linux-kernel - Re: [PATCH bpf-next 1/2] bpf: Add a bpf_kallsyms

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Message-ID: <20201129010705.7djnqmztkjhqlrdt@ast-mbp>
Date:   Sat, 28 Nov 2020 17:07:05 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Florent Revest <revest@...omium.org>
Cc:     bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, kpsingh@...omium.org, revest@...gle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next 1/2] bpf: Add a bpf_kallsyms_lookup helper

On Thu, Nov 26, 2020 at 05:57:47PM +0100, Florent Revest wrote:
> This helper exposes the kallsyms_lookup function to eBPF tracing
> programs. This can be used to retrieve the name of the symbol at an
> address. For example, when hooking into nf_register_net_hook, one can
> audit the name of the registered netfilter hook and potentially also
> the name of the module in which the symbol is located.
> 
> Signed-off-by: Florent Revest <revest@...gle.com>
> ---
>  include/uapi/linux/bpf.h       | 16 +++++++++++++
>  kernel/trace/bpf_trace.c       | 41 ++++++++++++++++++++++++++++++++++
>  tools/include/uapi/linux/bpf.h | 16 +++++++++++++
>  3 files changed, 73 insertions(+)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index c3458ec1f30a..670998635eac 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -3817,6 +3817,21 @@ union bpf_attr {
>   *		The **hash_algo** is returned on success,
>   *		**-EOPNOTSUP** if IMA is disabled or **-EINVAL** if
>   *		invalid arguments are passed.
> + *
> + * long bpf_kallsyms_lookup(u64 address, char *symbol, u32 symbol_size, char *module, u32 module_size)
> + *	Description
> + *		Uses kallsyms to write the name of the symbol at *address*
> + *		into *symbol* of size *symbol_sz*. This is guaranteed to be
> + *		zero terminated.
> + *		If the symbol is in a module, up to *module_size* bytes of
> + *		the module name is written in *module*. This is also
> + *		guaranteed to be zero-terminated. Note: a module name
> + *		is always shorter than 64 bytes.
> + *	Return
> + *		On success, the strictly positive length of the full symbol
> + *		name, If this is greater than *symbol_size*, the written
> + *		symbol is truncated.
> + *		On error, a negative value.

Looks like debug-only helper.
I cannot think of a way to use in production code.
What program suppose to do with that string?
Do string compare? BPF side doesn't have a good way to do string manipulations.
If you really need to print a symbolic name for a given address
I'd rather extend bpf_trace_printk() to support %pS