"Masami Hiramatsu (Google)" [off-list ref] 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 <redacted>
Cc: Naveen N Rao <naveen@kernel.org>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Paul Walmsley <redacted>
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 <redacted>
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>

[..]

[..]

+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/ (local))