linux-kernel - Re: [PATCH v17 11/16] fprobe: Rewrite fprobe on function-graph tracer

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Message-ID: <20241016101022.185f741b@gandalf.local.home>
Date: Wed, 16 Oct 2024 10:10:22 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Sven Schnelle <svens@...ux.ibm.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>, Alexei Starovoitov
 <alexei.starovoitov@...il.com>, Florent Revest <revest@...omium.org>,
 linux-trace-kernel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, bpf <bpf@...r.kernel.org>, Alexei
 Starovoitov <ast@...nel.org>, Jiri Olsa <jolsa@...nel.org>, Alan Maguire
 <alan.maguire@...cle.com>, Mark Rutland <mark.rutland@....com>,
 linux-arch@...r.kernel.org, Catalin Marinas <catalin.marinas@....com>, Will
 Deacon <will@...nel.org>, Huacai Chen <chenhuacai@...nel.org>, WANG Xuerui
 <kernel@...0n.name>, Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin
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 N Rao <naveen@...nel.org>, Madhavan Srinivasan <maddy@...ux.ibm.com>, Paul
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 Albert Ou <aou@...s.berkeley.edu>, Heiko Carstens <hca@...ux.ibm.com>,
 Vasily Gorbik <gor@...ux.ibm.com>, Alexander Gordeev
 <agordeev@...ux.ibm.com>, Christian Borntraeger
 <borntraeger@...ux.ibm.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo
 Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
 <dave.hansen@...ux.intel.com>, x86@...nel.org, "H. Peter Anvin"
 <hpa@...or.com>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Andrew
 Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v17 11/16] fprobe: Rewrite fprobe on function-graph
 tracer

On Wed, 16 Oct 2024 14:07:31 +0200
Sven Schnelle <svens@...ux.ibm.com> wrote:

> > +/* Return reserved data size in words */
> > +static inline int decode_fprobe_header(unsigned long val, struct fprobe **fp)
> > +{
> > +	unsigned long ptr;
> > +
> > +	ptr = (val & FPROBE_HEADER_PTR_MASK) | ~FPROBE_HEADER_PTR_MASK;
> > +	if (fp)
> > +		*fp = (struct fprobe *)ptr;
> > +	return val >> FPROBE_HEADER_PTR_BITS;
> > +}  
> 
> I think that still has the issue that the size is encoded in the
> leftmost fields of the pointer, which doesn't work on all
> architectures. I reported this already in v15
> (https://lore.kernel.org/all/yt9dmsjyx067.fsf@linux.ibm.com/)

From what you said in v15:

> I haven't yet fully understood why this logic is needed, but the
> WARN_ON_ONCE triggers on s390. I'm assuming this fails because fp always
> has the upper bits of the address set on x86 (and likely others). As an
> example, in my test setup, fp is 0x8feec218 on s390, while it is
> 0xffff888100add118 in x86-kvm.

Since we only need to save 4 bits for size, we could have what it is
replacing always be zero or always be f, depending on the arch. The
question then is, is s390's 4 MSBs always zero?

Thus we could make it be:

static inline int decode_fprobe_header(unsigned long val, struct fprobe **fp)
{
	unsigned long ptr;

	ptr = (val & FPROBE_HEADER_PTR_MASK) | FPROBE_HEADER_MSB_MASK;
	if (fp)
		*fp = (struct fprobe *)ptr;
	return val >> FPROBE_HEADER_PTR_BITS;
}

And define FPROBE_HEADER_MSB_MASK to be either:

For most archs:

#define FPROBE_HEADER_MSB_MASK	(0xf << FPROBE_HEADER_PTR_BITS)

or on s390:

#define FPROBE_HEADER_MSB_MASK	(0x0)

Would this work?

-- Steve