Reexec with /proc/self/exe on Linux (#11283) (#11455)

* Reexec with `/proc/self/exe` on Linux

* Add a check for qemu-user

* Add comment

lib/srv/reexec_linux.go

@@ -3,10 +3,49 @@
 package srv
 
 import (
+	"fmt"
+	"os"
 	"os/exec"
 	"syscall"
+
+	"golang.org/x/sys/unix"
 )
 
+func init() {
+	// errors in open/openat are signaled by returning -1, we don't really care
+	// about the specifics anyway so we can just ignore the error value
+	//
+	// we're opening with O_PATH rather than O_RDONLY because the binary might
+	// not actually be readable (but only executable)
+	fd1, _ := syscall.Open("/proc/self/exe", unix.O_PATH|syscall.O_NOCTTY|syscall.O_CLOEXEC, 0)
+	fd2, _ := syscall.Open("/proc/self/exe", unix.O_PATH|syscall.O_NOCTTY|syscall.O_CLOEXEC, 0)
+
+	// this can happen if both calls returned -1 or if we're in running in a
+	// version of qemu-user that's affected by this bug:
+	// https://gitlab.com/qemu-project/qemu/-/issues/927
+	if fd1 == fd2 {
+		return
+	}
+
+	// if one has failed but not the other we can't really trust what's
+	// happening anymore
+	if fd1 == -1 || fd2 == -1 {
+		syscall.Close(fd1)
+		syscall.Close(fd2)
+		return
+	}
+
+	syscall.Close(fd2)
+	// we must specify the path with our pid number instead of self, because
+	// file descriptors are shuffled and overwritten during (*exec.Cmd).Start()
+	// after forking
+	reexecPath = fmt.Sprintf("/proc/%d/fd/%d", os.Getpid(), fd1)
+}
+
+// reexecPath specifies a path to execute on reexec, overriding Path in the cmd
+// passed to reexecCommandOSTweaks, if not empty.
+var reexecPath string
+
 func reexecCommandOSTweaks(cmd *exec.Cmd) {
 	if cmd.SysProcAttr == nil {
 		cmd.SysProcAttr = new(syscall.SysProcAttr)
@@ -15,6 +54,13 @@ func reexecCommandOSTweaks(cmd *exec.Cmd) {
 	// cleaning up child processes, send a signal for graceful shutdown
 	// to children.
 	cmd.SysProcAttr.Pdeathsig = syscall.SIGQUIT
+
+	// replace the path on disk (which might not exist, or refer to an
+	// upgraded version of teleport) with reexecPath, which contains
+	// some path that refers to the specific binary we're running
+	if reexecPath != "" {
+		cmd.Path = reexecPath
+	}
 }
 
 func userCommandOSTweaks(cmd *exec.Cmd) {