linux-kernel - Re: [PATCH bpf-next v5 2/6] bpf/verifier: add bpf

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Date: Mon, 25 Mar 2024 09:42:08 +0100
From: Benjamin Tissoires <bentiss@...nel.org>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Kumar Kartikeya Dwivedi <memxor@...il.com>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>, 
	bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH bpf-next v5 2/6] bpf/verifier: add bpf_timer as a kfunc
 capable type

On Mar 24 2024, Alexei Starovoitov wrote:
> On Sat, Mar 23, 2024 at 9:57 PM Kumar Kartikeya Dwivedi
> <memxor@...il.com> wrote:
> 
> > > > >
> > > > > Observation is correct. The patch is buggy,
> > > > > but the suggestion to follow process_dynptr_func() will lead
> > > > > to unnecessary complexity.
> > > > > dynptr-s are on stack with plenty of extra checks.
> > > >
> > > > The suggestion was to call process_timer_func, not process_dynptr_func.
> > > >
> > > > > In this case bpf_timer is in map_value.
> > > > > Much simpler is to follow KF_ARG_PTR_TO_MAP approach.
> > > >
> > > > What I meant by the example was that dynptr handling does the same
> > > > thing for kfuncs and helpers (using the same function), so timer
> > > > arguments should do the same (i.e. use process_timer_func), which will
> > > > do all checks for constant offset (ensuring var_off is tnum_is_const)
> > > > and match it against btf_record->timer_off.
> > >
> > > I don't follow. Please elaborate with a patch.
> > > The var_off and off is a part of the bug, but it's not the biggest part of it.
> >
> > Not compile tested.

Compiles just fine :)

> 
> I see. All makes sense to me.
> 
> Benjamin,
> pls incorporate it in your set.
> 

OK, done!

I just had to revert to the following or KF_ARG_TIMER_ID was not
recognized by the verifier:

---
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 7ee20e9d14bd..a5e147468ac8 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10848,7 +10848,7 @@ BTF_ID(struct, bpf_list_head)
 BTF_ID(struct, bpf_list_node)
 BTF_ID(struct, bpf_rb_root)
 BTF_ID(struct, bpf_rb_node)
-BTF_ID(struct, bpf_timer)
+BTF_ID(struct, bpf_timer_kern)
 
 static bool __is_kfunc_ptr_arg_type(const struct btf *btf,
 				    const struct btf_param *arg, int type)
---

Cheers,
Benjamin