Re: [PATCHSET v2 0/6] Allow allocated direct descriptors

[Stefan Metzmacher <metze@samba.org>]

Hi Jens,

this looks very useful, thanks!

I have an additional feature request to make this even more useful...

IO_OP_ACCEPT allows a fixed descriptor for the listen socket
and then can generate a fixed descriptor for the accepted connection,
correct?

It would be extremely useful to also allow that pattern
for IO_OP_OPENAT[2], which currently is not able to get
a fixed descriptor for the dirfd argument (this also applies to
IO_OP_STATX, IO_OP_UNLINK and all others taking a dirfd).

Being able use such a sequence:

OPENTAT2(AT_FDCWD, "directory") => 1 (fixed)
STATX(1 (fixed))
FGETXATTR(1 (fixed)
OPENAT2(1 (fixed), "file") => 2 (fixed)
STATX(2 (fixed))
FGETXATTR(2 (fixed))
CLOSE(1 (fixed)
DUP( 2 (fixed)) => per-process fd for ("file")

I looked briefly how to implement that.
But set_nameidata() takes 'int dfd' to store the value
and it's used later somewhere deep down the stack.
And makes it too complex for me to create patches :-(

It would great if someone could have a look how to make this work.


> Currently using direct descriptors with open or accept requires the
> application to manage the descriptor space, picking which slot to use
> for any given file. However, there are cases where it's useful to just
> get a direct descriptor and not care about which value it is, instead
> just return it like a normal open or accept would.
> 
> This will also be useful for multishot accept support, where allocated
> direct descriptors are a requirement to make that feature work with
> these kinds of files.
> 
> This adds support for allocating a new fixed descriptor. This is chosen
> by passing in IORING_FILE_INDEX_ALLOC as the fixed slot, which is beyond
> any valid value for the file_index.

I guess that would not work for linked request, as we don't know the
actual id before submitting the requests.

Thanks!
metze