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

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

On Thu, 09 Jul 2026 14:06:32 +0200
"Markus Schneider-Pargmann" <msp@baylibre.com> wrote:

> Hi Masami,
> 
> On Mon Jun 22, 2026 at 6:56 AM CEST, Masami Hiramatsu wrote:
> > On Wed, 10 Jun 2026 11:20:24 +0200
> > "Markus Schneider-Pargmann" <msp@baylibre.com> wrote:
> >
> >> 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.
> >
> > Hi Markus,
> >
> > Ah, OK. CHERI makes pointers to non-long value. Are you sure
> > removing __packed makes fprobe working with CHERI? If so,
> > please describe it. Then I can pick it.
> 
> Sorry for the late response.
> 
> No, this patch alone doesn't make fprobe work with CHERI. __packed is
> problematic for CHERI because it sets the alignment to 1 which forces
> the compiler to do unaligned access operations even if the
> struct/pointer is aligned. Unaligned operations do not keep the CHERI
> capabilities. Without __packed the compiler knows everything is aligned
> and can use aligned operations and keep the CHERI capabilities. CHERI
> hardware does not support unaligned pointer capability access.
> 
> On its own it doesn't help CHERI, there are more patches that are more
> CHERI specific that are needed for fprobe to work on CHERI. I just
> thought it would be a great standalone patch for upstream.

OK, anyway, I would like to pick your patch but without fixes tag.
It may make no differences, so should be treated as a cleanup patch.
Let me pick it to probes/for-next.

Thank you,

> 
> Best
> Markus


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