rclone/sftp: Improve handling of ErrDot errors

Restic now yields a more informative error message when exec.ErrDot occurs.

changelog/unreleased/issue-3932

@@ -0,0 +1,15 @@
+Enhancement: Improve handling of ErrDot errors in rclone and sftp backends
+
+Since Go 1.19, restic can no longer implicitly run relative executables which
+are found in the current directory (e.g. `rclone` if it's found in `.`). This
+is a security feature of Go to prevent against running unintended and possibly
+harmful executables.
+
+The error message for this was just "cannot run executable found relative to
+current directory". This has now been improved to yield a more specific error
+message, informing the user how to explicitly allow running the executable
+using the `-o rclone.program` and `-o sftp.command` extended options with `./`.
+
+https://github.com/restic/restic/issues/3932
+https://pkg.go.dev/os/exec#hdr-Executables_in_the_current_directory
+https://go.dev/blog/path-security

doc/030_preparing_a_new_repo.rst

@@ -633,6 +633,13 @@ initiate a new repository in the path ``bar`` in the remote ``foo``:
 
 Restic takes care of starting and stopping rclone.
 
+.. note:: If you get an error message saying "cannot implicitly run relative
+          executable rclone found in current directory", this means that an
+          rclone executable was found in the current directory. For security
+          reasons restic will not run this implicitly, instead you have to
+          use the ``-o rclone.program=./rclone`` extended option to override
+          this security check and explicitly tell restic to use the executable.
+
 As a more concrete example, suppose you have configured a remote named
 ``b2prod`` for Backblaze B2 with rclone, with a bucket called ``yggdrasil``.
 You can then use rclone to list files in the bucket like this:

internal/backend/errdot_119.go

@@ -0,0 +1,20 @@
+//go:build go1.19
+// +build go1.19
+
+// This file provides a function to check whether an error from cmd.Start() is
+//  exec.ErrDot which was introduced in Go 1.19.
+// This function is needed so that we can perform this check only for Go 1.19 and
+//  up, whereas for older versions we use a dummy/stub in the file errdot_old.go.
+// Once the minimum Go version restic supports is 1.19, remove this file and
+//  replace any calls to it with the corresponding code as per below.
+
+package backend
+
+import (
+	"errors"
+	"os/exec"
+)
+
+func IsErrDot(err error) bool {
+	return errors.Is(err, exec.ErrDot)
+}

internal/backend/errdot_old.go

@@ -0,0 +1,13 @@
+//go:build !go1.19
+// +build !go1.19
+
+// This file provides a stub for IsErrDot() for Go versions below 1.19.
+// See the corresponding file errdot_119.go for more information.
+// Once the minimum Go version restic supports is 1.19, remove this file
+//  and perform the actions listed in errdot_119.go.
+
+package backend
+
+func IsErrDot(err error) bool {
+	return false
+}

internal/backend/rclone/backend.go

@@ -85,6 +85,9 @@ func run(command string, args ...string) (*StdioConn, *sync.WaitGroup, func() er
 		err = errW
 	}
 	if err != nil {
+		if backend.IsErrDot(err) {
+			return nil, nil, nil, errors.Errorf("cannot implicitly run relative executable %v found in current directory, use -o rclone.program=./<program> to override", cmd.Path)
+		}
 		return nil, nil, nil, err
 	}
 

internal/backend/sftp/sftp.go

@@ -80,7 +80,10 @@ func startClient(cfg Config) (*SFTP, error) {
 
 	bg, err := backend.StartForeground(cmd)
 	if err != nil {
-		return nil, errors.Wrap(err, "cmd.Start")
+		if backend.IsErrDot(err) {
+			return nil, errors.Errorf("cannot implicitly run relative executable %v found in current directory, use -o sftp.command=./<command> to override", cmd.Path)
+		}
+		return nil, err
 	}
 
 	// wait in a different goroutine