linux-kernel - Re: [PATCH v2 1/2] rust: pci: skip probing VFs if driver doesn't support VFs

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Message-ID: <bba17237-2401-4e9b-912b-29d31af748e1@kernel.org>
Date: Thu, 2 Oct 2025 23:14:47 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: John Hubbard <jhubbard@...dia.com>,
 Alexandre Courbot <acourbot@...dia.com>,
 Joel Fernandes <joelagnelf@...dia.com>, Timur Tabi <ttabi@...dia.com>,
 Alistair Popple <apopple@...dia.com>, Zhi Wang <zhiw@...dia.com>,
 Surath Mitra <smitra@...dia.com>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Alex Williamson
 <alex.williamson@...hat.com>, Bjorn Helgaas <bhelgaas@...gle.com>,
 Krzysztof Wilczyński <kwilczynski@...nel.org>,
 Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>,
 Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
 nouveau@...ts.freedesktop.org, linux-pci@...r.kernel.org,
 rust-for-linux@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] rust: pci: skip probing VFs if driver doesn't
 support VFs

On 10/2/25 11:04 PM, Jason Gunthorpe wrote:
> On Thu, Oct 02, 2025 at 09:36:17PM +0200, Danilo Krummrich wrote:
>> If we want to obtain the driver's private data from a device outside the scope
>> of bus callbacks, we always need to ensure that the device is guaranteed to be
>> bound and we also need to prove the type of the private data, since a device
>> structure can't be generic over its bound driver.
> 
> pci_iov_get_pf_drvdata() does both of these things - this is what it
> is for. Please don't open code it :(

It makes no sense to use it, both of those things will be ensured in a more
generic way in the base device implementation already (which is what I meant
with layering).

Both requirements are not specific to PCI, or the specific VF -> PF use-case.

In order to guarantee soundness both of those things have to be guaranteed for
any access to the driver's private data.

I will send some patches soon, I think this will make it obvious. :)
>>> Certain conditions may be workable, some drivers seem to have
>>> preferences not to call disable, though I think that is wrong :\
>>
>> I fully agree! I was told that this is because apparently some PF drivers are
>> only loaded to enable SR-IOV and then removed to shrink the potential attack
>> surface. Personally, I think that's slightly paranoid, if the driver would not
>> do anything else than enable / disable SR-IOV, but I think we can work around
>> this use-case if people really want it.
> 
> I've heard worse reasons than that. If that is the interest I'd
> suggest they should just use VFIO and leave a userspace stub
> process..

I'm not sure I follow your proposal, can you elaborate?