arm64/mm: Fix annotated branch unbootable kernel

[PATCH] arm64/mm: Fix annotated branch unbootable kernel

Posted by Breno Leitao 1 month, 1 week ago

The arm64 kernel doesn't boot with annotated branches
(PROFILE_ANNOTATED_BRANCHES) enabled and CONFIG_DEBUG_VIRTUAL together.

Bisecting it, I found that disabling branch profiling in arch/arm64/mm
solved the problem. Narrowing down a bit further, I found that
physaddr.c is the file that needs to have branch profiling disabled to
get the machine to boot.

I suspect that it might invoke some ftrace helper very early in the boot
process and ftrace is still not enabled(!?).

Disable branch profiling for physaddr.o to allow booting an arm64
machine with CONFIG_PROFILE_ANNOTATED_BRANCHES and
CONFIG_DEBUG_VIRTUAL together.

Cc: stable@vger.kernel.org
Fixes: ec6d06efb0bac ("arm64: Add support for CONFIG_DEBUG_VIRTUAL")
Signed-off-by: Breno Leitao <leitao@debian.org>
---
Another approach is to disable profiling on all arch/arm64 code, similarly to
x86, where DISABLE_BRANCH_PROFILING is called for all arch/x86 code. See
commit 2cbb20b008dba ("tracing: Disable branch profiling in noinstr
code").
---
 arch/arm64/mm/Makefile | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/arm64/mm/Makefile b/arch/arm64/mm/Makefile
index c26489cf96cd..8bfe2451ea26 100644
--- a/arch/arm64/mm/Makefile
+++ b/arch/arm64/mm/Makefile
@@ -14,5 +14,10 @@ obj-$(CONFIG_ARM64_MTE)		+= mteswap.o
 obj-$(CONFIG_ARM64_GCS)		+= gcs.o
 KASAN_SANITIZE_physaddr.o	+= n
 
+# Branch profiling isn't noinstr-safe
+ifdef CONFIG_TRACE_BRANCH_PROFILING
+CFLAGS_physaddr.o += -DDISABLE_BRANCH_PROFILING
+endif
+
 obj-$(CONFIG_KASAN)		+= kasan_init.o
 KASAN_SANITIZE_kasan_init.o	:= n

---
base-commit: c8ebd433459bcbf068682b09544e830acd7ed222
change-id: 20251231-annotated-75de3f33cd7b

Best regards,
--  
Breno Leitao <leitao@debian.org>

Re: [PATCH] arm64/mm: Fix annotated branch unbootable kernel

Posted by Will Deacon 1 month ago

On Wed, Dec 31, 2025 at 04:44:05AM -0800, Breno Leitao wrote:
> The arm64 kernel doesn't boot with annotated branches
> (PROFILE_ANNOTATED_BRANCHES) enabled and CONFIG_DEBUG_VIRTUAL together.
> 
> Bisecting it, I found that disabling branch profiling in arch/arm64/mm
> solved the problem. Narrowing down a bit further, I found that
> physaddr.c is the file that needs to have branch profiling disabled to
> get the machine to boot.
> 
> I suspect that it might invoke some ftrace helper very early in the boot
> process and ftrace is still not enabled(!?).
> 
> Disable branch profiling for physaddr.o to allow booting an arm64
> machine with CONFIG_PROFILE_ANNOTATED_BRANCHES and
> CONFIG_DEBUG_VIRTUAL together.
> 
> Cc: stable@vger.kernel.org
> Fixes: ec6d06efb0bac ("arm64: Add support for CONFIG_DEBUG_VIRTUAL")
> Signed-off-by: Breno Leitao <leitao@debian.org>
> ---
> Another approach is to disable profiling on all arch/arm64 code, similarly to
> x86, where DISABLE_BRANCH_PROFILING is called for all arch/x86 code. See
> commit 2cbb20b008dba ("tracing: Disable branch profiling in noinstr
> code").

Yes, let's start with arch/arm64/. We know that's safe and then if
somebody wants to make it finer-grained, it's on them to figure out a
way to do it without playing whack-a-mole.

Will

Re: [PATCH] arm64/mm: Fix annotated branch unbootable kernel

Posted by Steven Rostedt 4 weeks, 1 day ago

[ Resending with my kernel.org email, as I received a bunch of messages from gmail saying it's blocking me :-p ]

On Mon, 5 Jan 2026 21:15:40 +0000
Will Deacon <will@kernel.org> wrote:

> > Another approach is to disable profiling on all arch/arm64 code, similarly to
> > x86, where DISABLE_BRANCH_PROFILING is called for all arch/x86 code. See
> > commit 2cbb20b008dba ("tracing: Disable branch profiling in noinstr
> > code").  
> 
> Yes, let's start with arch/arm64/. We know that's safe and then if
> somebody wants to make it finer-grained, it's on them to figure out a
> way to do it without playing whack-a-mole.

OK, so by adding -DDISABLE_BRANCH_PROFILING to the Makefile configs and for
the files that were audited, could be opt-in?

CFLAGS_REMOVE_<autdit_file>.o = -DDISABLE_BRANCH_PROFILING

And add that for each file that has been fully audited?

-- Steve

Re: [PATCH] arm64/mm: Fix annotated branch unbootable kernel

Posted by Breno Leitao 3 weeks, 5 days ago

Hello Steven,

On Fri, Jan 09, 2026 at 02:50:22PM -0500, Steven Rostedt wrote:
> [ Resending with my kernel.org email, as I received a bunch of messages from gmail saying it's blocking me :-p ]
> 
> On Mon, 5 Jan 2026 21:15:40 +0000
> Will Deacon <will@kernel.org> wrote:
> 
> > > Another approach is to disable profiling on all arch/arm64 code, similarly to
> > > x86, where DISABLE_BRANCH_PROFILING is called for all arch/x86 code. See
> > > commit 2cbb20b008dba ("tracing: Disable branch profiling in noinstr
> > > code").  
> > 
> > Yes, let's start with arch/arm64/. We know that's safe and then if
> > somebody wants to make it finer-grained, it's on them to figure out a
> > way to do it without playing whack-a-mole.
> 
> OK, so by adding -DDISABLE_BRANCH_PROFILING to the Makefile configs and for
> the files that were audited, could be opt-in?

How to do the audit in this case? I suppose we want to disable branch
profiling for files that have any function that would eventually call
noinstr functions, right?

Re: [PATCH] arm64/mm: Fix annotated branch unbootable kernel

Posted by Steven Rostedt 3 weeks, 5 days ago

On Mon, 12 Jan 2026 01:42:47 -0800
Breno Leitao <leitao@debian.org> wrote:

> > OK, so by adding -DDISABLE_BRANCH_PROFILING to the Makefile configs and for
> > the files that were audited, could be opt-in?  
> 
> How to do the audit in this case? I suppose we want to disable branch
> profiling for files that have any function that would eventually call
> noinstr functions, right?

IIUC, noinstr is mostly used for the transition between user space and the
kernel (interrupts, exceptions, syscalls, etc). There shouldn't be any
random calls to noinstr functions unless it's going into user space, right?

-- Steve

Re: [PATCH] arm64/mm: Fix annotated branch unbootable kernel

Posted by Steven Rostedt 4 weeks, 1 day ago

On Mon, 5 Jan 2026 21:15:40 +0000
Will Deacon <will@kernel.org> wrote:

> > Another approach is to disable profiling on all arch/arm64 code, similarly to
> > x86, where DISABLE_BRANCH_PROFILING is called for all arch/x86 code. See
> > commit 2cbb20b008dba ("tracing: Disable branch profiling in noinstr
> > code").  
> 
> Yes, let's start with arch/arm64/. We know that's safe and then if
> somebody wants to make it finer-grained, it's on them to figure out a
> way to do it without playing whack-a-mole.

OK, so by adding -DDISABLE_BRANCH_PROFILING to the Makefile configs and for
the files that were audited, could be opt-in?

CFLAGS_REMOVE_<autdit_file>.o = -DDISABLE_BRANCH_PROFILING

And add that for each file that has been fully audited?

-- Steve