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

[Askar Safin <safinaskar@gmail.com>]

Joanne Koong <joannelkoong@gmail.com>:
> > 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).

-- 
Askar Safin