fix an error caused by fd reuse race when starting runc init

There is a race situation when we are opening a file, if there is a
small fd was closed at that time, maybe it will be reused by safeExe.
Because of Go stdlib fds shuffling bug, if the fd of safeExe is too
small, go stdlib will dup3 it to another fd, or dup3 a other fd to this
fd, then it will cause the fd type cmd.Path refers to a random path,
and it can lead to an error "permission denied" when starting the process.
Please see #4294 and <golang/go#61751>.
So we should not use the original fd of safeExe, but use the fd after
shuffled by Go stdlib. Because Go stdlib will guarantee this fd refers to
the correct file.

Signed-off-by: lfbzhm <lifubang@acmcoder.com>

Author: lifubang

libcontainer/container_linux.go

@@ -618,6 +618,8 @@ func (c *Container) newParentProcess(p *Process) (parentProcess, error) {
 		)
 	}
 
+	// TODO: After https://go-review.googlesource.com/c/go/+/515799 included
+	// in go versions supported by us, we can remove this logic.
 	if safeExe != nil {
 		// Due to a Go stdlib bug, we need to add safeExe to the set of
 		// ExtraFiles otherwise it is possible for the stdlib to clobber the fd
@@ -628,6 +630,18 @@ func (c *Container) newParentProcess(p *Process) (parentProcess, error) {
 		//
 		// See <https://github.com/golang/go/issues/61751>.
 		cmd.ExtraFiles = append(cmd.ExtraFiles, safeExe)
+
+		// There is a race situation when we are opening a file, if there is a
+		// small fd was closed at that time, maybe it will be reused by safeExe.
+		// Because of Go stdlib fds shuffling bug, if the fd of safeExe is too
+		// small, go stdlib will dup3 it to another fd, or dup3 a other fd to this
+		// fd, then it will cause the fd type cmd.Path refers to a random path,
+		// and it can lead to an error "permission denied" when starting the process.
+		// Please see #4294.
+		// So we should not use the original fd of safeExe, but use the fd after
+		// shuffled by Go stdlib. Because Go stdlib will guarantee this fd refers to
+		// the correct file.
+		cmd.Path = "/proc/self/fd/" + strconv.Itoa(stdioFdCount+len(cmd.ExtraFiles)-1)
 	}
 
 	// NOTE: when running a container with no PID namespace and the parent