linux-kernel - Re: [PATCH 3/3] kprobes: Allow probing on any address belonging to ftrace

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Message-Id: <20220221091552.f2b24bde8142df1d3fd63b42@kernel.org>
Date:   Mon, 21 Feb 2022 09:15:52 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>, <bpf@...r.kernel.org>,
        <linuxppc-dev@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] kprobes: Allow probing on any address belonging to
 ftrace

On Thu, 17 Feb 2022 17:06:25 +0530
"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> wrote:

> On certain architectures, ftrace can reserve multiple instructions at
> function entry. Rather than rejecting kprobe on addresses other than the
> exact ftrace call instruction, use the address returned by ftrace to
> probe at the correct address when CONFIG_KPROBES_ON_FTRACE is enabled.
> 
> Signed-off-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
> ---
>  kernel/kprobes.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 94cab8c9ce56cc..0a797ede3fdf37 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -1497,6 +1497,10 @@ bool within_kprobe_blacklist(unsigned long addr)
>  static kprobe_opcode_t *_kprobe_addr(kprobe_opcode_t *addr,
>  			const char *symbol_name, unsigned int offset)
>  {
> +#ifdef CONFIG_KPROBES_ON_FTRACE
> +	unsigned long ftrace_addr = 0;
> +#endif
> +
>  	if ((symbol_name && addr) || (!symbol_name && !addr))
>  		goto invalid;
>  
> @@ -1507,6 +1511,14 @@ static kprobe_opcode_t *_kprobe_addr(kprobe_opcode_t *addr,
>  	}
>  
>  	addr = (kprobe_opcode_t *)(((char *)addr) + offset);
> +
> +#ifdef CONFIG_KPROBES_ON_FTRACE
> +	if (addr)
> +		ftrace_addr = ftrace_location((unsigned long)addr);
> +	if (ftrace_addr)
> +		return (kprobe_opcode_t *)ftrace_addr;

As I said, this must be

if (ftrace_addr != addr)
	return -EILSEQ;

This will prevent users from being confused by the results of probing
that 'func' and 'func+4' are the same. (now only 'func' is allowed to
be probed.)

Thank you,

> +#endif
> +
>  	if (addr)
>  		return addr;
>  
> -- 
> 2.35.1
> 


-- 
Masami Hiramatsu <mhiramat@...nel.org>