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Date: Fri, 18 Feb 2022 12:15:19 +0530
From: "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
Subject: Re: [PATCH 3/3] kprobes: Allow probing on any address belonging to
 ftrace
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>, Daniel Borkmann
 <daniel@iogearbox.net>, Masami Hiramatsu <mhiramat@kernel.org>,
 Michael Ellerman <mpe@ellerman.id.au>, Steven Rostedt <rostedt@goodmis.org>
References: <cover.1645096227.git.naveen.n.rao@linux.vnet.ibm.com>
 <78480d05821d45e09fb234f61f9037e26d42f02d.1645096227.git.naveen.n.rao@linux.vnet.ibm.com>
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

Naveen N. Rao 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.
>=20
> Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
> ---
>  kernel/kprobes.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>=20
> 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 =3D 0;
> +#endif
> +
>  	if ((symbol_name && addr) || (!symbol_name && !addr))
>  		goto invalid;
> =20
> @@ -1507,6 +1511,14 @@ static kprobe_opcode_t *_kprobe_addr(kprobe_opcode=
_t *addr,
>  	}
> =20
>  	addr =3D (kprobe_opcode_t *)(((char *)addr) + offset);
> +
> +#ifdef CONFIG_KPROBES_ON_FTRACE
> +	if (addr)
> +		ftrace_addr =3D ftrace_location((unsigned long)addr);
> +	if (ftrace_addr)
> +		return (kprobe_opcode_t *)ftrace_addr;
> +#endif

One of the side effects of this is that we'll now allow probes on=20
non-instruction boundary within the full ftrace address range. It's not=20
too much of an issue since we ensure that the probe location eventually=20
lands on the actual ftrace instruction. But, I'm wondering if we should=20
instead allow architectures to opt-in to this, by making this be=20
architecture specific. Architectures can then do whatever validation is=20
necessary.


- Naveen