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

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Message-ID: <yt9da5f3gblm.fsf@linux.ibm.com>
Date: Wed, 16 Oct 2024 20:13:25 +0200
From: Sven Schnelle <svens@...ux.ibm.com>
To: Steven Rostedt <rostedt@...dmis.org>
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 <npiggin@...il.com>,
        Christophe Leroy
 <christophe.leroy@...roup.eu>,
        Naveen N Rao <naveen@...nel.org>,
        Madhavan Srinivasan <maddy@...ux.ibm.com>,
        Paul Walmsley
 <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        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
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        Borislav Petkov
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        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
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        Mathieu Desnoyers
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        Andrew Morton
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Subject: Re: [PATCH v17 11/16] fprobe: Rewrite fprobe on function-graph tracer

Steven Rostedt <rostedt@...dmis.org> writes:

> 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?

s390 has separate address spaces for kernel and userspace - so kernel
addresses could be anywhere. I don't know think the range should be
limited artifically because of some optimizations.