Re: [PATCH bpf-next v7 1/4] bpf: Relax tracing prog recursive attach rules

[Jiri Olsa <olsajiri@gmail.com>]

On Fri, Dec 08, 2023 at 07:55:53PM +0100, Dmitrii Dolgov wrote:

SNIP

> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index eb447b0a9423..e7393674ab94 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1414,6 +1414,7 @@ struct bpf_prog_aux {
>  	bool dev_bound; /* Program is bound to the netdev. */
>  	bool offload_requested; /* Program is bound and offloaded to the netdev. */
>  	bool attach_btf_trace; /* true if attaching to BTF-enabled raw tp */
> +	bool attach_tracing_prog; /* true if tracing another tracing program */
>  	bool func_proto_unreliable;
>  	bool sleepable;
>  	bool tail_call_reachable;
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 5e43ddd1b83f..d5470a5c8c6d 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -3039,6 +3039,7 @@ static void bpf_tracing_link_release(struct bpf_link *link)
>  
>  	bpf_trampoline_put(tr_link->trampoline);
>  
> +	link->prog->aux->attach_tracing_prog = false;

I think it'd be better to have this as part of the 'if (tr_link->tgt_prog)'
path below, because it's set only for that case

>  	/* tgt_prog is NULL if target is a kernel function */
>  	if (tr_link->tgt_prog)
>  		bpf_prog_put(tr_link->tgt_prog);
> @@ -3243,6 +3244,12 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
>  		goto out_unlock;
>  	}
>  
> +	/* Bookkeeping for managing the prog attachment chain */
> +	if (tgt_prog &&
> +		prog->type == BPF_PROG_TYPE_TRACING &&
> +		tgt_prog->type == BPF_PROG_TYPE_TRACING)
> +		prog->aux->attach_tracing_prog = true;

wrong indentation in here, please check the if conditions around

thanks,
jirka

> +
>  	link->tgt_prog = tgt_prog;
>  	link->trampoline = tr;
>  
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 8e7b6072e3f4..f8c15ce8fd05 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -20077,6 +20077,7 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
>  			    struct bpf_attach_target_info *tgt_info)
>  {
>  	bool prog_extension = prog->type == BPF_PROG_TYPE_EXT;
> +	bool prog_tracing = prog->type == BPF_PROG_TYPE_TRACING;
>  	const char prefix[] = "btf_trace_";
>  	int ret = 0, subprog = -1, i;
>  	const struct btf_type *t;
> @@ -20147,10 +20148,21 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
>  			bpf_log(log, "Can attach to only JITed progs\n");
>  			return -EINVAL;
>  		}
> -		if (tgt_prog->type == prog->type) {
> -			/* Cannot fentry/fexit another fentry/fexit program.
> -			 * Cannot attach program extension to another extension.
> -			 * It's ok to attach fentry/fexit to extension program.
> +		if (prog_tracing) {
> +			if (aux->attach_tracing_prog) {
> +				/*
> +				 * Target program is an fentry/fexit which is already attached
> +				 * to another tracing program. More levels of nesting
> +				 * attachment are not allowed.
> +				 */
> +				bpf_log(log, "Cannot nest tracing program attach more than once\n");
> +				return -EINVAL;
> +			}
> +		} else if (tgt_prog->type == prog->type) {
> +			/*
> +			 * To avoid potential call chain cycles, prevent attaching of a
> +			 * program extension to another extension. It's ok to attach
> +			 * fentry/fexit to extension program.
>  			 */
>  			bpf_log(log, "Cannot recursively attach\n");
>  			return -EINVAL;
> @@ -20163,16 +20175,15 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
>  			 * except fentry/fexit. The reason is the following.
>  			 * The fentry/fexit programs are used for performance
>  			 * analysis, stats and can be attached to any program
> -			 * type except themselves. When extension program is
> -			 * replacing XDP function it is necessary to allow
> -			 * performance analysis of all functions. Both original
> -			 * XDP program and its program extension. Hence
> -			 * attaching fentry/fexit to BPF_PROG_TYPE_EXT is
> -			 * allowed. If extending of fentry/fexit was allowed it
> -			 * would be possible to create long call chain
> -			 * fentry->extension->fentry->extension beyond
> -			 * reasonable stack size. Hence extending fentry is not
> -			 * allowed.
> +			 * type. When extension program is replacing XDP function
> +			 * it is necessary to allow performance analysis of all
> +			 * functions. Both original XDP program and its program
> +			 * extension. Hence attaching fentry/fexit to
> +			 * BPF_PROG_TYPE_EXT is allowed. If extending of
> +			 * fentry/fexit was allowed it would be possible to create
> +			 * long call chain fentry->extension->fentry->extension
> +			 * beyond reasonable stack size. Hence extending fentry
> +			 * is not allowed.
>  			 */
>  			bpf_log(log, "Cannot extend fentry/fexit\n");
>  			return -EINVAL;
> -- 
> 2.41.0
>