> On Nov 9, 2018, at 1:06 PM, Jann Horn <jannh@xxxxxxxxxx> wrote: > > +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) > <joel@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx >> Cc: john.stultz@xxxxxxxxxx >> Cc: tkjos@xxxxxxxxxx >> Cc: gregkh@xxxxxxxxxxxxxxxxxxx >> Cc: hch@xxxxxxxxxxxxx >> Reviewed-by: John Stultz <john.stultz@xxxxxxxxxx> >> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> >> --- > [...] >> diff --git a/mm/memfd.c b/mm/memfd.c >> index 2bb5e257080e..5ba9804e9515 100644 >> --- a/mm/memfd.c >> +++ b/mm/memfd.c > [...] >> @@ -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. Every now and then I try to write a patch to prevent using proc to reopen a file with greater permission than the original open. I like your idea to have a clean way to reopen a a memfd with reduced permissions. But I would make it a syscall instead and maybe make it only work for memfd at first. And the proc issue would need to be fixed, too.
