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[176.36.0.241]) by smtp.gmail.com with ESMTPSA id l20-20020a1709065a9400b009a168ab6ee2sm1117661ejq.164.2023.11.03.10.28.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 03 Nov 2023 10:28:04 -0700 (PDT) Message-ID: <8d2ada70affa8559cf954f64316b5ea0c9d1d348.camel@gmail.com> Subject: Re: [PATCH bpf-next 03/13] bpf: enhance BPF_JEQ/BPF_JNE is_branch_taken logic 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 Date: Fri, 03 Nov 2023 19:28:02 +0200 In-Reply-To: <20231103000822.2509815-4-andrii@kernel.org> References: <20231103000822.2509815-1-andrii@kernel.org> <20231103000822.2509815-4-andrii@kernel.org> Autocrypt: addr=eddyz87@gmail.com; prefer-encrypt=mutual; keydata=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 nYzhLWGcczc6J71q1Dje0l5vIPaSFOgwmWD4DA+WvuxM/shH4rtWeodbv iCTce6yYIygHgUAtJcHozAlgRrL0jz44cggBTcoeXp/atckXK546OugZPnl00J3qmm5uWAznU6T5YDv2vCvAMEbz69ib+kHtnOSBvR0Jb86UZZqSb4ATfwMOWe9htGTjKMb0QQOLK0mTcrk/TtymaG+T4Fsos0kgrxqjgfrxxEhYcVNW8v8HISmFGFbqsJmFbVtgk68BcU0wgF8oFxo7u+XYQDdKbI1uQGNBGKNNQEBDADbQIdo8L3sdSWGQtu+LnFqCZoAbYurZCmUjLV3df1b+sg+GJZvVTmMZnzDP/ADufcbjopBBjGTRAY4L76T2niu2EpjclMMM3mtrOc738Kr3+RvPjUupdkZ1ZEZaWpf4cZm+4wH5GUfyu5pmD5WXX2i1r9XaUjeVtebvbuXWmWI1ZDTfOkiz/6Z0GDSeQeEqx2PXYBcepU7S9UNWttDtiZ0+IH4DZcvyKPUcK3tOj4u8GvO3RnOrglERzNCM/WhVdG1+vgU9fXO83TB/PcfAsvxYSie7u792s/I+yA4XKKh82PSTvTzg2/4vEDGpI9yubkfXRkQN28w+HKF5qoRB8/L1ZW/brlXkNzA6SveJhCnH7aOF0Yezl6TfX27w1CW5Xmvfi7X33V/SPvo0tY1THrO1c+bOjt5F+2/K3tvejmXMS/I6URwa8n1e767y5ErFKyXAYRweE9zarEgpNZTuSIGNNAqK+SiLLXt51G7P30TVavIeB6s2lCt1QKt62ccLqUAEQEAAYkBvAQYAQoAJhYhBMfui64wKUMqp1wmBC4MZXpJYEQJBQJijTUBAhsMBQkDwmcAAAoJEC4MZXpJYEQJkRAMAKNvWVwtXm/WxWoiLnXyF2WGXKoDe5+itTLvBmKcV/b1OKZF1s90V7WfSBz712eFAynEzyeezPbwU8QBiTpZcHXwQni3IYKvsh7s 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 Thu, 2023-11-02 at 17:08 -0700, Andrii Nakryiko wrote: > Use 32-bit subranges to prune some 64-bit BPF_JEQ/BPF_JNE conditions > that otherwise would be "inconclusive" (i.e., is_branch_taken() would > return -1). This can happen, for example, when registers are initialized > as 64-bit u64/s64, then compared for inequality as 32-bit subregisters, > and then followed by 64-bit equality/inequality check. That 32-bit > inequality can establish some pattern for lower 32 bits of a register > (e.g., s< 0 condition determines whether the bit #31 is zero or not), > while overall 64-bit value could be anything (according to a value range > representation). >=20 > This is not a fancy quirky special case, but actually a handling that's > necessary to prevent correctness issue with BPF verifier's range > tracking: set_range_min_max() assumes that register ranges are > non-overlapping, and if that condition is not guaranteed by > is_branch_taken() we can end up with invalid ranges, where min > max. Acked-by: Eduard Zingerman >=20 > [0] https://lore.kernel.org/bpf/CACkBjsY2q1_fUohD7hRmKGqv1MV=3DeP2f6XK8= kjkYNw7BaiF8iQ@mail.gmail.com/ >=20 > Signed-off-by: Andrii Nakryiko > --- > kernel/bpf/verifier.c | 24 ++++++++++++++++++++++++ > 1 file changed, 24 insertions(+) >=20 > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index 2627461164ed..8691cacd3ad3 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -14214,6 +14214,18 @@ static int is_scalar_branch_taken(struct bpf_reg= _state *reg1, struct bpf_reg_sta > return 0; > if (smin1 > smax2 || smax1 < smin2) > return 0; > + if (!is_jmp32) { > + /* if 64-bit ranges are inconclusive, see if we can > + * utilize 32-bit subrange knowledge to eliminate > + * branches that can't be taken a priori > + */ > + if (reg1->u32_min_value > reg2->u32_max_value || > + reg1->u32_max_value < reg2->u32_min_value) > + return 0; > + if (reg1->s32_min_value > reg2->s32_max_value || > + reg1->s32_max_value < reg2->s32_min_value) > + return 0; > + } > break; > case BPF_JNE: > /* constants, umin/umax and smin/smax checks would be > @@ -14226,6 +14238,18 @@ static int is_scalar_branch_taken(struct bpf_reg= _state *reg1, struct bpf_reg_sta > return 1; > if (smin1 > smax2 || smax1 < smin2) > return 1; > + if (!is_jmp32) { > + /* if 64-bit ranges are inconclusive, see if we can > + * utilize 32-bit subrange knowledge to eliminate > + * branches that can't be taken a priori > + */ > + if (reg1->u32_min_value > reg2->u32_max_value || > + reg1->u32_max_value < reg2->u32_min_value) > + return 1; > + if (reg1->s32_min_value > reg2->s32_max_value || > + reg1->s32_max_value < reg2->s32_min_value) > + return 1; > + } > break; > case BPF_JSET: > if (!is_reg_const(reg2, is_jmp32)) {