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Message-ID: <20180427234817.g6zprnsoj3cfvzhb@ast-mbp>
Date: Fri, 27 Apr 2018 16:48:19 -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 v7 05/10] bpf/verifier: improve register value
range tracking with ARSH
On Wed, Apr 25, 2018 at 12:29:05PM -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>
> ---
> include/linux/tnum.h | 4 +++-
> kernel/bpf/tnum.c | 10 ++++++++++
> kernel/bpf/verifier.c | 41 +++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 54 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/tnum.h b/include/linux/tnum.h
> index 0d2d3da..c7dc2b5 100644
> --- a/include/linux/tnum.h
> +++ b/include/linux/tnum.h
> @@ -23,8 +23,10 @@ struct tnum tnum_range(u64 min, u64 max);
> /* Arithmetic and logical ops */
> /* Shift a tnum left (by a fixed shift) */
> struct tnum tnum_lshift(struct tnum a, u8 shift);
> -/* Shift a tnum right (by a fixed shift) */
> +/* Shift (rsh) a tnum right (by a fixed shift) */
> struct tnum tnum_rshift(struct tnum a, u8 shift);
> +/* Shift (arsh) a tnum right (by a fixed min_shift) */
> +struct tnum tnum_arshift(struct tnum a, u8 min_shift);
> /* Add two tnums, return @a + @b */
> struct tnum tnum_add(struct tnum a, struct tnum b);
> /* Subtract two tnums, return @a - @b */
> diff --git a/kernel/bpf/tnum.c b/kernel/bpf/tnum.c
> index 1f4bf68..938d412 100644
> --- a/kernel/bpf/tnum.c
> +++ b/kernel/bpf/tnum.c
> @@ -43,6 +43,16 @@ struct tnum tnum_rshift(struct tnum a, u8 shift)
> return TNUM(a.value >> shift, a.mask >> shift);
> }
>
> +struct tnum tnum_arshift(struct tnum a, u8 min_shift)
> +{
> + /* if a.value is negative, arithmetic shifting by minimum shift
> + * will have larger negative offset compared to more shifting.
> + * If a.value is nonnegative, arithmetic shifting by minimum shift
> + * will have larger positive offset compare to more shifting.
> + */
> + return TNUM((s64)a.value >> min_shift, (s64)a.mask >> min_shift);
> +}
> +
> struct tnum tnum_add(struct tnum a, struct tnum b)
> {
> u64 sm, sv, sigma, chi, mu;
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 6e3f859..573807f 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -2974,6 +2974,47 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
> /* We may learn something more from the var_off */
> __update_reg_bounds(dst_reg);
> break;
> + case BPF_ARSH:
> + if (umax_val >= insn_bitness) {
> + /* Shifts greater than 31 or 63 are undefined.
> + * This includes shifts by a negative number.
> + */
> + mark_reg_unknown(env, regs, insn->dst_reg);
> + break;
> + }
> +
> + /* BPF_ARSH is an arithmetic shift. The new range of
> + * smin_value and smax_value should take the sign
> + * into consideration.
> + *
> + * For example, if smin_value = -16, umin_val = 0
> + * and umax_val = 2, the new smin_value should be
> + * -16 >> 0 = -16 since -16 >> 2 = -4.
> + * If smin_value = 16, umin_val = 0 and umax_val = 2,
> + * the new smin_value should be 16 >> 2 = 4.
> + *
> + * Now suppose smax_value = -4, umin_val = 0 and
> + * umax_val = 2, the new smax_value should be
> + * -4 >> 2 = -1. If smax_value = 32 with the same
> + * umin_val/umax_val, the new smax_value should remain 32.
> + */
> + if (dst_reg->smin_value < 0)
> + dst_reg->smin_value >>= umin_val;
> + else
> + dst_reg->smin_value >>= umax_val;
> + if (dst_reg->smax_value < 0)
> + dst_reg->smax_value >>= umax_val;
> + else
> + dst_reg->smax_value >>= umin_val;
above sounds correct, but unnecessary, since we have this:
if ((src_known && (smin_val != smax_val || umin_val != umax_val)) mark_unknown
at the top.
Also would it work if we blow smin/smax just like umin/umax
and rely on tnum_arshift only?
When you rebase please document new helper in new man-page style.
Thanks
> + dst_reg->var_off = tnum_arshift(dst_reg->var_off, umin_val);
> +
> + /* blow away the dst_reg umin_value/umax_value and rely on
> + * dst_reg var_off to refine the result.
> + */
> + dst_reg->umin_value = 0;
> + dst_reg->umax_value = U64_MAX;
> + __update_reg_bounds(dst_reg);
> + break;
> default:
> mark_reg_unknown(env, regs, insn->dst_reg);
> break;
> --
> 2.9.5
>
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