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[176.36.0.241]) by smtp.gmail.com with ESMTPSA id d29-20020a50f69d000000b00522828d438csm1353781edn.7.2023.10.31.08.37.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 31 Oct 2023 08:37:36 -0700 (PDT) Message-ID: Subject: Re: [PATCH v5 bpf-next 05/23] bpf: derive subreg bounds from full bounds when upper 32 bits are constant 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 17:37:34 +0200 In-Reply-To: <20231027181346.4019398-6-andrii@kernel.org> References: <20231027181346.4019398-1-andrii@kernel.org> <20231027181346.4019398-6-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: > > Comments in code try to explain the idea behind why this is correct. > > Please check the code and comments. > >=20 > > Acked-by: Shung-Hsi Yu > > Signed-off-by: Andrii Nakryiko Acked-by: Eduard Zingerman > > --- > > kernel/bpf/verifier.c | 45 +++++++++++++++++++++++++++++++++++++++++++ > > 1 file changed, 45 insertions(+) > >=20 > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > > index 0f66e9092c38..5082ca1ea5dc 100644 > > --- a/kernel/bpf/verifier.c > > +++ b/kernel/bpf/verifier.c > > @@ -2324,6 +2324,51 @@ static void __update_reg_bounds(struct bpf_reg_s= tate *reg) > > /* Uses signed min/max values to inform unsigned, and vice-versa */ > > static void __reg32_deduce_bounds(struct bpf_reg_state *reg) > > { > > + /* If upper 32 bits of u64/s64 range don't change, we can use lower 3= 2 > > + * bits to improve our u32/s32 boundaries. > > + * > > + * E.g., the case where we have upper 32 bits as zero ([10, 20] in > > + * u64) is pretty trivial, it's obvious that in u32 we'll also have > > + * [10, 20] range. But this property holds for any 64-bit range as > > + * long as upper 32 bits in that entire range of values stay the same= . > > + * > > + * E.g., u64 range [0x10000000A, 0x10000000F] ([4294967306, 429496731= 1] > > + * in decimal) has the same upper 32 bits throughout all the values i= n > > + * that range. As such, lower 32 bits form a valid [0xA, 0xF] ([10, 1= 5]) > > + * range. > > + * > > + * Note also, that [0xA, 0xF] is a valid range both in u32 and in s32= , > > + * following the rules outlined below about u64/s64 correspondence > > + * (which equally applies to u32 vs s32 correspondence). In general i= t > > + * depends on actual hexadecimal values of 32-bit range. They can for= m > > + * only valid u32, or only valid s32 ranges in some cases. > > + * > > + * So we use all these insights to derive bounds for subregisters her= e. > > + */ > > + if ((reg->umin_value >> 32) =3D=3D (reg->umax_value >> 32)) { > > + /* u64 to u32 casting preserves validity of low 32 bits as > > + * a range, if upper 32 bits are the same > > + */ > > + reg->u32_min_value =3D max_t(u32, reg->u32_min_value, (u32)reg->umin= _value); > > + reg->u32_max_value =3D min_t(u32, reg->u32_max_value, (u32)reg->umax= _value); > > + > > + if ((s32)reg->umin_value <=3D (s32)reg->umax_value) { > > + reg->s32_min_value =3D max_t(s32, reg->s32_min_value, (s32)reg->umi= n_value); > > + reg->s32_max_value =3D min_t(s32, reg->s32_max_value, (s32)reg->uma= x_value); > > + } > > + } > > + if ((reg->smin_value >> 32) =3D=3D (reg->smax_value >> 32)) { > > + /* low 32 bits should form a proper u32 range */ > > + if ((u32)reg->smin_value <=3D (u32)reg->smax_value) { > > + reg->u32_min_value =3D max_t(u32, reg->u32_min_value, (u32)reg->smi= n_value); > > + reg->u32_max_value =3D min_t(u32, reg->u32_max_value, (u32)reg->sma= x_value); > > + } > > + /* low 32 bits should form a proper s32 range */ > > + if ((s32)reg->smin_value <=3D (s32)reg->smax_value) { > > + reg->s32_min_value =3D max_t(s32, reg->s32_min_value, (s32)reg->smi= n_value); > > + reg->s32_max_value =3D min_t(s32, reg->s32_max_value, (s32)reg->sma= x_value); > > + } > > + } > > /* if u32 range forms a valid s32 range (due to matching sign bit), > > * try to learn from that > > */