Re: [PATCH] arch: um: Don't rename vmap to kernel_vmap

[David Gow <davidgow@google.com>]

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On Mon, 24 Nov 2025 at 15:49, Johannes Berg <johannes@sipsolutions.net> wrote:
>
> On Sun, 2025-11-23 at 18:07 +0100, Miguel Ojeda wrote:
> > On Sat, Nov 22, 2025 at 9:32 AM David Gow <davidgow@google.com> wrote:
> > >
> > > In order to work around the existence of a vmap symbol in libpcap, the
> > > UML makefile unconditionally redefines vmap to kernel_vmap. However,
> > > this not only affects the actual vmap symbol, but also anything else
> > > named vmap, including a number of struct members in DRM.
> > >
> > > This would not be too much of a problem, since all uses are also
> > > updated, except we now have Rust DRM bindings, which expect the
> > > corresponding Rust structs to have 'vmap' names. Since the redefinition
> > > applies in bindgen, but not to Rust code, we end up with errors such as:
> > >
> > > error[E0560]: struct `drm_gem_object_funcs` has no fields named `vmap`
> > >   --> rust/kernel/drm/gem/mod.rs:210:9
> > >
> > > Since, as far as I can tell, we no longer actually link to libpcap, it
> > > should be safe to just remove this define unconditionally.
> > >
> > > (If it's not, we can possibly either disable DRM Rust bindings under
> > > UML, or move the redefinition of vmap behind some config option.)
> > >
> > > We also take this opportunity to update the comment.
> > >
> > > Signed-off-by: David Gow <davidgow@google.com>
> >
> > Nice, thanks for this!
> >
> > Yeah, I guess we would otherwise need to do the same kind of "wild"
> > macro replacement in Rust code to support this or conditional
> > compilation, and neither sounds good.
> >
> > If it is not actually needed, then this sounds like a win-win.
> >
> > It seems it was indeed gone in commit:
> >
> >     12b8e7e69aa7 ("um: Remove obsolete pcap driver")
>
> Indeed, that was just missed during the removal, we can't link to
> libpcap any more.
>
> How do we want to take this patch in, and where is it needed? I hadn't
> planned to send a UML PR to -rc still, but I guess I _can_ if needed?
> But if anyone else wants to line it up through a tree (rust related?)
> that has pending work anyway, that seems fair too. In which case:
>
> Acked-by: Johannes Berg <johannes@sipsolutions.net>
>
> Or it's not that urgent because all this came up in -next now? I didn't
> really see (or fully understand) all the build bug reports.
>

I'm happy for this to go in via any tree. (Worst-case, we could
possibly take it via KUnit, though I'd rather not, as it's not really
KUnit-related at all.)

The issue has actually been around since probably 6.16 (c284d3e42338
("rust: drm: gem: Add GEM object abstraction")), but since it only
applies to people building Rust graphics drivers against UML, which is
not super common, it seems like it's only come up in randconfig builds
so far.

-- David

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