> Thanks for reporting.
> The assembly code looks correct. So this mostly related to kernel.
> Will take a further look.
You are welcome, I've noticed your [PATCH bpf 1/2] bpf: fix an
incorrect branch elimination by verifier.
I'll try it with latest bpf tree later. Thank you all.
> For the llvm crash below, it should have been fixed in last llvm trunk.
> Please give a try. Thanks!
Get it, thanks. I'll try later.
Yonghong Song <yhs@xxxxxx> 于2020年6月30日周二 上午4:47写道：
>
>
>
> On 6/28/20 10:25 PM, Wenbo Zhang wrote:
> > I found in tp_btf program, direct access struct's pointer member's
> > behaviour isn't consistent with
> > BPF_CORE_READ. for example:
> >
> > SEC("tp_btf/block_rq_issue")
> > int BPF_PROG(tp_btf__block_rq_issue, struct request_queue *q,
> > struct request *rq)
> > {
> > /* After echo none > /sys/block/$dev/queue/scheduler,
> > * the $dev's q->elevator will be set to NULL.
> > */
> > if (!q->elevator)
> > bpf_printk("direct access: noop\n");
> > if (!BPF_CORE_READ(q, elevator))
> > bpf_printk("FROM CORE READ: noop\n");
> > return 0;
> > }
> >
> > Although its value is NULL, from trace_pipe I can only see
> >
> >> FROM CORE READ: noop
> >
> > So it seems `if (!q->elevator)` always return false.
>
> Thanks for reporting.
> The assembly code looks correct. So this mostly related to kernel.
> Will take a further look.
>
> For the llvm crash below, it should have been fixed in last llvm trunk.
> Please give a try. Thanks!
>
> >
> > I tested it with kernel 5.7.0-rc7+ and 5.8.0-rc1+, both have this problem.
> > clang version: clang version 10.0.0-4ubuntu1~18.04.1
> >
> > Reproduce step:
> > 1. Run this bpf prog;
> > 2. Run `cat /sys/kernel/debug/tracing/trace_pipe` in other window;
> > 3. Run `echo none > /sys/block/sdc/queue/scheduler`; # please replace
> > sdc to your device;
> > 4. Run `dd if=/dev/zero of=/dev/sdc bs=1MiB count=200 oflag=direct`;
> >
> >
> > The output of `llvm-objdump-10 -D bio.bpf.o` is:
> >
> >
> > bio.bpf.o: file format ELF64-BPF
> >
> >
> > Disassembly of section tp_btf/block_rq_issue:
> >
> > 0000000000000000 tp_btf__block_rq_issue:
> > 0: b7 02 00 00 08 00 00 00 r2 = 8
> > 1: 79 11 00 00 00 00 00 00 r1 = *(u64 *)(r1 + 0)
> > 2: bf 16 00 00 00 00 00 00 r6 = r1
> > 3: 0f 26 00 00 00 00 00 00 r6 += r2
> > 4: 79 11 08 00 00 00 00 00 r1 = *(u64 *)(r1 + 8)
> > 5: 55 01 0e 00 00 00 00 00 if r1 != 0 goto +14 <LBB0_2>
> > 6: b7 01 00 00 00 00 00 00 r1 = 0
> > 7: 73 1a fc ff 00 00 00 00 *(u8 *)(r10 - 4) = r1
> > 8: b7 01 00 00 6f 6f 70 0a r1 = 175140719
> > 9: 63 1a f8 ff 00 00 00 00 *(u32 *)(r10 - 8) = r1
> > 10: 18 01 00 00 63 63 65 73 00 00 00 00 73 3a 20 6e r1 =
> > 7935406810958488419 ll
> > 12: 7b 1a f0 ff 00 00 00 00 *(u64 *)(r10 - 16) = r1
> > 13: 18 01 00 00 64 69 72 65 00 00 00 00 63 74 20 61 r1 =
> > 6998721791186332004 ll
> > 15: 7b 1a e8 ff 00 00 00 00 *(u64 *)(r10 - 24) = r1
> > 16: bf a1 00 00 00 00 00 00 r1 = r10
> > 17: 07 01 00 00 e8 ff ff ff r1 += -24
> > 18: b7 02 00 00 15 00 00 00 r2 = 21
> > 19: 85 00 00 00 06 00 00 00 call 6
> >
> > 00000000000000a0 LBB0_2:
> > 20: bf a1 00 00 00 00 00 00 r1 = r10
> > 21: 07 01 00 00 e8 ff ff ff r1 += -24
> > 22: b7 02 00 00 08 00 00 00 r2 = 8
> > 23: bf 63 00 00 00 00 00 00 r3 = r6
> > 24: 85 00 00 00 04 00 00 00 call 4
> > 25: 79 a1 e8 ff 00 00 00 00 r1 = *(u64 *)(r10 - 24)
> > 26: 55 01 0e 00 00 00 00 00 if r1 != 0 goto +14 <LBB0_4>
> > 27: b7 01 00 00 0a 00 00 00 r1 = 10
> > 28: 6b 1a fc ff 00 00 00 00 *(u16 *)(r10 - 4) = r1
> > 29: b7 01 00 00 6e 6f 6f 70 r1 = 1886351214
> > 30: 63 1a f8 ff 00 00 00 00 *(u32 *)(r10 - 8) = r1
> > 31: 18 01 00 00 45 20 52 45 00 00 00 00 41 44 3a 20 r1 =
> > 2322243604989485125 ll
> > 33: 7b 1a f0 ff 00 00 00 00 *(u64 *)(r10 - 16) = r1
> > 34: 18 01 00 00 46 52 4f 4d 00 00 00 00 20 43 4f 52 r1 =
> > 5931033040285291078 ll
> > 36: 7b 1a e8 ff 00 00 00 00 *(u64 *)(r10 - 24) = r1
> > 37: bf a1 00 00 00 00 00 00 r1 = r10
> > 38: 07 01 00 00 e8 ff ff ff r1 += -24
> > 39: b7 02 00 00 16 00 00 00 r2 = 22
> > 40: 85 00 00 00 06 00 00 00 call 6
> >
> > 0000000000000148 LBB0_4:
> > 41: b7 00 00 00 00 00 00 00 r0 = 0
> > 42: 95 00 00 00 00 00 00 00 exit
> >
> > Disassembly of section license:
> >
> > 0000000000000000 LICENSE:
> > 0: 47 <unknown>
> > 0: 50 <unknown>
> > 0: 4c <unknown>
> > 0: 00 <unknown>
> >
> > Disassembly of section .rodata.str1.1:
> >
> > 0000000000000000 .rodata.str1.1:
> > 0: 64 69 72 65 63 74 20 61 w9 <<= 1629516899
> > 1: 63 63 65 73 73 3a 20 6e *(u32 *)(r3 + 29541) = r6
> > 2: 6f 6f 70 0a 00 46 52 4f <unknown>
> > 3: 4d 20 43 4f 52 45 20 52 <unknown>
> > 4: 45 41 44 3a 20 6e 6f 6f <unknown>
> > 5: 70 <unknown>
> > 5: 0a <unknown>
> > 5: 00 <unknown>
> >
> [...]
> > 709: 69 5f 72 65 6d 61 69 6e <unknown>
> > 710: 69 6e 67 00 62 69 5f 69 <unknown>
> > 711: 74 65 72 00 62 69 5f 65 w5 >>= 1700751714
> > 712: 6e 64 5f 69 6f 00 62 69 if
> >
> >
> > BTW, the llvm-objdump will core dump after output the above info:
> >
> > Stack dump:
> > 0. Program arguments: llvm-objdump-10 -D bio.bpf.o
> > /usr/lib/x86_64-linux-gnu/libLLVM-10.so.1(_ZN4llvm3sys15PrintStackTraceERNS_11raw_ostreamE+0x1f)[0x7f7636d5dc3f]
> > /usr/lib/x86_64-linux-gnu/libLLVM-10.so.1(_ZN4llvm3sys17RunSignalHandlersEv+0x50)[0x7f7636d5bf00]
> > /usr/lib/x86_64-linux-gnu/libLLVM-10.so.1(+0x978205)[0x7f7636d5e205]
> > /lib/x86_64-linux-gnu/libpthread.so.0(+0x12890)[0x7f76361d9890]
> > /usr/lib/x86_64-linux-gnu/libLLVM-10.so.1(+0x21bbed3)[0x7f76385a1ed3]
> > /usr/lib/x86_64-linux-gnu/libLLVM-10.so.1(+0x21baefb)[0x7f76385a0efb]
> > /usr/lib/x86_64-linux-gnu/libLLVM-10.so.1(+0x21bc0ce)[0x7f76385a20ce]
> > llvm-objdump-10[0x41b78c]
> > llvm-objdump-10[0x425278]
> > llvm-objdump-10[0x41f502]
> > llvm-objdump-10[0x41a473]
> > /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xe7)[0x7f763546db97]
> > llvm-objdump-10[0x41542a]
> > [1] 21636 segmentation fault (core dumped)
> >
> > llvm-objdump-10 --version
> > LLVM (https://urldefense.proofpoint.com/v2/url?u=http-3A__llvm.org_&d=DwIBaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=DA8e1B5r073vIqRrFz7MRA&m=zs4_mz-CGExwverPej7QEcaeDzsjcZfkD_GiyMQDJbE&s=1inYTci4noQ6dJN-mUYTlvU7OrTX3C7h-0Kn39reX-Y&e= ):
> > LLVM version 10.0.0
> >
> > Optimized build.
> > Default target: x86_64-pc-linux-gnu
> > Host CPU: broadwell
> >
> > Registered Targets:
> > aarch64 - AArch64 (little endian)
> > aarch64_32 - AArch64 (little endian ILP32)
> [...]
