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[PATCH v1 3/3] LoongArch: Fix unreachable instruction warnings about entry functions

Tiezhu Yang posted 3 patches 1 month ago
There is a newer version of this series
[PATCH v1 3/3] LoongArch: Fix unreachable instruction warnings about entry functions
Posted by Tiezhu Yang 1 month ago 
When compiling with LLVM and CONFIG_LTO_CLANG is set, there exist the
following objtool warnings:

  vmlinux.o: warning: objtool: kernel_entry+0x0: unreachable instruction
  vmlinux.o: warning: objtool: smpboot_entry+0x0: unreachable instruction

kernel_entry() and smpboot_entry() are in arch/loongarch/kernel/head.S,
there is "OBJECT_FILES_NON_STANDARD_head.o := y" to skip objtool checking
for head.o, but the STACK_FRAME_NON_STANDARD macro does not work for link
time validation of vmlinux.o according to objtool documentation, just give
a proper unwind hint to silence the warnings.

By the way, ASM_BUG() can be removed due to unnecessary, otherwise there
are following warnings:

  kernel_entry+0xf4: start_kernel() missing __noreturn
  in .c/.h or NORETURN() in noreturns.h

  smpboot_entry+0x68: start_secondary() missing __noreturn
  in .c/.h or NORETURN() in noreturns.h

This is because the previous instructions of kernel_entry+0xf4 and
smpboot_entry+0x68 are the 'bl' instructions, start_kernel() and
start_secondary() are the respective call destination symbols which
are noreturn functions, then the 'bl' instructions are already marked
as dead end in annotate_call_site().

For now, it is time to remove "OBJECT_FILES_NON_STANDARD_head.o := y"
in arch/loongarch/kernel/Makefile.

Link: https://lore.kernel.org/lkml/20250814083651.GR4067720@noisy.programming.kicks-ass.net/
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
---
 arch/loongarch/kernel/Makefile | 2 --
 arch/loongarch/kernel/head.S   | 6 ++----
 2 files changed, 2 insertions(+), 6 deletions(-)

diff --git a/arch/loongarch/kernel/Makefile b/arch/loongarch/kernel/Makefile
index 6f5a4574a911..4302c5b0a201 100644
--- a/arch/loongarch/kernel/Makefile
+++ b/arch/loongarch/kernel/Makefile
@@ -3,8 +3,6 @@
 # Makefile for the Linux/LoongArch kernel.
 #
 
-OBJECT_FILES_NON_STANDARD_head.o := y
-
 always-$(KBUILD_BUILTIN)	:= vmlinux.lds
 
 obj-y		+= head.o cpu-probe.o cacheinfo.o env.o setup.o entry.o genex.o \
diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
index e3865e92a917..a11880f3a7e1 100644
--- a/arch/loongarch/kernel/head.S
+++ b/arch/loongarch/kernel/head.S
@@ -42,6 +42,7 @@ SYM_DATA(kernel_fsize, .long _kernel_fsize);
 	.align 12
 
 SYM_CODE_START(kernel_entry)			# kernel entry point
+	UNWIND_HINT_UNDEFINED
 
 	/* Config direct window and set PG */
 	SETUP_DMWINS	t0
@@ -109,8 +110,6 @@ SYM_CODE_START(kernel_entry)			# kernel entry point
 #endif
 
 	bl		start_kernel
-	ASM_BUG()
-
 SYM_CODE_END(kernel_entry)
 
 #ifdef CONFIG_SMP
@@ -120,6 +119,7 @@ SYM_CODE_END(kernel_entry)
  * function after setting up the stack and tp registers.
  */
 SYM_CODE_START(smpboot_entry)
+	UNWIND_HINT_UNDEFINED
 
 	SETUP_DMWINS	t0
 	JUMP_VIRT_ADDR	t0, t1
@@ -142,8 +142,6 @@ SYM_CODE_START(smpboot_entry)
 	ld.d		tp, t0, CPU_BOOT_TINFO
 
 	bl		start_secondary
-	ASM_BUG()
-
 SYM_CODE_END(smpboot_entry)
 
 #endif /* CONFIG_SMP */
-- 
2.42.0
Re: [PATCH v1 3/3] LoongArch: Fix unreachable instruction warnings about entry functions
Posted by Josh Poimboeuf 4 weeks, 1 day ago 
On Mon, Sep 01, 2025 at 03:21:56PM +0800, Tiezhu Yang wrote:
> +++ b/arch/loongarch/kernel/Makefile
> @@ -3,8 +3,6 @@
>  # Makefile for the Linux/LoongArch kernel.
>  #
>  
> -OBJECT_FILES_NON_STANDARD_head.o := y
> -
>  always-$(KBUILD_BUILTIN)	:= vmlinux.lds
>  
>  obj-y		+= head.o cpu-probe.o cacheinfo.o env.o setup.o entry.o genex.o \
> diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
> index e3865e92a917..a11880f3a7e1 100644
> --- a/arch/loongarch/kernel/head.S
> +++ b/arch/loongarch/kernel/head.S
> @@ -42,6 +42,7 @@ SYM_DATA(kernel_fsize, .long _kernel_fsize);
>  	.align 12
>  
>  SYM_CODE_START(kernel_entry)			# kernel entry point
> +	UNWIND_HINT_UNDEFINED

Should this not be UNWIND_HINT_END_OF_STACK?

I notice Loongarch doesn't seem to use that anywhere.  How does any ORC
unwind succeed?  UNWIND_HINT_UNDEFINED sets an error condition which
should cause a livepatch transition to stall.

-- 
Josh
Re: [PATCH v1 3/3] LoongArch: Fix unreachable instruction warnings about entry functions
Posted by Tiezhu Yang 4 weeks, 1 day ago 
On 2025/9/4 上午3:22, Josh Poimboeuf wrote:
> On Mon, Sep 01, 2025 at 03:21:56PM +0800, Tiezhu Yang wrote:
>> +++ b/arch/loongarch/kernel/Makefile
>> @@ -3,8 +3,6 @@
>>   # Makefile for the Linux/LoongArch kernel.
>>   #
>>   
>> -OBJECT_FILES_NON_STANDARD_head.o := y
>> -
>>   always-$(KBUILD_BUILTIN)	:= vmlinux.lds
>>   
>>   obj-y		+= head.o cpu-probe.o cacheinfo.o env.o setup.o entry.o genex.o \
>> diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
>> index e3865e92a917..a11880f3a7e1 100644
>> --- a/arch/loongarch/kernel/head.S
>> +++ b/arch/loongarch/kernel/head.S
>> @@ -42,6 +42,7 @@ SYM_DATA(kernel_fsize, .long _kernel_fsize);
>>   	.align 12
>>   
>>   SYM_CODE_START(kernel_entry)			# kernel entry point
>> +	UNWIND_HINT_UNDEFINED
> 
> Should this not be UNWIND_HINT_END_OF_STACK?

Yes, makes sense, will do it in the next version.

> I notice Loongarch doesn't seem to use that anywhere.  How does any ORC
> unwind succeed?  UNWIND_HINT_UNDEFINED sets an error condition which
> should cause a livepatch transition to stall.

Actually, kernel_entry() or smpboot_entry() is recognized as the last
frame, because at this point is_entry_func() is true and 
state->stack_info.type = STACK_TYPE_UNKNOWN in unwind_next_frame() of
arch/loongarch/kernel/unwind_orc.c.

Call Trace:
[<90000000031a36b4>] show_stack+0x5c/0x180
[<900000000319d4b0>] dump_stack_lvl+0x6c/0x9c
[<900000000458d3e0>] nmi_cpu_backtrace+0x160/0x168
[<90000000031a3b6c>] handle_backtrace+0xc/0x40
[<90000000032b1078>] __flush_smp_call_function_queue+0xd0/0x330
[<90000000031aec40>] loongson_ipi_interrupt+0xb0/0x168
[<900000000325b4d4>] __handle_irq_event_percpu+0x54/0x1a8
[<900000000325b63c>] handle_irq_event_percpu+0x14/0x80
[<9000000003262a48>] handle_percpu_irq+0x50/0xa0
[<900000000325aad4>] generic_handle_domain_irq+0x2c/0x80
[<9000000003bc4d24>] handle_cpu_irq+0x64/0xa0
[<90000000045a350c>] handle_loongarch_irq+0x2c/0x48
[<90000000045a35e4>] do_vint+0xbc/0xe0
[<90000000031a1624>] handle_vint+0x144/0x1e4
[<900000000321e2e8>] _nohz_idle_balance.isra.0+0x230/0x3a0
[<90000000031cd004>] handle_softirqs+0x10c/0x298
[<90000000031cd2e8>] __irq_exit_rcu+0x100/0x160
[<90000000045a35a4>] do_vint+0x7c/0xe0
[<90000000045a55cc>] idle_exit+0x0/0x4
[<90000000045a55d8>] arch_cpu_idle+0x8/0x30
[<90000000045a5698>] default_idle_call+0x18/0x50
[<90000000032250b0>] do_idle+0xb8/0x130
[<9000000003225374>] cpu_startup_entry+0x2c/0x38
[<90000000045a6240>] kernel_entry_end+0xdc/0xe0
[<90000000045c0d44>] start_kernel+0x65c/0x660
[<90000000045a60f0>] kernel_entry+0xf0/0xf8

Thanks,
Tiezhu
Re: [PATCH v1 3/3] LoongArch: Fix unreachable instruction warnings about entry functions
Posted by Josh Poimboeuf 4 weeks ago 
On Thu, Sep 04, 2025 at 11:18:28AM +0800, Tiezhu Yang wrote:
> On 2025/9/4 上午3:22, Josh Poimboeuf wrote:
> > On Mon, Sep 01, 2025 at 03:21:56PM +0800, Tiezhu Yang wrote:
> > > +++ b/arch/loongarch/kernel/Makefile
> > > @@ -3,8 +3,6 @@
> > >   # Makefile for the Linux/LoongArch kernel.
> > >   #
> > > -OBJECT_FILES_NON_STANDARD_head.o := y
> > > -
> > >   always-$(KBUILD_BUILTIN)	:= vmlinux.lds
> > >   obj-y		+= head.o cpu-probe.o cacheinfo.o env.o setup.o entry.o genex.o \
> > > diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
> > > index e3865e92a917..a11880f3a7e1 100644
> > > --- a/arch/loongarch/kernel/head.S
> > > +++ b/arch/loongarch/kernel/head.S
> > > @@ -42,6 +42,7 @@ SYM_DATA(kernel_fsize, .long _kernel_fsize);
> > >   	.align 12
> > >   SYM_CODE_START(kernel_entry)			# kernel entry point
> > > +	UNWIND_HINT_UNDEFINED
> > 
> > Should this not be UNWIND_HINT_END_OF_STACK?
> 
> Yes, makes sense, will do it in the next version.
> 
> > I notice Loongarch doesn't seem to use that anywhere.  How does any ORC
> > unwind succeed?  UNWIND_HINT_UNDEFINED sets an error condition which
> > should cause a livepatch transition to stall.
> 
> Actually, kernel_entry() or smpboot_entry() is recognized as the last
> frame, because at this point is_entry_func() is true and
> state->stack_info.type = STACK_TYPE_UNKNOWN in unwind_next_frame() of
> arch/loongarch/kernel/unwind_orc.c.

I think you can get rid of is_entry_func() in favor of just using
UNWIND_HINT_END_OF_STACK at all the entry points.

-- 
Josh
Re: [PATCH v1 3/3] LoongArch: Fix unreachable instruction warnings about entry functions
Posted by Jinyang He 4 weeks, 1 day ago 
On 2025-09-04 03:22, Josh Poimboeuf wrote:

> On Mon, Sep 01, 2025 at 03:21:56PM +0800, Tiezhu Yang wrote:
>> +++ b/arch/loongarch/kernel/Makefile
>> @@ -3,8 +3,6 @@
>>   # Makefile for the Linux/LoongArch kernel.
>>   #
>>   
>> -OBJECT_FILES_NON_STANDARD_head.o := y
>> -
>>   always-$(KBUILD_BUILTIN)	:= vmlinux.lds
>>   
>>   obj-y		+= head.o cpu-probe.o cacheinfo.o env.o setup.o entry.o genex.o \
>> diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
>> index e3865e92a917..a11880f3a7e1 100644
>> --- a/arch/loongarch/kernel/head.S
>> +++ b/arch/loongarch/kernel/head.S
>> @@ -42,6 +42,7 @@ SYM_DATA(kernel_fsize, .long _kernel_fsize);
>>   	.align 12
>>   
>>   SYM_CODE_START(kernel_entry)			# kernel entry point
>> +	UNWIND_HINT_UNDEFINED
> Should this not be UNWIND_HINT_END_OF_STACK?
>
> I notice Loongarch doesn't seem to use that anywhere.  How does any ORC
> unwind succeed?
IIRC, unwinder stops when fail when call __kernel_text_address(bt_address),
or success when call 
arch/loongarch/kernel/unwind_orc.c::is_entry_func(bt_address).
> UNWIND_HINT_UNDEFINED sets an error condition which
> should cause a livepatch transition to stall.
>
Re: [PATCH v1 3/3] LoongArch: Fix unreachable instruction warnings about entry functions
Posted by Peter Zijlstra 1 month ago 
On Mon, Sep 01, 2025 at 03:21:56PM +0800, Tiezhu Yang wrote:
> When compiling with LLVM and CONFIG_LTO_CLANG is set, there exist the
> following objtool warnings:
> 
>   vmlinux.o: warning: objtool: kernel_entry+0x0: unreachable instruction
>   vmlinux.o: warning: objtool: smpboot_entry+0x0: unreachable instruction
> 
> kernel_entry() and smpboot_entry() are in arch/loongarch/kernel/head.S,
> there is "OBJECT_FILES_NON_STANDARD_head.o := y" to skip objtool checking
> for head.o, but the STACK_FRAME_NON_STANDARD macro does not work for link
> time validation of vmlinux.o according to objtool documentation, just give
> a proper unwind hint to silence the warnings.
> 
> By the way, ASM_BUG() can be removed due to unnecessary, otherwise there
> are following warnings:
> 
>   kernel_entry+0xf4: start_kernel() missing __noreturn
>   in .c/.h or NORETURN() in noreturns.h
> 
>   smpboot_entry+0x68: start_secondary() missing __noreturn
>   in .c/.h or NORETURN() in noreturns.h
> 
> This is because the previous instructions of kernel_entry+0xf4 and
> smpboot_entry+0x68 are the 'bl' instructions, start_kernel() and
> start_secondary() are the respective call destination symbols which
> are noreturn functions, then the 'bl' instructions are already marked
> as dead end in annotate_call_site().
> 
> For now, it is time to remove "OBJECT_FILES_NON_STANDARD_head.o := y"
> in arch/loongarch/kernel/Makefile.
> 
> Link: https://lore.kernel.org/lkml/20250814083651.GR4067720@noisy.programming.kicks-ass.net/
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>

Right, this looks good.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

> ---
>  arch/loongarch/kernel/Makefile | 2 --
>  arch/loongarch/kernel/head.S   | 6 ++----
>  2 files changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/loongarch/kernel/Makefile b/arch/loongarch/kernel/Makefile
> index 6f5a4574a911..4302c5b0a201 100644
> --- a/arch/loongarch/kernel/Makefile
> +++ b/arch/loongarch/kernel/Makefile
> @@ -3,8 +3,6 @@
>  # Makefile for the Linux/LoongArch kernel.
>  #
>  
> -OBJECT_FILES_NON_STANDARD_head.o := y
> -
>  always-$(KBUILD_BUILTIN)	:= vmlinux.lds
>  
>  obj-y		+= head.o cpu-probe.o cacheinfo.o env.o setup.o entry.o genex.o \
> diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
> index e3865e92a917..a11880f3a7e1 100644
> --- a/arch/loongarch/kernel/head.S
> +++ b/arch/loongarch/kernel/head.S
> @@ -42,6 +42,7 @@ SYM_DATA(kernel_fsize, .long _kernel_fsize);
>  	.align 12
>  
>  SYM_CODE_START(kernel_entry)			# kernel entry point
> +	UNWIND_HINT_UNDEFINED
>  
>  	/* Config direct window and set PG */
>  	SETUP_DMWINS	t0
> @@ -109,8 +110,6 @@ SYM_CODE_START(kernel_entry)			# kernel entry point
>  #endif
>  
>  	bl		start_kernel
> -	ASM_BUG()
> -
>  SYM_CODE_END(kernel_entry)
>  
>  #ifdef CONFIG_SMP
> @@ -120,6 +119,7 @@ SYM_CODE_END(kernel_entry)
>   * function after setting up the stack and tp registers.
>   */
>  SYM_CODE_START(smpboot_entry)
> +	UNWIND_HINT_UNDEFINED
>  
>  	SETUP_DMWINS	t0
>  	JUMP_VIRT_ADDR	t0, t1
> @@ -142,8 +142,6 @@ SYM_CODE_START(smpboot_entry)
>  	ld.d		tp, t0, CPU_BOOT_TINFO
>  
>  	bl		start_secondary
> -	ASM_BUG()
> -
>  SYM_CODE_END(smpboot_entry)
>  
>  #endif /* CONFIG_SMP */
> -- 
> 2.42.0
>

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