[PATCH] nios2: Implement _THIS_IP_ using inline asm

[PATCH] nios2: Implement _THIS_IP_ using inline asm

[Marco Elver]

Both GCC [1] and Clang [2] consider the generic version of _THIS_IP_ to
be broken:

        #define _THIS_IP_  ({ __label__ __here; __here: (unsigned long)&&__here; })

In particular, the address of a label is only expected to be used with a
computed goto.

While the generic version more or less works today, it is known to be
brittle and may break with current and future optimizations. For
example, Clang -O2 always returns 1 when this function is inlined:

        static inline unsigned long get_ip(void)
        { return ({ __label__ __here; __here: (unsigned long)&&__here; }); }

Fix it by overriding _THIS_IP_ in <asm/linkage.h> (which is included by
<linux/instruction_pointer.h>) using an architecture-specific inline asm
version. Additionally, avoiding taking the address of a label prevents
compilers from emitting spurious indirect branch targets (e.g. ENDBR or
BTI) under control-flow integrity schemes.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120071 [1]
Link: https://github.com/llvm/llvm-project/issues/138272 [2]
Signed-off-by: Marco Elver <elver@google.com>
---
 arch/nios2/include/asm/linkage.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/nios2/include/asm/linkage.h b/arch/nios2/include/asm/linkage.h
index 211302301a8a..c4073235852b 100644
--- a/arch/nios2/include/asm/linkage.h
+++ b/arch/nios2/include/asm/linkage.h
@@ -12,4 +12,6 @@
 #define __ALIGN .align 4
 #define __ALIGN_STR ".align 4"
 
+#define _THIS_IP_ ({ unsigned long __ip; asm volatile("nextpc %0" : "=r" (__ip)); __ip; })
+
 #endif
-- 
2.54.0.746.g67dd491aae-goog

Re: [PATCH] nios2: Implement _THIS_IP_ using inline asm

[David Laight]

On Thu, 21 May 2026 14:23:55 +0200
Marco Elver <elver@google.com> wrote:

> Both GCC [1] and Clang [2] consider the generic version of _THIS_IP_ to
> be broken:
> 
>         #define _THIS_IP_  ({ __label__ __here; __here: (unsigned long)&&__here; })
> 
> In particular, the address of a label is only expected to be used with a
> computed goto.
> 
> While the generic version more or less works today, it is known to be
> brittle and may break with current and future optimizations. For
> example, Clang -O2 always returns 1 when this function is inlined:
> 
>         static inline unsigned long get_ip(void)
>         { return ({ __label__ __here; __here: (unsigned long)&&__here; }); }
> 
> Fix it by overriding _THIS_IP_ in <asm/linkage.h> (which is included by
> <linux/instruction_pointer.h>) using an architecture-specific inline asm
> version. Additionally, avoiding taking the address of a label prevents
> compilers from emitting spurious indirect branch targets (e.g. ENDBR or
> BTI) under control-flow integrity schemes.
> 
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120071 [1]
> Link: https://github.com/llvm/llvm-project/issues/138272 [2]
> Signed-off-by: Marco Elver <elver@google.com>

I know more than enough about the Nios-II instruction set to know it
is the correct opcode, so: 

Reviewed-by: David Laight <david.laight.linux@gmail.com>

> ---
>  arch/nios2/include/asm/linkage.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/nios2/include/asm/linkage.h b/arch/nios2/include/asm/linkage.h
> index 211302301a8a..c4073235852b 100644
> --- a/arch/nios2/include/asm/linkage.h
> +++ b/arch/nios2/include/asm/linkage.h
> @@ -12,4 +12,6 @@
>  #define __ALIGN .align 4
>  #define __ALIGN_STR ".align 4"
>  
> +#define _THIS_IP_ ({ unsigned long __ip; asm volatile("nextpc %0" : "=r" (__ip)); __ip; })
> +
>  #endif

Re: [PATCH] nios2: Implement _THIS_IP_ using inline asm

[Dinh Nguyen]

On 5/21/26 11:33, David Laight wrote:
> On Thu, 21 May 2026 14:23:55 +0200
> Marco Elver <elver@google.com> wrote:
> 
>> Both GCC [1] and Clang [2] consider the generic version of _THIS_IP_ to
>> be broken:
>>
>>          #define _THIS_IP_  ({ __label__ __here; __here: (unsigned long)&&__here; })
>>
>> In particular, the address of a label is only expected to be used with a
>> computed goto.
>>
>> While the generic version more or less works today, it is known to be
>> brittle and may break with current and future optimizations. For
>> example, Clang -O2 always returns 1 when this function is inlined:
>>
>>          static inline unsigned long get_ip(void)
>>          { return ({ __label__ __here; __here: (unsigned long)&&__here; }); }
>>
>> Fix it by overriding _THIS_IP_ in <asm/linkage.h> (which is included by
>> <linux/instruction_pointer.h>) using an architecture-specific inline asm
>> version. Additionally, avoiding taking the address of a label prevents
>> compilers from emitting spurious indirect branch targets (e.g. ENDBR or
>> BTI) under control-flow integrity schemes.
>>
>> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120071 [1]
>> Link: https://github.com/llvm/llvm-project/issues/138272 [2]
>> Signed-off-by: Marco Elver <elver@google.com>
> 
> I know more than enough about the Nios-II instruction set to know it
> is the correct opcode, so:
> 
> Reviewed-by: David Laight <david.laight.linux@gmail.com>
> 
>> ---
>>   arch/nios2/include/asm/linkage.h | 2 ++
>>   1 file changed, 2 insertions(+)
>>

Applied!

Thanks,
Dinh