Re: [PATCH v3 1/1] tracing/kprobes: Return EADDRNOTAVAIL when func matches several symbols

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Thu, 24 Aug 2023 18:08:59 +0200
Francis Laniel <flaniel@linux.microsoft.com> wrote:

> Previously to this commit, if func matches several symbols, a kprobe, being
> either sysfs or PMU, would only be installed for the first matching address.
> This could lead to some misunderstanding when some BPF code was never called
> because it was attached to a function which was indeed not called, because
> the effectively called one has no kprobes attached.
> 
> So, this commit returns EADDRNOTAVAIL when func matches several symbols.
> This way, user needs to use address to remove the ambiguity.
> 
> Suggested-by: Masami Hiramatsu <mhiramat@kernel.org>
> Signed-off-by: Francis Laniel <flaniel@linux.microsoft.com>
> Link: https://lore.kernel.org/lkml/20230819101105.b0c104ae4494a7d1f2eea742@kernel.org/

Looks good to me!

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thank you!

> ---
>  kernel/trace/trace_kprobe.c | 61 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 61 insertions(+)
> 
> diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> index 23dba01831f7..2f393739e8cf 100644
> --- a/kernel/trace/trace_kprobe.c
> +++ b/kernel/trace/trace_kprobe.c
> @@ -705,6 +705,25 @@ static struct notifier_block trace_kprobe_module_nb = {
>  	.priority = 1	/* Invoked after kprobe module callback */
>  };
>  
> +static int count_symbols(void *data, unsigned long unused)
> +{
> +	unsigned int *count = data;
> +
> +	(*count)++;
> +
> +	return 0;
> +}
> +
> +static unsigned int number_of_same_symbols(char *func_name)
> +{
> +	unsigned int count;
> +
> +	count = 0;
> +	kallsyms_on_each_match_symbol(count_symbols, func_name, &count);
> +
> +	return count;
> +}
> +
>  static int __trace_kprobe_create(int argc, const char *argv[])
>  {
>  	/*
> @@ -836,6 +855,29 @@ static int __trace_kprobe_create(int argc, const char *argv[])
>  		}
>  	}
>  
> +	if (symbol) {
> +		unsigned int count;
> +
> +		count = number_of_same_symbols(symbol);
> +		if (count > 1) {
> +			/*
> +			 * Users should use ADDR to remove the ambiguity of
> +			 * using KSYM only.
> +			 */
> +			ret = -EADDRNOTAVAIL;
> +
> +			goto error;
> +		} else if (count == 0) {
> +			/*
> +			 * We can return ENOENT earlier than when register the
> +			 * kprobe.
> +			 */
> +			ret = -ENOENT;
> +
> +			goto error;
> +		}
> +	}
> +
>  	trace_probe_log_set_index(0);
>  	if (event) {
>  		ret = traceprobe_parse_event_name(&event, &group, gbuf,
> @@ -1699,6 +1741,7 @@ static int unregister_kprobe_event(struct trace_kprobe *tk)
>  }
>  
>  #ifdef CONFIG_PERF_EVENTS
> +
>  /* create a trace_kprobe, but don't add it to global lists */
>  struct trace_event_call *
>  create_local_trace_kprobe(char *func, void *addr, unsigned long offs,
> @@ -1709,6 +1752,24 @@ create_local_trace_kprobe(char *func, void *addr, unsigned long offs,
>  	int ret;
>  	char *event;
>  
> +	if (func) {
> +		unsigned int count;
> +
> +		count = number_of_same_symbols(func);
> +		if (count > 1)
> +			/*
> +			 * Users should use addr to remove the ambiguity of
> +			 * using func only.
> +			 */
> +			return ERR_PTR(-EADDRNOTAVAIL);
> +		else if (count == 0)
> +			/*
> +			 * We can return ENOENT earlier than when register the
> +			 * kprobe.
> +			 */
> +			return ERR_PTR(-ENOENT);
> +	}
> +
>  	/*
>  	 * local trace_kprobes are not added to dyn_event, so they are never
>  	 * searched in find_trace_kprobe(). Therefore, there is no concern of
> -- 
> 2.34.1
> 


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