linux-kernel - Re: [PATCH v5 11/34] function_graph: Have the instances use their own ftrace

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Message-ID: <ZZwEz8HsTa2IZE3L@FVFF77S0Q05N>
Date: Mon, 8 Jan 2024 14:21:03 +0000
From: Mark Rutland <mark.rutland@....com>
To: Masami Hiramatsu <mhiramat@...nel.org>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	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>,
	Sven Schnelle <svens@...ux.ibm.com>,
	Alexei Starovoitov <ast@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Alan Maguire <alan.maguire@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>, Guo Ren <guoren@...nel.org>
Subject: Re: [PATCH v5 11/34] function_graph: Have the instances use their
 own ftrace_ops for filtering

On Mon, Jan 08, 2024 at 12:25:55PM +0000, Mark Rutland wrote:
> We also have HAVE_FUNCTION_GRAPH_RET_ADDR_PTR, but since the return address is
> not on the stack at the point function-entry is intercepted we use the FP as
> the retp value -- in the absence of tail calls this will be different between a
> caller and callee.

Ah; I just spotted that this patch changed that in ftrace_graph_func(), which
is the source of the bug. 

As of this patch, we use the address of fregs->lr as the retp value, but the
unwinder still uses the FP value, and so when unwind_recover_return_address()
calls ftrace_graph_ret_addr(), the retp value won't match the expected entry on
the fgraph ret_stack, resulting in failing to find the expected entry.

Since the ftrace_regs only exist transiently during function entry/exit, it's
possible for a stackframe to reuse that same address on the stack, which would
result in finding a different entry by mistake.

The diff below restores the existing behaviour and fixes the issue for me.
Could you please fold that into this patch?

On a separate note, looking at how this patch changed arm64's
ftrace_graph_func(), do we need similar changes to arm64's
prepare_ftrace_return() for the old-style mcount based ftrace?

Mark.

---->8----
diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
index 205937e04ece..329092ce06ba 100644
--- a/arch/arm64/kernel/ftrace.c
+++ b/arch/arm64/kernel/ftrace.c
@@ -495,7 +495,7 @@ void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
        if (bit < 0)
                return;

-       if (!function_graph_enter_ops(*parent, ip, fregs->fp, parent, gops))
+       if (!function_graph_enter_ops(*parent, ip, fregs->fp, (void *)fregs->fp, gops))
                *parent = (unsigned long)&return_to_handler;

        ftrace_test_recursion_unlock(bit);