On Tue, Oct 09, 2018 at 03:20:41PM -0700, Joel Fernandes (Google) wrote:
> One of the main usecases Android has is the ability to create a region
> and mmap it as writeable, then drop its protection for "future" writes
> while keeping the existing already mmap'ed writeable-region active.
s/drop/add/ ?
Otherwise this doesn't make much sense to me.
> This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
> To support the usecase, this patch adds a new F_SEAL_FS_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:
Where does the FS come from? I'd rather expect this to be implemented
as a 'force' style flag that applies the seal even if the otherwise
required precondition is not met.
> Note: This seal will also prevent growing and shrinking of the memfd.
> This is not something we do in Android so it does not affect us, however
> I have mentioned this behavior of the seal in the manpage.
This seems odd, as that is otherwise split into the F_SEAL_SHRINK /
F_SEAL_GROW flags.
> static int memfd_add_seals(struct file *file, unsigned int seals)
> {
> @@ -219,6 +220,9 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
> }
> }
>
> + if ((seals & F_SEAL_FS_WRITE) && !(*file_seals & F_SEAL_FS_WRITE))
> + file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE);
> +
This seems to lack any synchronization for f_mode.
