netdev - Re: [PATCH v3 bpf-next 1/5] bpf: Add support for forcing kfunc args to be referenced

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Date:   Wed, 18 May 2022 23:36:13 +0530
From:   Kumar Kartikeya Dwivedi <memxor@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, davem@...emloft.net, kuba@...nel.org,
        edumazet@...gle.com, pabeni@...hat.com, pablo@...filter.org,
        fw@...len.de, netfilter-devel@...r.kernel.org,
        lorenzo.bianconi@...hat.com, brouer@...hat.com, toke@...hat.com
Subject: Re: [PATCH v3 bpf-next 1/5] bpf: Add support for forcing kfunc args
 to be referenced

On Wed, May 18, 2022 at 11:28:12PM IST, Yonghong Song wrote:
>
>
> On 5/18/22 3:43 AM, Lorenzo Bianconi wrote:
> > From: Kumar Kartikeya Dwivedi <memxor@...il.com>
> >
> > Similar to how we detect mem, size pairs in kfunc, teach verifier to
> > treat __ref suffix on argument name to imply that it must be a
> > referenced pointer when passed to kfunc. This is required to ensure that
> > kfunc that operate on some object only work on acquired pointers and not
> > normal PTR_TO_BTF_ID with same type which can be obtained by pointer
> > walking. Release functions need not specify such suffix on release
> > arguments as they are already expected to receive one referenced
> > argument.
> >
> > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
> > Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> > ---
> >   kernel/bpf/btf.c   | 40 ++++++++++++++++++++++++++++++----------
> >   net/bpf/test_run.c |  5 +++++
> >   2 files changed, 35 insertions(+), 10 deletions(-)
> >
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index 2f0b0440131c..83a354732d96 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -6021,18 +6021,13 @@ static bool __btf_type_is_scalar_struct(struct bpf_verifier_log *log,
> >   	return true;
> >   }
> > -static bool is_kfunc_arg_mem_size(const struct btf *btf,
> > -				  const struct btf_param *arg,
> > -				  const struct bpf_reg_state *reg)
> > +static bool btf_param_match_suffix(const struct btf *btf,
> > +				   const struct btf_param *arg,
> > +				   const char *suffix)
> >   {
> > -	int len, sfx_len = sizeof("__sz") - 1;
> > -	const struct btf_type *t;
> > +	int len, sfx_len = strlen(suffix);
> >   	const char *param_name;
> > -	t = btf_type_skip_modifiers(btf, arg->type, NULL);
> > -	if (!btf_type_is_scalar(t) || reg->type != SCALAR_VALUE)
> > -		return false;
> > -
> >   	/* In the future, this can be ported to use BTF tagging */
> >   	param_name = btf_name_by_offset(btf, arg->name_off);
> >   	if (str_is_empty(param_name))
> > @@ -6041,12 +6036,31 @@ static bool is_kfunc_arg_mem_size(const struct btf *btf,
> >   	if (len < sfx_len)
> >   		return false;
> >   	param_name += len - sfx_len;
> > -	if (strncmp(param_name, "__sz", sfx_len))
> > +	if (strncmp(param_name, suffix, sfx_len))
> >   		return false;
> >   	return true;
> >   }
> > +static bool is_kfunc_arg_ref(const struct btf *btf,
> > +			     const struct btf_param *arg)
> > +{
> > +	return btf_param_match_suffix(btf, arg, "__ref");
>
> Do we also need to do btf_type_skip_modifiers and to ensure
> the type after skipping modifiers are a pointer type?
> The current implementation should work for
> bpf_kfunc_call_test_ref(), but with additional checking
> we may avoid some accidental mistakes.
>

The point where this check happens, arg[i].type is already known to be a pointer
type, after skipping modifiers.

> > +}
> > +
> > +static bool is_kfunc_arg_mem_size(const struct btf *btf,
> > +				  const struct btf_param *arg,
> > +				  const struct bpf_reg_state *reg)
> > +{
> > +	const struct btf_type *t;
> > +
> > +	t = btf_type_skip_modifiers(btf, arg->type, NULL);
> > +	if (!btf_type_is_scalar(t) || reg->type != SCALAR_VALUE)
> > +		return false;
> > +
> > +	return btf_param_match_suffix(btf, arg, "__sz");
> > +}
> > +
> >   static int btf_check_func_arg_match(struct bpf_verifier_env *env,
> >   				    const struct btf *btf, u32 func_id,
> >   				    struct bpf_reg_state *regs,
> > @@ -6115,6 +6129,12 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
> >   			return -EINVAL;
> >   		}
> > +		/* Check if argument must be a referenced pointer */
> > +		if (is_kfunc && is_kfunc_arg_ref(btf, args + i) && !reg->ref_obj_id) {
> > +			bpf_log(log, "R%d must be referenced\n", regno);
> > +			return -EINVAL;
> > +		}
> > +
> >   		ref_t = btf_type_skip_modifiers(btf, t->type, &ref_id);
> >   		ref_tname = btf_name_by_offset(btf, ref_t->name_off);
> > diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> > index 4d08cca771c7..adbc7dd18511 100644
> > --- a/net/bpf/test_run.c
> > +++ b/net/bpf/test_run.c
> > @@ -690,6 +690,10 @@ noinline void bpf_kfunc_call_test_mem_len_fail2(u64 *mem, int len)
> >   {
> >   }
> > +noinline void bpf_kfunc_call_test_ref(struct prog_test_ref_kfunc *p__ref)
> > +{
> > +}
> > +
> >   __diag_pop();
> [...]

--
Kartikeya