Re: [PATCH 0/3] vmsplice: make vmsplice a trivial wrapper for preadv2/pwritev2

[Joanne Koong]

On Mon, Jun 15, 2026 at 9:15 PM Askar Safin <safinaskar@xxxxxxxxx> wrote:
>
> Joanne Koong <joannelkoong@xxxxxxxxx>:
> > > speaking of fuse_dev_splice……_write actually, this series has broken
> > > xdg-document-portal!
> > >
> > > https://github.com/flatpak/xdg-desktop-portal/issues/2026
> > >
> > > Specifically what happens is that the EINVAL is returned due to oh.len
> > > != nbytes:
> > >
> > > fuse_dev_do_write: oh.len 16400 != nbytes 15526
> > >
> > > (where 16400 == 16384 (read len) + 16, 15526 == 15510 (file len) + 16)
> > >
> > > After reverting the series, there is no error because oh.len
> > > becomes 15526 too.
> >
> > I think this is because of how libfuse handles eof / short reads. When
> > it detects a short read, it fixes up the header length after the
> > header was already vmspliced to the pipe because it assumes vmsplice
> > mapped the header's page into the pipe by reference. It assumes that
> > modifying the header length in place gets then reflected in what the
> > pipe later splices out.
> >
> > The logic for this happens in fuse_send_data_iov() [1]:
> > a) sets out->len = headerlen (16) + len (16384) = 16400 in the
> > stack-allocated fuse_out_header
> > b) vmsplices the header to the pipe
> > c) splices the backing file to the pipe. if this hits EOF, it'll get
> > back 15510 instead of 16384
> > d) detects the short read [2], fixes up the stack out->len = 16 + 15510 = 15526
> > e) splices the pipe to /dev/fuse
> >
> > After this patch, step b) is a straight copy which means step d)'s
> > fixup doesn't modify what's in the pipe. This could be fixed up in
> > libfuse to not depend on modify-after-vmsplice, but I don't think this
> > helps for applications using already-released libfuse versions. I
> > think this patch needs to be reverted.
> >
> > Thanks,
> > Joanne
> >
> > [1] https://github.com/libfuse/libfuse/blob/master/lib/fuse_lowlevel.c#L846
> > [2] https://github.com/libfuse/libfuse/blob/master/lib/fuse_lowlevel.c#L956
>
> Uh, this is very unfortunate. But I still want to remove vmsplice.
> Maybe we can somehow save my patchsets? For example, let's return EINVAL
> for this particular combination (writable pipe + SPLICE_F_NONBLOCK).

writable pipe + SPLICE_F_NONBLOCK is a valid vmsplice call today, so I
think returning -EINVAL would still cause regressions. It happens to
be a workaround for libfuse only because libfuse falls back to
writev() when vmsplice fails, but I don't think we can assume other
callers have the same fallback.

Thanks,
Joanne

>
> --
> Askar Safin