Messages in this thread | | | Date | Tue, 27 Dec 2022 21:23:52 -0800 | Subject | Re: WARNING in __mark_chain_precision | From | Yonghong Song <> |
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On 12/20/22 4:30 PM, Andrii Nakryiko wrote: > On Mon, Dec 19, 2022 at 11:13 AM <sdf@google.com> wrote: >> >> On 12/19, Hao Sun wrote: >>> Hi, >> >>> The following backtracking bug can be triggered on the latest bpf-next and >>> Linux 6.1 with the C prog provided. I don't have enough knowledge about >>> this part in the verifier, don't know how to fix this. >> >> Maybe something related to commit be2ef8161572 ("bpf: allow precision >> tracking >> for programs with subprogs") and/or the related ones? >> >> >>> This can be reproduced on: >> >>> HEAD commit: 0e43662e61f2 tools/resolve_btfids: Use pkg-config to locate >>> libelf >>> git tree: bpf-next >>> console log: https://pastebin.com/raw/45hZ7iqm >>> kernel config: https://pastebin.com/raw/0pu1CHRm >>> C reproducer: https://pastebin.com/raw/tqsiezvT >> >>> func#0 @0 >>> 0: R1=ctx(off=0,imm=0) R10=fp0 >>> 0: (18) r2 = 0x8000000000000 ; R2_w=2251799813685248 >>> 2: (18) r6 = 0xffff888027358000 ; >>> R6_w=map_ptr(off=0,ks=3032,vs=3664,imm=0) >>> 4: (18) r7 = 0xffff88802735a000 ; >>> R7_w=map_ptr(off=0,ks=156,vs=2624,imm=0) >>> 6: (18) r8 = 0xffff88802735e000 ; >>> R8_w=map_ptr(off=0,ks=2396,vs=76,imm=0) >>> 8: (18) r9 = 0x8e9700000000 ; R9_w=156779191205888 >>> 10: (36) if w9 >= 0xffffffe3 goto pc+1 >>> last_idx 10 first_idx 0 >>> regs=200 stack=0 before 8: (18) r9 = 0x8e9700000000 >>> 11: R9_w=156779191205888 >>> 11: (85) call #0 >>> 12: (cc) w2 s>>= w7 > > w2 should have been set to NOT_INIT (because r1-r5 are clobbered by > calls) and rejected here as !read_ok (see check_reg_arg()) before > attempting to mark precision for r2. Can you please try to debug and > understand why that didn't happen here?
The verifier is doing the right thing here and the 'call #0' does implicitly cleared r1-r5.
So for 'w2 s>>= w7', since w2 is used, the verifier tries to find its definition by backtracing. It encountered 'call #0', which clears r1-r5. Since w2 is not defined, the verifier issued the error: BUG regs 4
The specific verifier code is in backtrack_insn():
} else if (class == BPF_JMP || class == BPF_JMP32) { if (opcode == BPF_CALL) { if (insn->src_reg == BPF_PSEUDO_CALL) return -ENOTSUPP; /* BPF helpers that invoke callback subprogs are * equivalent to BPF_PSEUDO_CALL above */ if (insn->src_reg == 0 && is_callback_calling_function(insn->imm)) return -ENOTSUPP; /* regular helper call sets R0 */ *reg_mask &= ~1; if (*reg_mask & 0x3f) { /* if backtracing was looking for registers R1-R5 * they should have been found already. */ verbose(env, "BUG regs %x\n", *reg_mask); WARN_ONCE(1, "verifier backtracking bug"); return -EFAULT; } }
Note that the above mask '0x3f' which corresponds to registers r1-r5. If it tries to find the definition for any of them, 'BUG regs <mask>' will be printed out.
> > >>> last_idx 12 first_idx 12 >>> parent didn't have regs=4 stack=0 marks: R1=ctx(off=0,imm=0) >>> R2_rw=P2251799813685248 R6_w=map_ptr(off=0,ks=3032,vs=3664,imm=0) >>> R7_rw=map_ptr(off=0,ks=156,vs=2624,imm=0) >>> R8_w=map_ptr(off=0,ks=2396,vs=76,imm=0) R9_w=156779191205888 R10=fp0 >>> last_idx 11 first_idx 0 >>> regs=4 stack=0 before 11: (85) call #0 >>> BUG regs 4 >>> processed 8 insns (limit 1000000) max_states_per_insn 0 total_states 1 >>> peak_states 1 mark_read 1 >> >>> ------------[ cut here ]------------ >>> verifier backtracking bug >>> WARNING: CPU: 6 PID: 8646 at kernel/bpf/verifier.c:2756 backtrack_insn >>> kernel/bpf/verifier.c:2756 [inline] >>> WARNING: CPU: 6 PID: 8646 at kernel/bpf/verifier.c:2756 >>> __mark_chain_precision+0x1baf/0x1d70 kernel/bpf/verifier.c:3065 >>> Modules linked in: >>> CPU: 6 PID: 8646 Comm: a.out Not tainted 6.1.0-09634-g0e43662e61f2 #146 >>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux >>> 1.16.1-1-1 04/01/2014 >>> RIP: 0010:backtrack_insn kernel/bpf/verifier.c:2756 [inline] >>> RIP: 0010:__mark_chain_precision+0x1baf/0x1d70 kernel/bpf/verifier.c:3065 >>> Code: 0d 31 ff 89 de e8 91 ec ed ff 84 db 0f 85 ef fe ff ff e8 b4 f0 >>> ed ff 48 c7 c7 e0 8f 53 8a c6 05 28 71 ab 0d 01 e8 83 b3 1e 08 <0f> 0b >>> e9 50 f8 ff ff 48 8b 74 24 38 48 c7 c7 80 d0 63 8d e8 49 46 >>> RSP: 0018:ffffc9001463f1a0 EFLAGS: 00010282 >>> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000 >>> RDX: ffff888020470000 RSI: ffffffff816662c0 RDI: fffff520028c7e26 >>> RBP: 0000000000000004 R08: 0000000000000005 R09: 0000000000000000 >>> R10: 0000000080000000 R11: 0000000000000000 R12: 0000000000000020 >>> R13: dffffc0000000000 R14: 000000000000000b R15: ffff88802be74000 >>> FS: 00007fd3daeb8440(0000) GS:ffff888063980000(0000) >>> knlGS:0000000000000000 >>> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >>> CR2: 0000000020000240 CR3: 0000000017394000 CR4: 0000000000750ee0 >>> PKRU: 55555554 >>> Call Trace: >>> <TASK> >>> mark_chain_precision kernel/bpf/verifier.c:3165 [inline] >>> adjust_reg_min_max_vals+0x981/0x58d0 kernel/bpf/verifier.c:10715 >>> check_alu_op+0x380/0x1820 kernel/bpf/verifier.c:10928 >>> do_check kernel/bpf/verifier.c:13821 [inline] >>> do_check_common+0x1c3b/0xe520 kernel/bpf/verifier.c:16289 >>> do_check_main kernel/bpf/verifier.c:16352 [inline] >>> bpf_check+0x83b4/0xb310 kernel/bpf/verifier.c:16936 >>> bpf_prog_load+0xf7a/0x21a0 kernel/bpf/syscall.c:2619 >>> __sys_bpf+0xf03/0x5840 kernel/bpf/syscall.c:4979 >>> __do_sys_bpf kernel/bpf/syscall.c:5083 [inline] >>> __se_sys_bpf kernel/bpf/syscall.c:5081 [inline] >>> __x64_sys_bpf+0x78/0xc0 kernel/bpf/syscall.c:5081 >>> do_syscall_x64 arch/x86/entry/common.c:50 [inline] >>> do_syscall_64+0x38/0xb0 arch/x86/entry/common.c:80 >>> entry_SYSCALL_64_after_hwframe+0x63/0xcd >>> RIP: 0033:0x7fd3da8e4469 >>> Code: 00 f3 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 >>> 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d >>> 01 f0 ff ff 73 01 c3 48 8b 0d ff 49 2b 00 f7 d8 64 89 01 48 >>> RSP: 002b:00007fff090c1a78 EFLAGS: 00000246 ORIG_RAX: 0000000000000141 >>> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fd3da8e4469 >>> RDX: 0000000000000080 RSI: 0000000020000840 RDI: 0000000000000005 >>> RBP: 00007fff090c2a90 R08: 00007fd3da92e160 R09: 0000000000000000 >>> R10: 0000000000000000 R11: 0000000000000246 R12: 0000561aefc006c0 >>> R13: 00007fff090c2b70 R14: 0000000000000000 R15: 0000000000000000 >>> </TASK>
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