[PATCH] tracing: fprobe: Remove __packed from generic __fprobe_header

[PATCH] tracing: fprobe: Remove __packed from generic __fprobe_header

[Markus Schneider-Pargmann (The Capable Hub)]

fp pointer and unsigned long have the same size on all relevant
architectures that build Linux. Furthermore this struct is only used in
architectures that do not set ARCH_DEFINE_ENCODE_FPROBE_HEADER which is
set only for 64bit architectures (apart from LoongArch).

Both fields are aligned on these architectures so the struct with
__packed and without it are the same.

Remove the __packed as it is unnecessary.

Fixes: 4346ba160409 ("fprobe: Rewrite fprobe on function-graph tracer")
Signed-off-by: Markus Schneider-Pargmann (The Capable Hub) <msp@baylibre.com>
---
 kernel/trace/fprobe.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
index cc49ebd2a773..21751dcdb7b9 100644
--- a/kernel/trace/fprobe.c
+++ b/kernel/trace/fprobe.c
@@ -181,7 +181,7 @@ static inline void read_fprobe_header(unsigned long *stack,
 struct __fprobe_header {
 	struct fprobe *fp;
 	unsigned long size_words;
-} __packed;
+};
 
 #define FPROBE_HEADER_SIZE_IN_LONG	SIZE_IN_LONG(sizeof(struct __fprobe_header))
 

---
base-commit: 254f49634ee16a731174d2ae34bc50bd5f45e731
change-id: 20260427-topic-fprobe-packed-v7-1-f44f9bbdedf6

Best regards,
--  
Markus Schneider-Pargmann (The Capable Hub) <msp@baylibre.com>

Re: [PATCH] tracing: fprobe: Remove __packed from generic __fprobe_header

[Markus Schneider-Pargmann]

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Hi,

On Tue Apr 28, 2026 at 10:30 AM CEST, Markus Schneider-Pargmann (The Capable Hub) wrote:
> fp pointer and unsigned long have the same size on all relevant
> architectures that build Linux. Furthermore this struct is only used in
> architectures that do not set ARCH_DEFINE_ENCODE_FPROBE_HEADER which is
> set only for 64bit architectures (apart from LoongArch).
>
> Both fields are aligned on these architectures so the struct with
> __packed and without it are the same.
>
> Remove the __packed as it is unnecessary.

Friendly ping on this.

Best
Markus

>
> Fixes: 4346ba160409 ("fprobe: Rewrite fprobe on function-graph tracer")
> Signed-off-by: Markus Schneider-Pargmann (The Capable Hub) <msp@baylibre.com>
> ---
>  kernel/trace/fprobe.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> index cc49ebd2a773..21751dcdb7b9 100644
> --- a/kernel/trace/fprobe.c
> +++ b/kernel/trace/fprobe.c
> @@ -181,7 +181,7 @@ static inline void read_fprobe_header(unsigned long *stack,
>  struct __fprobe_header {
>  	struct fprobe *fp;
>  	unsigned long size_words;
> -} __packed;
> +};
>  
>  #define FPROBE_HEADER_SIZE_IN_LONG	SIZE_IN_LONG(sizeof(struct __fprobe_header))
>  
>
> ---
> base-commit: 254f49634ee16a731174d2ae34bc50bd5f45e731
> change-id: 20260427-topic-fprobe-packed-v7-1-f44f9bbdedf6
>
> Best regards,
> --  
> Markus Schneider-Pargmann (The Capable Hub) <msp@baylibre.com>


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Re: [PATCH] tracing: fprobe: Remove __packed from generic __fprobe_header

[Masami Hiramatsu]

Hi Markus,

Thanks for ping me.

On Tue, 28 Apr 2026 10:30:29 +0200
"Markus Schneider-Pargmann (The Capable Hub)" <msp@baylibre.com> wrote:

> fp pointer and unsigned long have the same size on all relevant
> architectures that build Linux. Furthermore this struct is only used in
> architectures that do not set ARCH_DEFINE_ENCODE_FPROBE_HEADER which is
> set only for 64bit architectures (apart from LoongArch).
> 
> Both fields are aligned on these architectures so the struct with
> __packed and without it are the same.
> 
> Remove the __packed as it is unnecessary.
> 
> Fixes: 4346ba160409 ("fprobe: Rewrite fprobe on function-graph tracer")

NOTE: This is not a Fix, but just cleanup or minor update. Or, you have
any problem with this __packed attribute?

Unless there is no problem (or any concern), I would like to keep this
as it is.

Thank you,

> Signed-off-by: Markus Schneider-Pargmann (The Capable Hub) <msp@baylibre.com>
> ---
>  kernel/trace/fprobe.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> index cc49ebd2a773..21751dcdb7b9 100644
> --- a/kernel/trace/fprobe.c
> +++ b/kernel/trace/fprobe.c
> @@ -181,7 +181,7 @@ static inline void read_fprobe_header(unsigned long *stack,
>  struct __fprobe_header {
>  	struct fprobe *fp;
>  	unsigned long size_words;
> -} __packed;
> +};
>  
>  #define FPROBE_HEADER_SIZE_IN_LONG	SIZE_IN_LONG(sizeof(struct __fprobe_header))
>  
> 
> ---
> base-commit: 254f49634ee16a731174d2ae34bc50bd5f45e731
> change-id: 20260427-topic-fprobe-packed-v7-1-f44f9bbdedf6
> 
> Best regards,
> --  
> Markus Schneider-Pargmann (The Capable Hub) <msp@baylibre.com>
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>

Re: [PATCH] tracing: fprobe: Remove __packed from generic __fprobe_header

[Markus Schneider-Pargmann]

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Hi Masami,

On Wed Jun 10, 2026 at 10:17 AM CEST, Masami Hiramatsu wrote:
> Hi Markus,
>
> Thanks for ping me.
>
> On Tue, 28 Apr 2026 10:30:29 +0200
> "Markus Schneider-Pargmann (The Capable Hub)" <msp@baylibre.com> wrote:
>
>> fp pointer and unsigned long have the same size on all relevant
>> architectures that build Linux. Furthermore this struct is only used in
>> architectures that do not set ARCH_DEFINE_ENCODE_FPROBE_HEADER which is
>> set only for 64bit architectures (apart from LoongArch).
>> 
>> Both fields are aligned on these architectures so the struct with
>> __packed and without it are the same.
>> 
>> Remove the __packed as it is unnecessary.
>> 
>> Fixes: 4346ba160409 ("fprobe: Rewrite fprobe on function-graph tracer")
>
> NOTE: This is not a Fix, but just cleanup or minor update. Or, you have
> any problem with this __packed attribute?

Thanks, yes it is not fixing a bug, I can remove this.

>
> Unless there is no problem (or any concern), I would like to keep this
> as it is.

There is currently no problem with __packed in the upstream kernel. I
just thought this would be a good cleanup to remove the unnecessary
attribute. I am working on CHERI architectures where pointers have
capabilities. __packed breaks these capability tags and therefore
doesn't work on CHERI. When looking into why this struct has a __packed
attribute I didn't see a reason, so I thought this would be a good patch
for upstream as well even though CHERI is not yet relevant for upstream
linux.

Best
Markus

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Re: [PATCH] tracing: fprobe: Remove __packed from generic __fprobe_header

[David Laight]

On Wed, 10 Jun 2026 17:17:40 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> Hi Markus,
> 
> Thanks for ping me.
> 
> On Tue, 28 Apr 2026 10:30:29 +0200
> "Markus Schneider-Pargmann (The Capable Hub)" <msp@baylibre.com> wrote:
> 
> > fp pointer and unsigned long have the same size on all relevant
> > architectures that build Linux. Furthermore this struct is only used in
> > architectures that do not set ARCH_DEFINE_ENCODE_FPROBE_HEADER which is
> > set only for 64bit architectures (apart from LoongArch).
> > 
> > Both fields are aligned on these architectures so the struct with
> > __packed and without it are the same.
> > 
> > Remove the __packed as it is unnecessary.
> > 
> > Fixes: 4346ba160409 ("fprobe: Rewrite fprobe on function-graph tracer")  
> 
> NOTE: This is not a Fix, but just cleanup or minor update. Or, you have
> any problem with this __packed attribute?
> 
> Unless there is no problem (or any concern), I would like to keep this
> as it is.

There is likely to be a difference on architectures that fault misaligned
accesses.
On those gcc will use multiple byte-sized accesses (and a log of shifts etc)
for code that accesses those members because it will assume that the
structure itself can be misaligned.

So you only want __packed on structures that might be misaligned and those
that contain misaligned members.

If the structure is only guaranteed to be 32bit aligned then use __packed
__aligned(4) so that two 32bit accesses get used instead of 8 8bit ones.

-- David

> 
> Thank you,
> 
> > Signed-off-by: Markus Schneider-Pargmann (The Capable Hub) <msp@baylibre.com>
> > ---
> >  kernel/trace/fprobe.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> > index cc49ebd2a773..21751dcdb7b9 100644
> > --- a/kernel/trace/fprobe.c
> > +++ b/kernel/trace/fprobe.c
> > @@ -181,7 +181,7 @@ static inline void read_fprobe_header(unsigned long *stack,
> >  struct __fprobe_header {
> >  	struct fprobe *fp;
> >  	unsigned long size_words;
> > -} __packed;
> > +};
> >  
> >  #define FPROBE_HEADER_SIZE_IN_LONG	SIZE_IN_LONG(sizeof(struct __fprobe_header))
> >  
> > 
> > ---
> > base-commit: 254f49634ee16a731174d2ae34bc50bd5f45e731
> > change-id: 20260427-topic-fprobe-packed-v7-1-f44f9bbdedf6
> > 
> > Best regards,
> > --  
> > Markus Schneider-Pargmann (The Capable Hub) <msp@baylibre.com>
> >   
> 
>