On Fri, Nov 09, 2018 at 08:02:14PM +0000, Michael Tirado wrote:
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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.

IMO the best solution would handle the issue at memfd creation time by
removing the race condition.

I agree, this is another idea I'm exploring. We could add a new .open
callback to shmem_file_operations and check for seals there.

thanks,

 - Joel