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

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Message-Id: <20240111224720.12d2062d360641be25deb9d2@kernel.org>
Date: Thu, 11 Jan 2024 22:47:20 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Mark Rutland <mark.rutland@....com>
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, 8 Jan 2024 15:03:21 +0000
Mark Rutland <mark.rutland@....com> wrote:

> On Mon, Jan 08, 2024 at 02:21:03PM +0000, Mark Rutland wrote:
> > 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);
> 
> Thinking some more, this line gets excessively long when we pass the fregs too,
> so it's probably worth adding a local variable for fp, i.e. the diff below.

Yeah, that will be better to keep the line short.

Thank you,

> 
> Mark.
> 
> ---->8----
> diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
> index 205937e04ece..d4e142ef4686 100644
> --- a/arch/arm64/kernel/ftrace.c
> +++ b/arch/arm64/kernel/ftrace.c
> @@ -481,8 +481,9 @@ void prepare_ftrace_return(unsigned long self_addr, unsigned long *parent,
>  void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
>                        struct ftrace_ops *op, struct ftrace_regs *fregs)
>  {
> -       unsigned long *parent = &fregs->lr;
>         struct fgraph_ops *gops = container_of(op, struct fgraph_ops, ops);
> +       unsigned long *parent = &fregs->lr;
> +       unsigned long fp = fregs->fp;
>         int bit;
>  
>         if (unlikely(ftrace_graph_is_dead()))
> @@ -495,7 +496,7 @@ 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, fp, (void *)fp, gops))
>                 *parent = (unsigned long)&return_to_handler;
>  
>         ftrace_test_recursion_unlock(bit);
> 


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