The patch titled
Subject: mm: Add an F_SEAL_FUTURE_WRITE seal to memfd
has been added to the -mm tree. Its filename is
mm-add-an-f_seal_future_write-seal-to-memfd.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-add-an-f_seal_future_write-seal-to-memfd.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-add-an-f_seal_future_write-seal-to-memfd.patch
Before you just go and hit "reply", please:
a) Consider who else should be cc'ed
b) Prefer to cc a suitable mailing list as well
c) Ideally: find the original patch on the mailing list and do a
reply-to-all to that, adding suitable additional cc's
*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
The -mm tree is included into linux-next and is updated
there every 3-4 working days
------------------------------------------------------
From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx>
Subject: mm: Add an F_SEAL_FUTURE_WRITE seal to memfd
Android uses ashmem for sharing memory regions. We are looking forward to
migrating all usecases of ashmem to memfd so that we can possibly remove
the ashmem driver in the future from staging while also benefiting from
using memfd and contributing to it. Note staging drivers are also not ABI
and generally can be removed at anytime.
One of the main usecases Android has is the ability to create a region and
mmap it as writeable, then add protection against making any "future"
writes while keeping the existing already mmap'ed writeable-region active.
This allows us to implement a usecase where receivers of the shared
memory buffer can get a read-only view, while the sender continues to
write to the buffer. See CursorWindow documentation in Android for more
details:
https://developer.android.com/reference/android/database/CursorWindow
This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal
which prevents any future mmap and write syscalls from succeeding while
keeping the existing mmap active. The following program shows the seal
working in action:
#include <stdio.h>
#include <errno.h>
#include <sys/mman.h>
#include <linux/memfd.h>
#include <linux/fcntl.h>
#include <asm/unistd.h>
#include <unistd.h>
#define F_SEAL_FUTURE_WRITE 0x0010
#define REGION_SIZE (5 * 1024 * 1024)
int memfd_create_region(const char *name, size_t size)
{
int ret;
int fd = syscall(__NR_memfd_create, name, MFD_ALLOW_SEALING);
if (fd < 0) return fd;
ret = ftruncate(fd, size);
if (ret < 0) { close(fd); return ret; }
return fd;
}
int main() {
int ret, fd;
void *addr, *addr2, *addr3, *addr1;
ret = memfd_create_region("test_region", REGION_SIZE);
printf("ret=%d\n", ret);
fd = ret;
// Create map
addr = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
if (addr == MAP_FAILED)
printf("map 0 failed\n");
else
printf("map 0 passed\n");
if ((ret = write(fd, "test", 4)) != 4)
printf("write failed even though no future-write seal "
"(ret=%d errno =%d)\n", ret, errno);
else
printf("write passed\n");
addr1 = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
if (addr1 == MAP_FAILED)
perror("map 1 prot-write failed even though no seal\n");
else
printf("map 1 prot-write passed as expected\n");
ret = fcntl(fd, F_ADD_SEALS, F_SEAL_FUTURE_WRITE |
F_SEAL_GROW |
F_SEAL_SHRINK);
if (ret == -1)
printf("fcntl failed, errno: %d\n", errno);
else
printf("future-write seal now active\n");
if ((ret = write(fd, "test", 4)) != 4)
printf("write failed as expected due to future-write seal\n");
else
printf("write passed (unexpected)\n");
addr2 = mmap(0, REGION_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
if (addr2 == MAP_FAILED)
perror("map 2 prot-write failed as expected due to seal\n");
else
printf("map 2 passed\n");
addr3 = mmap(0, REGION_SIZE, PROT_READ, MAP_SHARED, fd, 0);
if (addr3 == MAP_FAILED)
perror("map 3 failed\n");
else
printf("map 3 prot-read passed as expected\n");
}
The output of running this program is as follows:
ret=3
map 0 passed
write passed
map 1 prot-write passed as expected
future-write seal now active
write failed as expected due to future-write seal
map 2 prot-write failed as expected due to seal
: Permission denied
map 3 prot-read passed as expected
Link: http://lkml.kernel.org/r/20181108041537.39694-1-joel@xxxxxxxxxxxxxxxxx
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Reviewed-by: John Stultz <john.stultz@xxxxxxxxxx>
Cc: John Reck <jreck@xxxxxxxxxx>
Cc: Todd Kjos <tkjos@xxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Daniel Colascione <dancol@xxxxxxxxxx>
Cc: J. Bruce Fields <bfields@xxxxxxxxxxxx>
Cc: Jeff Layton <jlayton@xxxxxxxxxx>
Cc: Khalid Aziz <khalid.aziz@xxxxxxxxxx>
Cc: Lei Yang <Lei.Yang@xxxxxxxxxxxxx>
Cc: Marc-Andr Lureau <marcandre.lureau@xxxxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Shuah Khan <shuah@xxxxxxxxxx>
Cc: Valdis Kletnieks <valdis.kletnieks@xxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/uapi/linux/fcntl.h | 1 +
mm/memfd.c | 22 +++++++++++++++++++++-
2 files changed, 22 insertions(+), 1 deletion(-)
--- a/include/uapi/linux/fcntl.h~mm-add-an-f_seal_future_write-seal-to-memfd
+++ a/include/uapi/linux/fcntl.h
@@ -41,6 +41,7 @@
#define F_SEAL_SHRINK 0x0002 /* prevent file from shrinking */
#define F_SEAL_GROW 0x0004 /* prevent file from growing */
#define F_SEAL_WRITE 0x0008 /* prevent writes */
+#define F_SEAL_FUTURE_WRITE 0x0010 /* prevent future writes while mapped */
/* (1U << 31) is reserved for signed error codes */
/*
--- a/mm/memfd.c~mm-add-an-f_seal_future_write-seal-to-memfd
+++ a/mm/memfd.c
@@ -131,7 +131,8 @@ static unsigned int *memfd_file_seals_pt
#define F_ALL_SEALS (F_SEAL_SEAL | \
F_SEAL_SHRINK | \
F_SEAL_GROW | \
- F_SEAL_WRITE)
+ F_SEAL_WRITE | \
+ F_SEAL_FUTURE_WRITE)
static int memfd_add_seals(struct file *file, unsigned int seals)
{
@@ -200,6 +201,25 @@ static int memfd_add_seals(struct file *
}
}
+ if ((seals & F_SEAL_FUTURE_WRITE) &&
+ !(*file_seals & F_SEAL_FUTURE_WRITE)) {
+ /*
+ * The FUTURE_WRITE seal also prevents growing and shrinking
+ * so we need them to be already set, or requested now.
+ */
+ int test_seals = (seals | *file_seals) &
+ (F_SEAL_GROW | F_SEAL_SHRINK);
+
+ if (test_seals != (F_SEAL_GROW | F_SEAL_SHRINK)) {
+ error = -EINVAL;
+ goto unlock;
+ }
+
+ spin_lock(&file->f_lock);
+ file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
+ spin_unlock(&file->f_lock);
+ }
+
*file_seals |= seals;
error = 0;
_
Patches currently in -mm which might be from joel@xxxxxxxxxxxxxxxxx are
mm-treewide-remove-unused-address-argument-from-pte_alloc-functions-v2.patch
mm-speed-up-mremap-by-20x-on-large-regions-v5.patch
mm-speed-up-mremap-by-20x-on-large-regions-v5-fix.patch
mm-select-have_move_pmd-in-x86-for-faster-mremap.patch
mm-add-an-f_seal_future_write-seal-to-memfd.patch
selftests-memfd-add-tests-for-f_seal_future_write-seal.patch
