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

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

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

> "Masami Hiramatsu (Google)" <mhiramat@kernel.org> writes:
> 
> > From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> >
> > Rewrite fprobe implementation on function-graph tracer.
> > Major API changes are:
> >  -  'nr_maxactive' field is deprecated.
> >  -  This depends on CONFIG_DYNAMIC_FTRACE_WITH_ARGS or
> >     !CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS, and
> >     CONFIG_HAVE_FUNCTION_GRAPH_FREGS. So currently works only
> >     on x86_64.
> >  -  Currently the entry size is limited in 15 * sizeof(long).
> >  -  If there is too many fprobe exit handler set on the same
> >     function, it will fail to probe.
> >
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Cc: Mark Rutland <mark.rutland@arm.com>
> > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > Cc: Will Deacon <will@kernel.org>
> > Cc: Huacai Chen <chenhuacai@kernel.org>
> > Cc: WANG Xuerui <kernel@xen0n.name>
> > Cc: Michael Ellerman <mpe@ellerman.id.au>
> > Cc: Nicholas Piggin <npiggin@gmail.com>
> > Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
> > Cc: Naveen N Rao <naveen@kernel.org>
> > Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
> > Cc: Paul Walmsley <paul.walmsley@sifive.com>
> > Cc: Palmer Dabbelt <palmer@dabbelt.com>
> > Cc: Albert Ou <aou@eecs.berkeley.edu>
> > Cc: Heiko Carstens <hca@linux.ibm.com>
> > Cc: Vasily Gorbik <gor@linux.ibm.com>
> > Cc: Alexander Gordeev <agordeev@linux.ibm.com>
> > Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
> > Cc: Sven Schnelle <svens@linux.ibm.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Borislav Petkov <bp@alien8.de>
> > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > Cc: x86@kernel.org
> > Cc: "H. Peter Anvin" <hpa@zytor.com>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> >
> [..]
> 
> > diff --git a/include/linux/fprobe.h b/include/linux/fprobe.h
> > index ef609bcca0f9..2d06bbd99601 100644
> > --- a/include/linux/fprobe.h
> > +++ b/include/linux/fprobe.h
> > @@ -5,10 +5,11 @@
> [..]
> > +static inline unsigned long encode_fprobe_header(struct fprobe *fp, int size_words)
> > +{
> > +	if (WARN_ON_ONCE(size_words > MAX_FPROBE_DATA_SIZE_WORD ||
> > +	    ((unsigned long)fp & ~FPROBE_HEADER_PTR_MASK) !=
> > +	    ~FPROBE_HEADER_PTR_MASK)) {
> > +		return 0;
> >  	}
> > +	return ((unsigned long)size_words << FPROBE_HEADER_PTR_BITS) |
> > +		((unsigned long)fp & FPROBE_HEADER_PTR_MASK);
> > +}
> > +
> > +/* 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/)

Oops, thanks for reporting. I should missed that.

> 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.

Ah, so s390 kernel/user memory layout is something like 4G/4G?
Hmm, this encode expects the leftmost 4bit is filled. For the
architecture which has 32bit address space, we may be possible to
use "unsigned long long" for 'val' on shadow stack (and use the
first 32bit for fp and another 32bit for size).

Anyway, I need to redesign it depending on architecture.

Thank you!

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