From 03e304544b9b01c175813215f5930a07152be448 Mon Sep 17 00:00:00 2001
From: 6t8k <6t8k@noreply.codeberg.org>
Date: Sat, 14 Oct 2023 23:25:48 +0200
Subject: [PATCH] cursor: memfd_create: try MFD_NOEXEC_SEAL

Effective from Linux 6.3 onward, this creates the memfd without execute
permissions and prevents that setting from ever being changed. A
run-time fallback is made to not using MFD_NOEXEC_SEAL when a
libwayland-cursor compiled on Linux >= 6.3 is run on Linux < 6.3.

This is a defense-in-depth security measure and silences a respective
kernel warning; see: https://lwn.net/Articles/918106/

This implementation is adopted from dnkl's `foot` terminal emulator.

Signed-off-by: 6t8k <6t8k@noreply.codeberg.org>
---
 cursor/os-compatibility.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/cursor/os-compatibility.c b/cursor/os-compatibility.c
index f2445ced..2b54bae2 100644
--- a/cursor/os-compatibility.c
+++ b/cursor/os-compatibility.c
@@ -40,6 +40,11 @@
 #include <sys/mman.h>
 #endif
 
+/* Fallback to no flag when missing the definition */
+#ifndef MFD_NOEXEC_SEAL
+#define MFD_NOEXEC_SEAL 0
+#endif
+
 #include "os-compatibility.h"
 
 #ifndef HAVE_MKOSTEMP
@@ -124,7 +129,21 @@ os_create_anonymous_file(off_t size)
 	int fd;
 
 #ifdef HAVE_MEMFD_CREATE
-	fd = memfd_create("wayland-cursor", MFD_CLOEXEC | MFD_ALLOW_SEALING);
+	/*
+	* Linux kernels older than 6.3 reject MFD_NOEXEC_SEAL with EINVAL.
+	* Try first *with* it, and if that fails, try again *without* it.
+	*/
+	errno = 0;
+	fd = memfd_create(
+		"wayland-cursor",
+		MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_NOEXEC_SEAL);
+
+	if (fd < 0 && errno == EINVAL && MFD_NOEXEC_SEAL != 0) {
+		fd = memfd_create(
+			"wayland-cursor",
+			MFD_CLOEXEC | MFD_ALLOW_SEALING);
+	}
+
 	if (fd >= 0) {
 		/* We can add this seal before calling posix_fallocate(), as
 		 * the file is currently zero-sized anyway.