Re: bpf: test_sysctl run failed on Debian9

Andrey Ignatov
2020-04-23T21:32:21Z

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Ma Xinjian <max.xinjian@intel.com> [Thu, 2020-04-23 02:12 -0700]:
> Hi,  Andrey.
>
> I noticed you add test_sysctl to tools/bpf, so drop this problem to you.
>
> When I run selftests: bpf: test_sysctl, failed with "(test_sysctl.c:1490:
> errno: Permission denied) >>> Loading program (./test_sysctl_prog.o) error."
>
>
> Testing env: "Debian GNU/Linux 9 (stretch)"
>
> kernel: 5.7.0-rc2    5.7.0-rc1  5.6  both failed
>
>
> Whole run log and kconfig please see the attatchment.
>
> Error info
>
> ```
>
> root@vm-snb-42 /usr/src/perf_selftests-x86_64-rhel-7.6-kselftests-ae83d0b416db002fe95601e7f97f64b59514d936/tools/testing/selftests/bpf#
> ./test_sysctl
>
> Test case: sysctl wrong attach_type .. [PASS]
> Test case: sysctl:read allow all .. [PASS]
> Test case: sysctl:read deny all .. [PASS]
>
> [snip]
>
> libbpf: -- END LOG --
> libbpf: failed to load program 'cgroup/sysctl'
> libbpf: failed to load object './test_sysctl_prog.o'
> (test_sysctl.c:1490: errno: Permission denied) >>> Loading program
> (./test_sysctl_prog.o) error.
>
> Test case: C prog: read tcp_mem .. [FAIL]
> Summary: 37 PASSED, 3 FAILED

Hi Ma,

I can not reproduce it. I built 5.7.0-rc2 with your config (with minor
changes to just make it work with my qemu-setup, specifically
CONFIG_EXT4_FS=y), built tests and run it, no failures:

root@arch-fb-vm1:/home/rdna/bpf-next/tools/testing/selftests/bpf uname -srm
Linux 5.7.0-rc2 x86_64
root@arch-fb-vm1:/home/rdna/bpf-next/tools/testing/selftests/bpf ./test_sysctl
...
Test case: C prog: deny all writes .. [PASS]
Test case: C prog: deny access by name .. [PASS]
Test case: C prog: read tcp_mem .. [PASS]
Summary: 40 PASSED, 0 FAILED

Thouhg I see that test_sysctl_prog.o program I have differs from what
you have. I attach test_sysctl_prog_xlated.gz with my program.

Specifically the difference starts after the first call to bpf_strtoul.

The code from my prog looks like this (from the first call to
bpf_strtoul to the second call to bpf_strtoul):

71: (85) call bpf_strtoul#110448
72: (bc) w7 = w0
; if (ret <= 0 || ret > MAX_ULONG_STR_LEN)
73: (bc) w1 = w7
74: (04) w1 += -1
75: (26) if w1 > 0xe goto pc-21
; off += ret & MAX_ULONG_STR_LEN;
76: (54) w7 &= 15
77: (bf) r1 = r10
78: (07) r1 += -64
; ret = bpf_strtoul(value + off, MAX_ULONG_STR_LEN, 0,
79: (0f) r1 += r7
; tcp_mem + i);
80: (bf) r4 = r10
81: (07) r4 += -80
; ret = bpf_strtoul(value + off, MAX_ULONG_STR_LEN, 0,
82: (b7) r2 = 15
83: (b7) r3 = 0
84: (85) call bpf_strtoul#110448

It can be seen that r1 points to stack-64 + r7 that is known to be <15.

This is basically `value + (ret & MAX_ULONG_STR_LEN)` from the C code.

The code from your version of program looks like this:

70: (85) call bpf_strtoul#106
last_idx 70 first_idx 63
regs=4 stack=0 before 69: (b7) r3 = 0
regs=4 stack=0 before 68: (b7) r2 = 15
71: (bc) w7 = w0
; if (ret <= 0 || ret > MAX_ULONG_STR_LEN)
72: (bc) w1 = w7
73: (04) w1 += -1
74: (26) if w1 > 0xe goto pc+14
R0=inv(id=0) R1_w=inv(id=0,umax_value=14,var_off=(0x0; 0xf)) R6=inv0 R7_w=inv(id=0,smax_value=2147483647,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R10=fp0 fp-8=mmmmmmmm fp-16=mmmmmmmm fp-24=mmmmmmmm fp-32=mmmmmmmm fp-40=mmmmmmmm fp-48=mmmmmmmm fp-56=mmmmmmmm fp-64=mmmmmmmm fp-72=00000000 fp-80=00000000 fp-88=mmmmmmmm
; ret = bpf_strtoul(value + off, MAX_ULONG_STR_LEN, 0,
75: (bc) w2 = w7
76: (67) r2 <<= 32
77: (77) r2 >>= 32
78: (bf) r1 = r10
;
79: (07) r1 += -64
; ret = bpf_strtoul(value + off, MAX_ULONG_STR_LEN, 0,
80: (0f) r1 += r2
last_idx 80 first_idx 71
regs=4 stack=0 before 79: (07) r1 += -64
regs=4 stack=0 before 78: (bf) r1 = r10
regs=4 stack=0 before 77: (77) r2 >>= 32
regs=4 stack=0 before 76: (67) r2 <<= 32
regs=4 stack=0 before 75: (bc) w2 = w7
regs=80 stack=0 before 74: (26) if w1 > 0xe goto pc+14
regs=80 stack=0 before 73: (04) w1 += -1
regs=80 stack=0 before 72: (bc) w1 = w7
regs=80 stack=0 before 71: (bc) w7 = w0
R0_rw=invP(id=0) R6=inv0 R7=ctx(id=0,off=0,imm=0) R10=fp0 fp-8=mmmmmmmm fp-16=mmmmmmmm fp-24=mmmmmmmm fp-32=mmmmmmmm fp-40=mmmmmmmm fp-48=mmmmmmmm fp-56=mmmmmmmm fp-64=mmmmmmmm fp-72=00000000 fp-80=00000000 fp-88=mmmmmmmm
parent didn't have regs=1 stack=0 marks
last_idx 70 first_idx 63
regs=1 stack=0 before 70: (85) call bpf_strtoul#106
; tcp_mem + i);
81: (bf) r4 = r10
82: (07) r4 += -80
; ret = bpf_strtoul(value + off, MAX_ULONG_STR_LEN, 0,
83: (b7) r2 = 15
84: (b7) r3 = 0
85: (85) call bpf_strtoul#106
R1 unbounded indirect variable offset stack access
processed 88 insns (limit 1000000) max_states_per_insn 0 total_states 7 peak_states 7 mark_read 6

In this case r1 points to stack-64 + r2 and it seems to me that the
state of r2 is the problem, but I don't undesrtand why.

32LSB of r2 seems to be known because:
71: (bc) w7 = w0
72: (bc) w1 = w7
73: (04) w1 += -1
74: (26) if w1 > 0xe goto pc+14
75: (bc) w2 = w7

and 32MSB of r2 are cleared because:
76: (67) r2 <<= 32
77: (77) r2 >>= 32

So it seems to me that r2 has to be known and in (0x0; 0xf) range.

To summarize:

* I see that your program differs what may mean you have different
environemnt where you build BPF proga (e.g. different clang/llvm
version), FWIW I have clang version 9.0.20190721 (though it's heavily
patched facebook version).

* I'm not quite sure why verifier rejects your version of the program --
here we would need more eyes. I'm cc'ing Alexei and Daniel.

--
Andrey Ignatov

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