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[176.36.0.241]) by smtp.gmail.com with ESMTPSA id u11-20020a1709063b8b00b009829d2e892csm1176267ejf.15.2023.10.31.08.37.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 31 Oct 2023 08:37:48 -0700 (PDT) Message-ID: Subject: Re: [PATCH v5 bpf-next 06/23] bpf: add special smin32/smax32 derivation from 64-bit bounds From: Eduard Zingerman To: Andrii Nakryiko , bpf@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net, martin.lau@kernel.org Cc: kernel-team@meta.com, Shung-Hsi Yu Date: Tue, 31 Oct 2023 17:37:47 +0200 In-Reply-To: <20231027181346.4019398-7-andrii@kernel.org> References: <20231027181346.4019398-1-andrii@kernel.org> <20231027181346.4019398-7-andrii@kernel.org> Autocrypt: addr=eddyz87@gmail.com; prefer-encrypt=mutual; keydata=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 nYzhLWGcczc6J71q1Dje0l5vIPaSFOgwmWD4DA+WvuxM/shH4rtWeodbv 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 t1iq+gsfnXbPz5AnS598ScZI1oP7OrPSFJkt/z4acEbOQDQs8aUqrd46PV jsdqGvKnXZxzylux29UTNby4jTlz9pNJM+wPrDRmGfchLDUmf6CffaUYCbu4FiId+9+dcTCDvxbABRy1C3OJ8QY7cxfJ+pEZW18fRJ0XCl/fiV/ecAOfB3HsqgTzAn555h0rkFgay0hAvMU/mAW/CFNSIxV397zm749ZNLA0L2dMy1AKuOqH+/B+/ImBfJMDjmdyJQ8WU/OFRuGLdqOd2oZrA1iuPIa+yUYyZkaZfz/emQwpIL1+Q4p1R/OplA4yc301AqruXXUcVDbEB+joHW3hy5FwK5t5OwTKatrSJBkydSF9zdXy98fYzGniRyRA65P0Ix/8J3BYB4edY2/w0Ip/mdYsYQljBY0A== Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.50.0 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Fri, 2023-10-27 at 11:13 -0700, Andrii Nakryiko wrote: > Add a special case where we can derive valid s32 bounds from umin/umax > or smin/smax by stitching together negative s32 subrange and > non-negative s32 subrange. That requires upper 32 bits to form a [N, N+1] > range in u32 domain (taking into account wrap around, so 0xffffffff > to 0x00000000 is a valid [N, N+1] range in this sense). See code comment > for concrete examples. >=20 > Acked-by: Shung-Hsi Yu > Signed-off-by: Andrii Nakryiko Acked-by: Eduard Zingerman fwiw, an alternative explanation might be arithmetic based. Suppose: . there are numbers a, b, c . 2**31 <=3D b < 2**32 . 0 <=3D c < 2**31 . umin =3D 2**32 * a + b . umax =3D 2**32 * (a + 1) + c The number of values in the range represented by [umin; umax] is: . N =3D umax - umin + 1 =3D 2**32 + c - b + 1 . min(N) =3D 2**32 + 0 - (2**32-1) + 1 =3D 2 . max(N) =3D 2**32 + (2**31 - 1) - 2**31 + 1 =3D 2**32 Hence [(s32)b; (s32)c] form a valid range. At-least that's how I convinced myself. > --- > kernel/bpf/verifier.c | 23 +++++++++++++++++++++++ > 1 file changed, 23 insertions(+) >=20 > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index 5082ca1ea5dc..38d21d0e46bd 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -2369,6 +2369,29 @@ static void __reg32_deduce_bounds(struct bpf_reg_s= tate *reg) > reg->s32_max_value =3D min_t(s32, reg->s32_max_value, (s32)reg->smax_= value); > } > } > + /* Special case where upper bits form a small sequence of two > + * sequential numbers (in 32-bit unsigned space, so 0xffffffff to > + * 0x00000000 is also valid), while lower bits form a proper s32 range > + * going from negative numbers to positive numbers. E.g., let's say we > + * have s64 range [-1, 1] ([0xffffffffffffffff, 0x0000000000000001]). > + * Possible s64 values are {-1, 0, 1} ({0xffffffffffffffff, > + * 0x0000000000000000, 0x00000000000001}). Ignoring upper 32 bits, > + * we still get a valid s32 range [-1, 1] ([0xffffffff, 0x00000001]). > + * Note that it doesn't have to be 0xffffffff going to 0x00000000 in > + * upper 32 bits. As a random example, s64 range > + * [0xfffffff0ffffff00; 0xfffffff100000010], forms a valid s32 range > + * [-16, 16] ([0xffffff00; 0x00000010]) in its 32 bit subregister. > + */ > + if ((u32)(reg->umin_value >> 32) + 1 =3D=3D (u32)(reg->umax_value >> 32= ) && > + (s32)reg->umin_value < 0 && (s32)reg->umax_value >=3D 0) { > + reg->s32_min_value =3D max_t(s32, reg->s32_min_value, (s32)reg->umin_v= alue); > + reg->s32_max_value =3D min_t(s32, reg->s32_max_value, (s32)reg->umax_v= alue); > + } > + if ((u32)(reg->smin_value >> 32) + 1 =3D=3D (u32)(reg->smax_value >> 32= ) && > + (s32)reg->smin_value < 0 && (s32)reg->smax_value >=3D 0) { > + reg->s32_min_value =3D max_t(s32, reg->s32_min_value, (s32)reg->smin_v= alue); > + reg->s32_max_value =3D min_t(s32, reg->s32_max_value, (s32)reg->smax_v= alue); > + } > /* if u32 range forms a valid s32 range (due to matching sign bit), > * try to learn from that > */