Re: [RFC PATCH v2 6/6] bpf: Enable kprobe_multi feature if CONFIG_FPROBE is enabled

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Aug 07, 2023 at 03:49:33PM +0900, Masami Hiramatsu (Google) wrote:
> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> Enable kprobe_multi feature if CONFIG_FPROBE is enabled. The pt_regs is
> converted from ftrace_regs by ftrace_partial_regs(), thus some registers
> may always returns 0. But it should be enough for function entry (access
> arguments) and exit (access return value).
> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> ---
>  kernel/trace/bpf_trace.c |   22 +++++++++-------------
>  1 file changed, 9 insertions(+), 13 deletions(-)
> 
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 99c5f95360f9..0725272a3de2 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -2460,7 +2460,7 @@ static int __init bpf_event_init(void)
>  fs_initcall(bpf_event_init);
>  #endif /* CONFIG_MODULES */
>  
> -#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> +#ifdef CONFIG_FPROBE
>  struct bpf_kprobe_multi_link {
>  	struct bpf_link link;
>  	struct fprobe fp;
> @@ -2482,6 +2482,8 @@ struct user_syms {
>  	char *buf;
>  };
>  
> +static DEFINE_PER_CPU(struct pt_regs, bpf_kprobe_multi_pt_regs);
> +
>  static int copy_user_syms(struct user_syms *us, unsigned long __user *usyms, u32 cnt)
>  {
>  	unsigned long __user usymbol;
> @@ -2623,13 +2625,14 @@ static u64 bpf_kprobe_multi_entry_ip(struct bpf_run_ctx *ctx)
>  
>  static int
>  kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link,
> -			   unsigned long entry_ip, struct pt_regs *regs)
> +			   unsigned long entry_ip, struct ftrace_regs *fregs)
>  {
>  	struct bpf_kprobe_multi_run_ctx run_ctx = {
>  		.link = link,
>  		.entry_ip = entry_ip,
>  	};
>  	struct bpf_run_ctx *old_run_ctx;
> +	struct pt_regs *regs;
>  	int err;
>  
>  	if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1)) {
> @@ -2639,6 +2642,7 @@ kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link,
>  
>  	migrate_disable();
>  	rcu_read_lock();
> +	regs = ftrace_partial_regs(fregs, this_cpu_ptr(&bpf_kprobe_multi_pt_regs));

you did check for !regs when returned from ftrace_get_regs, why don't we need
to check it in here? both ftrace_partial_regs and ftrace_get_regs call
arch_ftrace_get_regs on x86

also also I can't find the place ensuring fregs->regs.cs != 0 for FL_SAVE_REGS
flag as stated in arch_ftrace_get_regs, any hint?

thanks,
jirka


>  	old_run_ctx = bpf_set_run_ctx(&run_ctx.run_ctx);
>  	err = bpf_prog_run(link->link.prog, regs);
>  	bpf_reset_run_ctx(old_run_ctx);
> @@ -2656,13 +2660,9 @@ kprobe_multi_link_handler(struct fprobe *fp, unsigned long fentry_ip,
>  			  void *data)
>  {
>  	struct bpf_kprobe_multi_link *link;
> -	struct pt_regs *regs = ftrace_get_regs(fregs);
> -
> -	if (!regs)
> -		return 0;
>  
>  	link = container_of(fp, struct bpf_kprobe_multi_link, fp);
> -	kprobe_multi_link_prog_run(link, get_entry_ip(fentry_ip), regs);
> +	kprobe_multi_link_prog_run(link, get_entry_ip(fentry_ip), fregs);
>  	return 0;
>  }
>  
> @@ -2672,13 +2672,9 @@ kprobe_multi_link_exit_handler(struct fprobe *fp, unsigned long fentry_ip,
>  			       void *data)
>  {
>  	struct bpf_kprobe_multi_link *link;
> -	struct pt_regs *regs = ftrace_get_regs(fregs);
> -
> -	if (!regs)
> -		return;
>  
>  	link = container_of(fp, struct bpf_kprobe_multi_link, fp);
> -	kprobe_multi_link_prog_run(link, get_entry_ip(fentry_ip), regs);
> +	kprobe_multi_link_prog_run(link, get_entry_ip(fentry_ip), fregs);
>  }
>  
>  static int symbols_cmp_r(const void *a, const void *b, const void *priv)
> @@ -2918,7 +2914,7 @@ int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *pr
>  	kvfree(cookies);
>  	return err;
>  }
> -#else /* !CONFIG_DYNAMIC_FTRACE_WITH_REGS */
> +#else /* !CONFIG_FPROBE */
>  int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
>  {
>  	return -EOPNOTSUPP;
>