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

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

On Tue, 8 Aug 2023 00:08:11 +0200
Jiri Olsa <olsajiri@gmail.com> wrote:

> 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

Good catch! I think ftrace_partial_regs must not return NULL (unless getting
invalid parameter, e.g. fregs == NULL).

> 
> 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?

Oops, I misread that part. Maybe ftrace_partial_regs must forcibly return
ftrace_regs::regs if HAVE_PT_REGS_COMPAT_FTRACE_REGS=y because it does not
care the regs is partial or not.

Thank you!

> 
> 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;
> > 


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