On Sat, Nov 10, 2018 at 04:26:46AM -0800, Daniel Colascione wrote: > On Friday, November 9, 2018, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > > > On Fri, Nov 09, 2018 at 10:19:03PM +0100, Jann Horn wrote: > > > On Fri, Nov 9, 2018 at 10:06 PM Jann Horn <jannh@xxxxxxxxxx> wrote: > > > > 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. > > > > > > My favorite approach would be to forbid open() on memfds, hope that > > > nobody notices the tiny API break, and then add an ioctl for "reopen > > > this memfd with reduced permissions" - but that's just my personal > > > opinion. > > > > I did something along these lines and it fixes the issue, but I forbid open > > of memfd only when the F_SEAL_FUTURE_WRITE seal is in place. So then its > > not > > an ABI break because this is a brand new seal. That seems the least > > intrusive > > solution and it works. Do you mind testing it and I'll add your and > > Tested-by > > to the new fix? The patch is based on top of this series. > > > > Please don't forbid reopens entirely. You're taking a feature that works > generally (reopens) and breaking it in one specific case (memfd write > sealed files). The open modes are available in .open in the struct file: > you can deny *only* opens for write instead of denying reopens generally. Yes, as we discussed over chat already, I will implement it that way. Also lets continue to discuss Andy's concerns he raised on the other thread. thanks, - Joel
