netdev - Re: [PATCH bpf-next v8 05/10] bpf/verifier: improve register value range tracking with ARSH

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Message-ID: <20180428165247.cvrnfiz3y7phjnbz@ast-mbp>
Date:   Sat, 28 Apr 2018 09:52:49 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     ast@...com, daniel@...earbox.net, netdev@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH bpf-next v8 05/10] bpf/verifier: improve register value
 range tracking with ARSH

On Sat, Apr 28, 2018 at 12:02:00AM -0700, Yonghong Song wrote:
> When helpers like bpf_get_stack returns an int value
> and later on used for arithmetic computation, the LSH and ARSH
> operations are often required to get proper sign extension into
> 64-bit. For example, without this patch:
>     54: R0=inv(id=0,umax_value=800)
>     54: (bf) r8 = r0
>     55: R0=inv(id=0,umax_value=800) R8_w=inv(id=0,umax_value=800)
>     55: (67) r8 <<= 32
>     56: R8_w=inv(id=0,umax_value=3435973836800,var_off=(0x0; 0x3ff00000000))
>     56: (c7) r8 s>>= 32
>     57: R8=inv(id=0)
> With this patch:
>     54: R0=inv(id=0,umax_value=800)
>     54: (bf) r8 = r0
>     55: R0=inv(id=0,umax_value=800) R8_w=inv(id=0,umax_value=800)
>     55: (67) r8 <<= 32
>     56: R8_w=inv(id=0,umax_value=3435973836800,var_off=(0x0; 0x3ff00000000))
>     56: (c7) r8 s>>= 32
>     57: R8=inv(id=0, umax_value=800,var_off=(0x0; 0x3ff))
> With better range of "R8", later on when "R8" is added to other register,
> e.g., a map pointer or scalar-value register, the better register
> range can be derived and verifier failure may be avoided.
> 
> In our later example,
>     ......
>     usize = bpf_get_stack(ctx, raw_data, max_len, BPF_F_USER_STACK);
>     if (usize < 0)
>         return 0;
>     ksize = bpf_get_stack(ctx, raw_data + usize, max_len - usize, 0);
>     ......
> Without improving ARSH value range tracking, the register representing
> "max_len - usize" will have smin_value equal to S64_MIN and will be
> rejected by verifier.
> 
> Signed-off-by: Yonghong Song <yhs@...com>

Acked-by: Alexei Starovoitov <ast@...nel.org>