2022-07-29 06:12:46

by Zeng Jingxiang

Subject: [PATCH] bpf/verifier: fix control flow issues in __reg64_bound_u32()

From: Zeng Jingxiang <[email protected]>

This greater-than-or-equal-to-zero comparison of an unsigned value
is always true. "a >= U32_MIN".
1632 return a >= U32_MIN && a <= U32_MAX;

Fixes: b9979db83401 ("bpf: Fix propagation of bounds from 64-bit min/max into 32-bit and var_off.")
Signed-off-by: Zeng Jingxiang <[email protected]>
---
kernel/bpf/verifier.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 0efbac0fd126..dd67108fb1d7 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1629,7 +1629,7 @@ static bool __reg64_bound_s32(s64 a)

static bool __reg64_bound_u32(u64 a)
{
- return a >= U32_MIN && a <= U32_MAX;
+ return a <= U32_MAX;
}

static void __reg_combine_64_into_32(struct bpf_reg_state *reg)
--
2.27.0

2022-07-29 17:19:05

by Yonghong Song

Subject: Re: [PATCH] bpf/verifier: fix control flow issues in __reg64_bound_u32()

On 7/28/22 10:49 PM, Zeng Jingxiang wrote:
> From: Zeng Jingxiang <[email protected]>
>
> This greater-than-or-equal-to-zero comparison of an unsigned value
> is always true. "a >= U32_MIN".
> 1632 return a >= U32_MIN && a <= U32_MAX;
>
> Fixes: b9979db83401 ("bpf: Fix propagation of bounds from 64-bit min/max into 32-bit and var_off.")
> Signed-off-by: Zeng Jingxiang <[email protected]>
> ---
> kernel/bpf/verifier.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 0efbac0fd126..dd67108fb1d7 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -1629,7 +1629,7 @@ static bool __reg64_bound_s32(s64 a)
>
> static bool __reg64_bound_u32(u64 a)
> {
> - return a >= U32_MIN && a <= U32_MAX;
> + return a <= U32_MAX;
> }

I cannot find the related link. But IIRC, Alexei commented that
the code is written this way to express the intention (within
32bit bounds) so this patch is unnecessary...

>
> static void __reg_combine_64_into_32(struct bpf_reg_state *reg)

2022-07-29 17:20:00

by Hao Luo

Subject: Re: [PATCH] bpf/verifier: fix control flow issues in __reg64_bound_u32()

On Fri, Jul 29, 2022 at 10:15 AM Yonghong Song <[email protected]> wrote:
>
>
>
> On 7/28/22 10:49 PM, Zeng Jingxiang wrote:
> > From: Zeng Jingxiang <[email protected]>
> >
> > This greater-than-or-equal-to-zero comparison of an unsigned value
> > is always true. "a >= U32_MIN".
> > 1632 return a >= U32_MIN && a <= U32_MAX;
> >
> > Fixes: b9979db83401 ("bpf: Fix propagation of bounds from 64-bit min/max into 32-bit and var_off.")
> > Signed-off-by: Zeng Jingxiang <[email protected]>
> > ---
> > kernel/bpf/verifier.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 0efbac0fd126..dd67108fb1d7 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -1629,7 +1629,7 @@ static bool __reg64_bound_s32(s64 a)
> >
> > static bool __reg64_bound_u32(u64 a)
> > {
> > - return a >= U32_MIN && a <= U32_MAX;
> > + return a <= U32_MAX;
> > }
>
> I cannot find the related link. But IIRC, Alexei commented that
> the code is written this way to express the intention (within
> 32bit bounds) so this patch is unnecessary...
>

Yeah, I agree with Yonghong. I was about to reply.

Jingxiang, you are absolutely correct that a <= U32_MAX is redundant,
but I feel having both sides checked explicitly makes code more
readable.

> >
> > static void __reg_combine_64_into_32(struct bpf_reg_state *reg)