Use memfd_secret for storing password buffer where available

[Michael Stapelberg]

See https://lwn.net/Articles/865256/ for background. This will only become available with the upcoming Linux 5.14 release.

[Michael Stapelberg]

There hasn’t been a new release of the linux manpages since 5.13, but you can view the current memfd_secret manpage like so:

curl https://raw.githubusercontent.com/mkerrisk/man-pages/master/man2/memfd_secret.2 | man /dev/stdin

I tried calling memfd_secret on my Linux 5.17 system like so:

--- i/i3lock.c
+++ w/i3lock.c
@@ -11,6 +11,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <pwd.h>
+#include <sys/syscall.h>
 #include <sys/types.h>
 #include <string.h>
 #include <unistd.h>
@@ -67,7 +68,7 @@ static bool pam_cleanup;
 #endif
 int input_position = 0;
 /* Holds the password you enter (in UTF-8). */
-static char password[512];
+static char *password = NULL;
 static bool beep = false;
 bool debug_mode = false;
 bool unlock_indicator = true;
@@ -1085,6 +1086,19 @@ int main(int argc, char *argv[]) {
         errx(EXIT_FAILURE, "PAM: %s", pam_strerror(pam_handle, ret));
 #endif
 
+#if defined(__linux__)
+    int secret_fd = syscall(SYS_memfd_secret, 0);
+    if (secret_fd == -1)
+      err(EXIT_FAILURE, "memfd_secret");
+    printf("secret_fd = %d\n", secret_fd);
+    exit(1);
+#else
+    password = malloc(512);
+    if (password == NULL) {
+      err(EXIT_FAILURE, "malloc");
+    }
+#endif
+
 /* Using mlock() as non-super-user seems only possible in Linux.
  * Users of other operating systems should use encrypted swap/no swap
  * (or remove the ifdef and run i3lock as super-user).

…but had to discover that memfd_secret returns -ENOSYS on my machine.

This seems to be because it needs to be explicitly enabled by passing the secretmem_enable= option on the kernel command line.

Given that it’s not enabled by default, I don’t think there is much sense in trying to use it. Vanishingly few systems will have it available. We can revisit if/when Linux enables this by default.