lkml.org 
[lkml]   [2021]   [Jun]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: Sealed memfd & no-fault mmap
    On Thu, Jun 03, 2021 at 01:14:47PM +0000, Simon Ser wrote:
    > On Saturday, May 29th, 2021 at 10:15 PM, Hugh Dickins <hughd@google.com> wrote:
    >
    > > And IIUC it would have to be the recipient (Wayland compositor) doing
    > > the NOFAULT business, because (going back to the original mail) we are
    > > only considering this so that Wayland might satisfy clients who predate
    > > or refuse Linux-only APIs. So, an ioctl (or fcntl, as sealing chose)
    > > at the client end cannot be expected; and could not be relied on anyway.
    >
    > Yes, that is correct.
    >
    > > NOFAULT? Does BSD use "fault" differently, and in Linux terms we
    > > would say NOSIGBUS to mean the same?
    > >
    > > Can someone point to a specification of BSD's __MAP_NOFAULT?
    > > Searching just found me references to bugs.
    >
    > __MAP_NOFAULT isn't documented, sadly. The commit that introduces the
    > flag [1] is the best we're going to get, I think.
    >
    > > What mainly worries me about the suggestion is: what happens to the
    > > zero page inserted into NOFAULT mappings, when later a page for that
    > > offset is created and added to page cache?
    >
    > Not 100% sure exactly this means what I think it means, but from my PoV,
    > it's fine if the contents of an expanded shm file aren't visible from the
    > process that has mapped it with MAP_NOFAULT/MAP_NOSIGBUS. In other words,
    > it's fine if:
    >
    > - The client sets up a 1KiB shm file and sends it to the compositor.
    > - The compositor maps it with MAP_NOFAULT/MAP_NOSIGBUS.
    > - The client expands the file to 2KiB and writes interesting data in it.
    > - The compositor still sees zeros past the 1KiB mark. The compositor needs
    > to unmap and re-map the file to see the data past the 1KiB mark.
    >
    > If the MAP_NOFAULT/MAP_NOSIGBUS flag only affects the mapping itself and
    > nothing else, this should be fine?

    This is going to operate at a page boundary, so the example you gave
    will work. How about this:

    - The client sets up a 1KiB shm file and sends it to the compositor.
    - The client expands the file to 5KiB
    - The compositor sees the new data up to 4KiB but zeroes past the 4KiB
    mark.

    Does that still make userspace happy?

    \
     
     \ /
      Last update: 2021-06-03 15:59    [W:2.235 / U:0.040 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site