Re: [PATCH bpf-next 2/3] bpf: Add get_func_[arg|ret|arg_cnt] helpers

[Jiri Olsa <jolsa@redhat.com>]

On Mon, Dec 06, 2021 at 11:39:00AM -0800, John Fastabend wrote:
> Jiri Olsa wrote:
> > Adding following helpers for tracing programs:
> > 
> > Get n-th argument of the traced function:
> >   long bpf_get_func_arg(void *ctx, u32 n, u64 *value)
> > 
> > Get return value of the traced function:
> >   long bpf_get_func_ret(void *ctx, u64 *value)
> > 
> > Get arguments count of the traced funtion:
> >   long bpf_get_func_arg_cnt(void *ctx)
> > 
> > The trampoline now stores number of arguments on ctx-8
> > address, so it's easy to verify argument index and find
> > return value argument's position.
> > 
> > Moving function ip address on the trampoline stack behind
> > the number of functions arguments, so it's now stored on
> > ctx-16 address if it's needed.
> > 
> > All helpers above are inlined by verifier.
> > 
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index c26871263f1f..d5a3791071d6 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -4983,6 +4983,31 @@ union bpf_attr {
> >   *	Return
> >   *		The number of loops performed, **-EINVAL** for invalid **flags**,
> >   *		**-E2BIG** if **nr_loops** exceeds the maximum number of loops.
> > + *
> > + * long bpf_get_func_arg(void *ctx, u32 n, u64 *value)
> > + *	Description
> > + *		Get **n**-th argument (zero based) of the traced function (for tracing programs)
> > + *		returned in **value**.
> > + *
> > + *	Return
> > + *		0 on success.
> > + *		**-EINVAL** if n >= arguments count of traced function.
> > + *
> > + * long bpf_get_func_ret(void *ctx, u64 *value)
> > + *	Description
> > + *		Get return value of the traced function (for tracing programs)
> > + *		in **value**.
> > + *
> > + *	Return
> > + *		0 on success.
> > + *		**-EINVAL** for tracing programs other than BPF_TRACE_FEXIT or BPF_MODIFY_RETURN.
> 
> 
> Can we just throw a verifier error if the program type doesn't support
> this? Then weget a void and ther is no error case.

we discussed this with Andrii in previous version:
  https://lore.kernel.org/bpf/CAEf4BzbauHaDDJvGpx4oCRddd4KWpb4PkxUiUJvx-CXqEN2sdQ@mail.gmail.com/

having it this way will allow us for example to use one program
for both fentry and fexit hooks

> 
> > + *
> > + * long bpf_get_func_arg_cnt(void *ctx)
> > + *	Description
> > + *		Get number of arguments of the traced function (for tracing programs).
> > + *
> > + *	Return
> > + *		The number of arguments of the traced function.
> >   */
> >  #define __BPF_FUNC_MAPPER(FN)		\
> >  	FN(unspec),			\
> > @@ -5167,6 +5192,9 @@ union bpf_attr {
> >  	FN(kallsyms_lookup_name),	\
> >  	FN(find_vma),			\
> >  	FN(loop),			\
> > +	FN(get_func_arg),		\
> > +	FN(get_func_ret),		\
> > +	FN(get_func_arg_cnt),		\
> >  	/* */
> >  
> >  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 6522ffdea487..cf6853d3a8e9 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -12974,6 +12974,7 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env,
> >  static int do_misc_fixups(struct bpf_verifier_env *env)
> >  {
> >  	struct bpf_prog *prog = env->prog;
> > +	enum bpf_attach_type eatype = prog->expected_attach_type;
> >  	bool expect_blinding = bpf_jit_blinding_enabled(prog);
> >  	enum bpf_prog_type prog_type = resolve_prog_type(prog);
> >  	struct bpf_insn *insn = prog->insnsi;
> > @@ -13344,11 +13345,79 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
> >  			continue;
> >  		}
> >  
> 
> [...]
> 
> > +		/* Implement get_func_arg_cnt inline. */
> > +		if (prog_type == BPF_PROG_TYPE_TRACING &&
> > +		    insn->imm == BPF_FUNC_get_func_arg_cnt) {
> > +			/* Load nr_args from ctx - 8 */
> > +			insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
> > +
> > +			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, 1);
> > +			if (!new_prog)
> > +				return -ENOMEM;
> 
> How does this handle the !x86 case? The code above only touches the x86
> jit? Perhaps its obvious with some code digging, but its not to me from
> the patch description and code here.

right, it assumes all 3 helpers are called from trampoline only 

so I think I mis-placed the verifier's bpf_func_proto 'get' for all 3 helpers,
it should be in tracing_prog_func_proto rather than in bpf_tracing_func_proto
plus perhaps check for proper attach type

thanks
jirka

> 
> > +
> > +			env->prog = prog = new_prog;
> > +			insn      = new_prog->insnsi + i + delta;
> > +			continue;
> > +		}
> > +
>