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

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

[David Gow]

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>
---
 arch/um/Makefile | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/arch/um/Makefile b/arch/um/Makefile
index 7be0143b5ba3..721b652ffb65 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -46,19 +46,17 @@ ARCH_INCLUDE	:= -I$(srctree)/$(SHARED_HEADERS)
 ARCH_INCLUDE	+= -I$(srctree)/$(HOST_DIR)/um/shared
 KBUILD_CPPFLAGS += -I$(srctree)/$(HOST_DIR)/um
 
-# -Dvmap=kernel_vmap prevents anything from referencing the libpcap.o symbol so
-# named - it's a common symbol in libpcap, so we get a binary which crashes.
-#
-# Same things for in6addr_loopback and mktime - found in libc. For these two we
-# only get link-time error, luckily.
+# -Dstrrchr=kernel_strrchr (as well as the various in6addr symbols) prevents
+#  anything from referencing
+# libc symbols with the same name, which can cause a linker error.
 #
 # -Dlongjmp=kernel_longjmp prevents anything from referencing the libpthread.a
 # embedded copy of longjmp, same thing for setjmp.
 #
-# These apply to USER_CFLAGS to.
+# These apply to USER_CFLAGS too.
 
 KBUILD_CFLAGS += $(CFLAGS) $(CFLAGS-y) -D__arch_um__ \
-	$(ARCH_INCLUDE) $(MODE_INCLUDE) -Dvmap=kernel_vmap	\
+	$(ARCH_INCLUDE) $(MODE_INCLUDE)	\
 	-Dlongjmp=kernel_longjmp -Dsetjmp=kernel_setjmp \
 	-Din6addr_loopback=kernel_in6addr_loopback \
 	-Din6addr_any=kernel_in6addr_any -Dstrrchr=kernel_strrchr \
-- 
2.52.0.rc2.455.g230fcf2819-goog

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

[Miguel Ojeda]

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")

So it sounds reasonable to me assuming I am not missing anything,
which I may be... FWIW:

Acked-by: Miguel Ojeda <ojeda@kernel.org>

Cheers,
Miguel

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

[Johannes Berg]

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.

johannes

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

[David Gow]

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

[Johannes Berg]

On Tue, 2025-11-25 at 15:36 +0800, David Gow wrote:
> > 
> > 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.

Oh, interesting, OK. I guess then given that it's not super important
and how late we're in the game, I'll just throw it into the (relatively
small) pile we have for UML for -next. Given that we removed the pcap
driver in 6.11 (12b8e7e69aa7a) I guess we could even ask stable to take
it, but it's not even that important until someone wants to test the
rust DRM stuff in kunit or something :)

johannes

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

[David Gow]

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On Tue, 25 Nov 2025 at 15:40, Johannes Berg <johannes@sipsolutions.net> wrote:
>
> On Tue, 2025-11-25 at 15:36 +0800, David Gow wrote:
> > >
> > > 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.
>
> Oh, interesting, OK. I guess then given that it's not super important
> and how late we're in the game, I'll just throw it into the (relatively
> small) pile we have for UML for -next. Given that we removed the pcap
> driver in 6.11 (12b8e7e69aa7a) I guess we could even ask stable to take
> it, but it's not even that important until someone wants to test the
> rust DRM stuff in kunit or something :)
>

Sounds good to me. Throwing the Fixes: tag in wouldn't hurt at least
(and I do think some of the Rust DRM stuff is growing some KUnit
tests, so it'll be nice to have going forward, even if they can be
tested under other architectures).

Cheers,
-- David

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