Re: [PATCHSET][RFC] struct fd and memory safety

Jason Gunthorpe
2024-08-06T17:59:01Z

On Tue, Jul 30, 2024 at 06:09:27AM +0100, Al Viro wrote:

> * ib_uverbs_open_xrcd(). FWIW, a closer look shows that the
> damn thing is buggy - it accepts _any_ descriptor and pins the associated
> inode. mount tmpfs, open a file there, feed it to that, unmount and
> watch the show...

What happens? There is still an igrab() while it is in the red black
tree?

> AFAICS, that's done for the sake of libibverbs and
> I've no idea how it's actually used - all examples I'd been able to
> find use -1 for descriptor here. Needs to be discussed with infiniband
> folks (Sean Hefty?). For now, leave that as-is.

The design seems insane, but it is what it is from 20 years ago..

Userspace can affiliate this "xrc domain" with a file in the
filesystem. Any file. That is actually a deliberate part of the API.

This is done as some ugly way to pass xrc domain object from process A
to process B. IIRC the idea is process A will affiliate the object
with a file and then B will be able to access the shared object if B
is able to open the file.

It looks like the code keeps a red/black tree of this association, and
holds an igrab while the inode is in that tree..

Jason