From 423a033ba7bf9b38a4843ae9609019cc108647cf Mon Sep 17 00:00:00 2001
From: Kyle Evans <kevans@FreeBSD.org>
Date: Fri, 10 Jul 2020 00:45:16 +0000
Subject: [PATCH] memfd_create: turn on SHM_GROW_ON_WRITE

memfd_create fds will no longer require an ftruncate(2) to set the size;
they'll grow (to the extent that it's possible) upon write(2)-like syscalls.

Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D25502
---
 lib/libc/sys/shm_open.c       |  2 +-
 sys/compat/linux/linux_file.c |  2 +-
 tests/sys/kern/memfd_test.c   | 17 +++++++++++++----
 3 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/lib/libc/sys/shm_open.c b/lib/libc/sys/shm_open.c
index 7eb546552c30..cf89505b3d56 100644
--- a/lib/libc/sys/shm_open.c
+++ b/lib/libc/sys/shm_open.c
@@ -84,7 +84,7 @@ memfd_create(const char *name, unsigned int flags)
 	/* We've already validated that we're sufficiently sized. */
 	snprintf(memfd_name, NAME_MAX + 1, "%s%s", MEMFD_NAME_PREFIX, name);
 	oflags = O_RDWR;
-	shmflags = 0;
+	shmflags = SHM_GROW_ON_WRITE;
 	if ((flags & MFD_CLOEXEC) != 0)
 		oflags |= O_CLOEXEC;
 	if ((flags & MFD_ALLOW_SEALING) != 0)
diff --git a/sys/compat/linux/linux_file.c b/sys/compat/linux/linux_file.c
index c4160983bcec..377e0ef4d34a 100644
--- a/sys/compat/linux/linux_file.c
+++ b/sys/compat/linux/linux_file.c
@@ -1758,7 +1758,7 @@ linux_memfd_create(struct thread *td, struct linux_memfd_create_args *args)
 	if ((flags & MFD_HUGETLB) != 0)
 		return (ENOSYS);
 	oflags = O_RDWR;
-	shmflags = 0;
+	shmflags = SHM_GROW_ON_WRITE;
 	if ((flags & MFD_CLOEXEC) != 0)
 		oflags |= O_CLOEXEC;
 	if ((flags & MFD_ALLOW_SEALING) != 0)
diff --git a/tests/sys/kern/memfd_test.c b/tests/sys/kern/memfd_test.c
index 5d5b8f9e6084..3784c4f0c840 100644
--- a/tests/sys/kern/memfd_test.c
+++ b/tests/sys/kern/memfd_test.c
@@ -30,6 +30,7 @@ __FBSDID("$FreeBSD$");
 
 #include <sys/fcntl.h>
 #include <sys/mman.h>
+#include <sys/stat.h>
 
 #include <atf-c.h>
 #include <errno.h>
@@ -38,18 +39,26 @@ __FBSDID("$FreeBSD$");
 ATF_TC_WITHOUT_HEAD(basic);
 ATF_TC_BODY(basic, tc)
 {
+	struct stat sb;
 	int fd;
 	char buf[8];
 
 	ATF_REQUIRE((fd = memfd_create("...", 0)) != -1);
 
-	/* File size should be initially 0 */
-	ATF_REQUIRE(write(fd, buf, sizeof(buf)) == 0);
+	/* write(2) should grow us out automatically. */
+	ATF_REQUIRE(write(fd, buf, sizeof(buf)) == sizeof(buf));
+	ATF_REQUIRE(fstat(fd, &sb) == 0);
+	ATF_REQUIRE(sb.st_size == sizeof(buf));
 
 	/* ftruncate(2) must succeed without seals */
-	ATF_REQUIRE(ftruncate(fd, sizeof(buf) - 1) == 0);
+	ATF_REQUIRE(ftruncate(fd, 2 * (sizeof(buf) - 1)) == 0);
 
-	ATF_REQUIRE(write(fd, buf, sizeof(buf)) == sizeof(buf) - 1);
+	/* write(2) again must not be limited by ftruncate(2) size. */
+	ATF_REQUIRE(write(fd, buf, sizeof(buf)) == sizeof(buf));
+
+	/* Sanity check. */
+	ATF_REQUIRE(fstat(fd, &sb) == 0);
+	ATF_REQUIRE(sb.st_size == 2 * sizeof(buf));
 
 	close(fd);
 }