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[176.36.0.241]) by smtp.gmail.com with ESMTPSA id s18-20020a5d4ed2000000b00336421f1818sm3978765wrv.112.2023.12.14.08.26.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 14 Dec 2023 08:26:14 -0800 (PST) Message-ID: <73d021e3f77161668aae833e478b210ed5cd2f4d.camel@gmail.com> Subject: Re: [Bug Report] bpf: incorrectly pruning runtime execution path From: Eduard Zingerman To: Hao Sun Cc: Andrii Nakryiko , Alexei Starovoitov , Andrii Nakryiko , Daniel Borkmann , bpf , Linux Kernel Mailing List Date: Thu, 14 Dec 2023 18:26:13 +0200 In-Reply-To: <9dee19c7d39795242c15b2f7aa56fb4a6c3ebffa.camel@gmail.com> References: <480a5cfefc23446f7c82c5b87eef6306364132b9.camel@gmail.com> <917DAD9F-8697-45B8-8890-D33393F6CDF1@gmail.com> <9dee19c7d39795242c15b2f7aa56fb4a6c3ebffa.camel@gmail.com> 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 3.50.1 MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2023-12-14 at 17:10 +0200, Eduard Zingerman wrote: > [...] > > The reason why retval checks fails is that the way you disable dead > > code removal pass is not complete. Disable opt_remove_dead_code() > > just prevent the instruction #30 from being removed, but also note > > opt_hard_wire_dead_code_branches(), which convert conditional jump > > into unconditional one, so #30 is still skipped. > >=20 > > > Note that I tried this test with two functions: > > > - bpf_get_current_cgroup_id, with this function I get retval 2, not 4= :) > > > - bpf_get_prandom_u32, with this function I get a random retval each = time. > > >=20 > > > What is the expectation when 'bpf_get_current_cgroup_id' is used? > > > That it is some known (to us) number, but verifier treats it as unkno= wn scalar? > > >=20 > >=20 > > Either one would work, but to make #30 always taken, r0 should be > > non-zero. >=20 > Oh, thank you, I made opt_hard_wire_dead_code_branches() a noop, > replaced r0 =3D 0x4 by r0 /=3D 0 and see "divide error: 0000 [#1] PREEMPT= SMP NOPTI" > error in the kernel log on every second or third run of the test > (when using prandom). >=20 > Working to minimize the test case will share results a bit later. Here is the minimized version of the test: https://gist.github.com/eddyz87/fb4d3c7d5aabdc2ae247ed73fefccd32 If executed several times: ./test_progs -vvv -a verifier_and/pruning_test it eventually crashes VM with the following error: [ 2.039066] divide error: 0000 [#1] PREEMPT SMP NOPTI ... [ 2.039987] Call Trace: [ 2.039987] [ 2.039987] ? die+0x36/0x90 [ 2.039987] ? do_trap+0xdb/0x100 [ 2.039987] ? bpf_prog_32cfdb2c00b08250_pruning_test+0x4d/0x60 [ 2.039987] ? do_error_trap+0x7d/0x110 [ 2.039987] ? bpf_prog_32cfdb2c00b08250_pruning_test+0x4d/0x60 [ 2.039987] ? exc_divide_error+0x38/0x50 [ 2.039987] ? bpf_prog_32cfdb2c00b08250_pruning_test+0x4d/0x60 [ 2.039987] ? asm_exc_divide_error+0x1a/0x20 [ 2.039987] ? bpf_prog_32cfdb2c00b08250_pruning_test+0x4d/0x60 [ 2.039987] bpf_test_run+0x1b5/0x350 [ 2.039987] ? bpf_test_run+0x115/0x350 ... I'll continue debugging this a bit later today.