netdev - Re: [PATCH bpf-next 12/15] bpf: Preserve boundaries and track scalars on narrowing fill

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Message-ID: <ZZhA3LBjXEuCQH5Q@mail.gmail.com>
Date: Fri, 5 Jan 2024 19:48:25 +0200
From: Maxim Mikityanskiy <maxtram95@...il.com>
To: Eduard Zingerman <eddyz87@...il.com>
Cc: bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>,
	John Fastabend <john.fastabend@...il.com>,
	Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	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>,
	"David S. Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	linux-kselftest@...r.kernel.org, netdev@...r.kernel.org,
	Maxim Mikityanskiy <maxim@...valent.com>
Subject: Re: [PATCH bpf-next 12/15] bpf: Preserve boundaries and track
 scalars on narrowing fill

On Thu, 04 Jan 2024 at 04:27:00 +0200, Eduard Zingerman wrote:
> On Wed, 2023-12-20 at 23:40 +0200, Maxim Mikityanskiy wrote:
> 
> [...]
> 
> The two tests below were added by the following commit:
> ef979017b837 ("bpf: selftest: Add verifier tests for <8-byte scalar spill and refill")
> 
> As far as I understand, the original intent was to check the behavior
> for stack read/write with non-matching size.
> I think these tests are redundant after patch #13. Wdyt?

_6_offset_to_skb_data is for sure not redundant. I don't test a partial
fill from the most significant bits in my patch 13.

u16_offset_to_skb_data is somewhat similar to
fill_32bit_after_spill_64bit, but they aren't exactly the same: the
former spills (u32)20 and fills (u16)20 (the same value), while my test
spills (u64)0xXXXXXXXX00000000 and fills (u32)0 (the most significant
bits are stripped). Maybe u16_offset_to_skb_data is redundant, but more
coverage is better than less coverage, isn't it?

> > diff --git a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
> > index 809a09732168..de03e72e07a9 100644
> > --- a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
> > +++ b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
> > @@ -217,7 +217,7 @@ __naked void uninit_u32_from_the_stack(void)
> >  
> >  SEC("tc")
> >  __description("Spill a u32 const scalar.  Refill as u16.  Offset to skb->data")
> > -__failure __msg("invalid access to packet")
> > +__success __retval(0)
> >  __naked void u16_offset_to_skb_data(void)
> >  {
> >  	asm volatile ("					\
> > @@ -225,19 +225,24 @@ __naked void u16_offset_to_skb_data(void)
> >  	r3 = *(u32*)(r1 + %[__sk_buff_data_end]);	\
> >  	w4 = 20;					\
> >  	*(u32*)(r10 - 8) = r4;				\
> > -	r4 = *(u16*)(r10 - 8);				\
> > +	r4 = *(u16*)(r10 - %[offset]);			\
> >  	r0 = r2;					\
> > -	/* r0 += r4 R0=pkt R2=pkt R3=pkt_end R4=umax=65535 */\
> > +	/* r0 += r4 R0=pkt R2=pkt R3=pkt_end R4=20 */\
> >  	r0 += r4;					\
> > -	/* if (r0 > r3) R0=pkt,umax=65535 R2=pkt R3=pkt_end R4=umax=65535 */\
> > +	/* if (r0 > r3) R0=pkt,off=20 R2=pkt R3=pkt_end R4=20 */\
> >  	if r0 > r3 goto l0_%=;				\
> > -	/* r0 = *(u32 *)r2 R0=pkt,umax=65535 R2=pkt R3=pkt_end R4=20 */\
> > +	/* r0 = *(u32 *)r2 R0=pkt,off=20 R2=pkt R3=pkt_end R4=20 */\
> >  	r0 = *(u32*)(r2 + 0);				\
> >  l0_%=:	r0 = 0;						\
> >  	exit;						\
> >  "	:
> >  	: __imm_const(__sk_buff_data, offsetof(struct __sk_buff, data)),
> > -	  __imm_const(__sk_buff_data_end, offsetof(struct __sk_buff, data_end))
> > +	  __imm_const(__sk_buff_data_end, offsetof(struct __sk_buff, data_end)),
> > +#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> > +	  __imm_const(offset, 8)
> > +#else
> > +	  __imm_const(offset, 6)
> > +#endif
> >  	: __clobber_all);
> >  }
> >  
> > @@ -270,7 +275,7 @@ l0_%=:	r0 = 0;						\
> >  }
> >  
> >  SEC("tc")
> > -__description("Spill a u32 const scalar.  Refill as u16 from fp-6.  Offset to skb->data")
> > +__description("Spill a u32 const scalar.  Refill as u16 from MSB.  Offset to skb->data")
> >  __failure __msg("invalid access to packet")
> >  __naked void _6_offset_to_skb_data(void)
> >  {
> > @@ -279,7 +284,7 @@ __naked void _6_offset_to_skb_data(void)
> >  	r3 = *(u32*)(r1 + %[__sk_buff_data_end]);	\
> >  	w4 = 20;					\
> >  	*(u32*)(r10 - 8) = r4;				\
> > -	r4 = *(u16*)(r10 - 6);				\
> > +	r4 = *(u16*)(r10 - %[offset]);			\
> >  	r0 = r2;					\
> >  	/* r0 += r4 R0=pkt R2=pkt R3=pkt_end R4=umax=65535 */\
> >  	r0 += r4;					\
> > @@ -291,7 +296,12 @@ l0_%=:	r0 = 0;						\
> >  	exit;						\
> >  "	:
> >  	: __imm_const(__sk_buff_data, offsetof(struct __sk_buff, data)),
> > -	  __imm_const(__sk_buff_data_end, offsetof(struct __sk_buff, data_end))
> > +	  __imm_const(__sk_buff_data_end, offsetof(struct __sk_buff, data_end)),
> > +#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> > +	  __imm_const(offset, 6)
> > +#else
> > +	  __imm_const(offset, 8)
> > +#endif
> >  	: __clobber_all);
> >  }
> >  
> 
> 
>