optional filter applied (use nil
 // for no filter). This can be useful in tests or benchmarks
 // that provide fake mountinfo data, or when a source other
-// than /proc/self/mountinfo needs to be read from.
+// than /proc/thread-self/mountinfo needs to be read from.
 //
 // This function is Linux-specific.
 func GetMountsFromReader(r io.Reader, filter FilterFunc) ([]*Info, error) {
@@ -127,8 +131,32 @@ func GetMountsFromReader(r io.Reader, filter FilterFunc) ([]*Info, error) {
 	return out, nil
 }
 
-func parseMountTable(filter FilterFunc) ([]*Info, error) {
-	f, err := os.Open("/proc/self/mountinfo")
+var (
+	haveProcThreadSelf     bool
+	haveProcThreadSelfOnce sync.Once
+)
+
+func parseMountTable(filter FilterFunc) (_ []*Info, err error) {
+	haveProcThreadSelfOnce.Do(func() {
+		_, err := os.Stat("/proc/thread-self/mountinfo")
+		haveProcThreadSelf = err == nil
+	})
+
+	// We need to lock ourselves to the current OS thread in order to make sure
+	// that the thread referenced by /proc/thread-self stays alive until we
+	// finish parsing the file.
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	var f *os.File
+	if haveProcThreadSelf {
+		f, err = os.Open("/proc/thread-self/mountinfo")
+	} else {
+		// On pre-3.17 kernels (such as CentOS 7), we don't have
+		// /proc/thread-self/ so we need to manually construct
+		// /proc/self/task/<tid>/ as a fallback.
+		f, err = os.Open("/proc/self/task/" + strconv.Itoa(unix.Gettid()) + "/mountinfo")
+	}
 	if err != nil {
 		return nil, err
 	}