Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd
From: Jann Horn <jannh@google.com>
Date: 2018-11-09 21:06:31
Also in:
linux-fsdevel, linux-kselftest, linux-mm, lkml
Possibly related (same subject, not in this thread)
- 2018-11-10 · Re: [PATCH v3 resend 1/2] mm: Add an F_SEAL_FUTURE_WRITE seal to memfd · Joel Fernandes <hidden>
+linux-api for API addition +hughd as FYI since this is somewhat related to mm/shmem On Fri, Nov 9, 2018 at 9:46 PM Joel Fernandes (Google) [off-list ref] wrote:
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.
Please CC linux-api@ on patches like this. If you had done that, I might have criticized your v1 patch instead of your v3 patch...
The following program shows the seal working in action:
[...]
Cc: jreck@google.com Cc: john.stultz@linaro.org Cc: tkjos@google.com Cc: gregkh@linuxfoundation.org Cc: hch@infradead.org Reviewed-by: John Stultz <redacted> Signed-off-by: Joel Fernandes (Google) <redacted> ---
[...]
quoted hunk
diff --git a/mm/memfd.c b/mm/memfd.c index 2bb5e257080e..5ba9804e9515 100644 --- a/mm/memfd.c +++ b/mm/memfd.c
[...]
quoted hunk
@@ -219,6 +220,25 @@ static int memfd_add_seals(struct file *file, unsigned int seals) } } + 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); + }
So you're fiddling around with the file, but not the inode? How are
you preventing code like the following from re-opening the file as
writable?
$ cat memfd.c
#define _GNU_SOURCE
#include <unistd.h>
#include <sys/syscall.h>
#include <printf.h>
#include <fcntl.h>
#include <err.h>
#include <stdio.h>
int main(void) {
int fd = syscall(__NR_memfd_create, "testfd", 0);
if (fd == -1) err(1, "memfd");
char path[100];
sprintf(path, "/proc/self/fd/%d", fd);
int fd2 = open(path, O_RDWR);
if (fd2 == -1) err(1, "reopen");
printf("reopen successful: %d\n", fd2);
}
$ gcc -o memfd memfd.c
$ ./memfd
reopen successful: 4
$
That aside: I wonder whether a better API would be something that
allows you to create a new readonly file descriptor, instead of
fiddling with the writability of an existing fd.