[PATCH] um: Allow multiple symbol definitions for GCOV builds

[PATCH] um: Allow multiple symbol definitions for GCOV builds

[Alex Hung]

GCOV-enabled UML builds link GCC's libgcov into the final vmlinux
image. libgcov provides helper symbols such as mangle_path(), which
collide with kernel symbols of the same name (for example the one in
fs/seq_file.o). This makes the link fail:

  ld: libgcov.a(_gcov.o): in function `mangle_path':
  multiple definition of `mangle_path';
  fs/seq_file.o: first defined here

Pass --allow-multiple-definition to the linker when CONFIG_GCOV is set
so the final UML link succeeds while keeping the kernel-provided
definition.

Assisted-by: Copilot:Claude-Opus-4.8
Signed-off-by: Alex Hung <alex.hung@amd.com>
---
 arch/um/Makefile | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/um/Makefile b/arch/um/Makefile
index 721b652ffb65..09c8354c8b65 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -137,6 +137,13 @@ ifeq ($(CONFIG_LD_IS_BFD),y)
 LDFLAGS_EXECSTACK += $(call ld-option,--no-warn-rwx-segments)
 endif
 
+# GCC's libgcov defines symbols that can collide with UML kernel code, such as
+# mangle_path(). Allow the final link to keep the kernel definitions when GCOV
+# pulls in libgcov.
+ifdef CONFIG_GCOV
+	KBUILD_LDFLAGS += --allow-multiple-definition
+endif
+
 LD_FLAGS_CMDLINE = $(foreach opt,$(KBUILD_LDFLAGS) $(LDFLAGS_EXECSTACK),-Wl,$(opt))
 
 # Used by link-vmlinux.sh which has special support for um link
-- 
2.43.0

Re: [PATCH] um: Allow multiple symbol definitions for GCOV builds

[Johannes Berg]

On Wed, 2026-07-08 at 16:40 -0600, Alex Hung wrote:
> GCOV-enabled UML builds link GCC's libgcov into the final vmlinux
> image. libgcov provides helper symbols such as mangle_path(), which
> collide with kernel symbols of the same name (for example the one in
> fs/seq_file.o). This makes the link fail:
> 
>   ld: libgcov.a(_gcov.o): in function `mangle_path':
>   multiple definition of `mangle_path';
>   fs/seq_file.o: first defined here
> 
> Pass --allow-multiple-definition to the linker when CONFIG_GCOV is set
> so the final UML link succeeds while keeping the kernel-provided
> definition.
> 

I cannot see how that would possibly result in correct behaviour (beyond
being able to link)??

johannes

Re: [PATCH] um: Allow multiple symbol definitions for GCOV builds

[Alex Hung]

On 7/21/26 15:58, Johannes Berg wrote:
> On Wed, 2026-07-08 at 16:40 -0600, Alex Hung wrote:
>> GCOV-enabled UML builds link GCC's libgcov into the final vmlinux
>> image. libgcov provides helper symbols such as mangle_path(), which
>> collide with kernel symbols of the same name (for example the one in
>> fs/seq_file.o). This makes the link fail:
>>
>>    ld: libgcov.a(_gcov.o): in function `mangle_path':
>>    multiple definition of `mangle_path';
>>    fs/seq_file.o: first defined here
>>
>> Pass --allow-multiple-definition to the linker when CONFIG_GCOV is set
>> so the final UML link succeeds while keeping the kernel-provided
>> definition.
>>
> 
> I cannot see how that would possibly result in correct behaviour (beyond
> being able to link)??


Hi Johannes,

Good point. --allow-multiple-definition silences the linker error, but 
picking kernel or libgcov's mangle_path() will break the other at runtime.

I saw your previous patch 
(https://lkml.iu.edu/hypermail/linux/kernel/2103.1/06604.html) to rename 
mangle_path() to seq_mangle_path(). How about reviving it to avoid name 
conflicts instead?

Alex

> 
> johannes

Re: [PATCH] um: Allow multiple symbol definitions for GCOV builds

[Johannes Berg]

Hi,

On Tue, 2026-07-21 at 16:23 -0600, Alex Hung wrote:
> > 
> > I cannot see how that would possibly result in correct behaviour (beyond
> > being able to link)??

> Good point. --allow-multiple-definition silences the linker error, but 
> picking kernel or libgcov's mangle_path() will break the other at runtime.
> 
> I saw your previous patch 
> (https://lkml.iu.edu/hypermail/linux/kernel/2103.1/06604.html) to rename 
> mangle_path() to seq_mangle_path(). How about reviving it to avoid name 
> conflicts instead?

Honestly, I completely forgot about that. Looks like I ran into the
typical "nobody wants to merge it" problem ...

johannes

Re: [PATCH] um: Allow multiple symbol definitions for GCOV builds

[Alex Hung]

On 7/22/26 08:02, Johannes Berg wrote:
> Hi,
> 
> On Tue, 2026-07-21 at 16:23 -0600, Alex Hung wrote:
>>>
>>> I cannot see how that would possibly result in correct behaviour (beyond
>>> being able to link)??
> 
>> Good point. --allow-multiple-definition silences the linker error, but
>> picking kernel or libgcov's mangle_path() will break the other at runtime.
>>
>> I saw your previous patch
>> (https://lkml.iu.edu/hypermail/linux/kernel/2103.1/06604.html) to rename
>> mangle_path() to seq_mangle_path(). How about reviving it to avoid name
>> conflicts instead?
> 
> Honestly, I completely forgot about that. Looks like I ran into the
> typical "nobody wants to merge it" problem ...
> 
> johannes

Is there any concerns or blockers to merge them? I sure want to revive them.

The series may also have a solution for the other patch I sent 
(https://lore.kernel.org/linux-um/eea36454-cbbe-4690-980d-b4ec9423f3a0@amd.com/T/#u).

I will test the series and report here later.

Re: [PATCH] um: Allow multiple symbol definitions for GCOV builds

[Johannes Berg]

On Wed, 2026-07-22 at 10:09 -0600, Alex Hung wrote:
> 
> On 7/22/26 08:02, Johannes Berg wrote:
> > Hi,
> > 
> > On Tue, 2026-07-21 at 16:23 -0600, Alex Hung wrote:
> > > > 
> > > > I cannot see how that would possibly result in correct behaviour (beyond
> > > > being able to link)??
> > 
> > > Good point. --allow-multiple-definition silences the linker error, but
> > > picking kernel or libgcov's mangle_path() will break the other at runtime.
> > > 
> > > I saw your previous patch
> > > (https://lkml.iu.edu/hypermail/linux/kernel/2103.1/06604.html) to rename
> > > mangle_path() to seq_mangle_path(). How about reviving it to avoid name
> > > conflicts instead?
> > 
> > Honestly, I completely forgot about that. Looks like I ran into the
> > typical "nobody wants to merge it" problem ...
> > 
> > johannes
> 
> Is there any concerns or blockers to merge them?

I don't _think_ so, I guess it ended up mostly procedural, since it was
across different maintainers etc., and I lost interest because we
stopped trying to use gcov.

> I sure want to revive them.

Sure, sounds reasonable, feel free, of course!

> The series may also have a solution for the other patch I sent 
> (https://lore.kernel.org/linux-um/eea36454-cbbe-4690-980d-b4ec9423f3a0@amd.com/T/#u).

That one we should just apply for ARCH=um, seems reasonable to me.

> I will test the series and report here later.
> 

Thanks. Please just resend whatever you think should be included, maybe
first all the UML stuff separately.

johannes

Re: [PATCH] um: Allow multiple symbol definitions for GCOV builds

[Alex Hung]

On 7/22/26 10:14, Johannes Berg wrote:
> On Wed, 2026-07-22 at 10:09 -0600, Alex Hung wrote:
>>
>> On 7/22/26 08:02, Johannes Berg wrote:
>>> Hi,
>>>
>>> On Tue, 2026-07-21 at 16:23 -0600, Alex Hung wrote:
>>>>>
>>>>> I cannot see how that would possibly result in correct behaviour (beyond
>>>>> being able to link)??
>>>
>>>> Good point. --allow-multiple-definition silences the linker error, but
>>>> picking kernel or libgcov's mangle_path() will break the other at runtime.
>>>>
>>>> I saw your previous patch
>>>> (https://lkml.iu.edu/hypermail/linux/kernel/2103.1/06604.html) to rename
>>>> mangle_path() to seq_mangle_path(). How about reviving it to avoid name
>>>> conflicts instead?
>>>
>>> Honestly, I completely forgot about that. Looks like I ran into the
>>> typical "nobody wants to merge it" problem ...
>>>
>>> johannes
>>
>> Is there any concerns or blockers to merge them?
> 
> I don't _think_ so, I guess it ended up mostly procedural, since it was
> across different maintainers etc., and I lost interest because we
> stopped trying to use gcov.
> 
>> I sure want to revive them.
> 
> Sure, sounds reasonable, feel free, of course!
> 
>> The series may also have a solution for the other patch I sent
>> (https://lore.kernel.org/linux-um/eea36454-cbbe-4690-980d-b4ec9423f3a0@amd.com/T/#u).
> 
> That one we should just apply for ARCH=um, seems reasonable to me.
> 
>> I will test the series and report here later.
>>
> 
> Thanks. Please just resend whatever you think should be included, maybe
> first all the UML stuff separately.

Hi Johannes,

I think the two patches are relevant today

   um: fix CONFIG_GCOV for built-in code
   seq_file: rename mangle_path to seq_mangle_path

I will update the patch date and add my SoB, and then send them to 
mailing list, if that's okay.

> 
> johannes

Re: [PATCH] um: Allow multiple symbol definitions for GCOV builds

[Johannes Berg]

On Thu, 2026-07-23 at 18:44 -0600, Alex Hung wrote:
> 
> 
> I think the two patches are relevant today
> 
>    um: fix CONFIG_GCOV for built-in code
>    seq_file: rename mangle_path to seq_mangle_path
> 
> I will update the patch date and add my SoB, and then send them to 
> mailing list, if that's okay.

Sure. The first is probably the same as your prior RFC anyway?

I'd probably send them separately - good chance they might (need to) go
through different trees.

johannes