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

[Sven Schnelle <svens@linux.ibm.com>]

"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/)