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4daf0b327f
Yonghong Song says:
====================
LLVM commit [1] introduced hoistMinMax optimization like
(i < VIRTIO_MAX_SGS) && (i < out_sgs)
to
upper = MIN(VIRTIO_MAX_SGS, out_sgs)
... i < upper ...
and caused the verification failure. Commit [2] workarounded the issue by
adding some bpf assembly code to prohibit the above optimization.
This patch improved verifier such that verification can succeed without
the above workaround.
Without [2], the current verifier will hit the following failures:
...
119: (15) if r1 == 0x0 goto pc+1
The sequence of 8193 jumps is too complex.
verification time 525829 usec
stack depth 64
processed 156616 insns (limit 1000000) max_states_per_insn 8 total_states 1754 peak_states 1712 mark_read 12
-- END PROG LOAD LOG --
libbpf: prog 'trace_virtqueue_add_sgs': failed to load: -14
libbpf: failed to load object 'loop6.bpf.o'
...
The failure is due to verifier inadequately handling '<const> <cond_op> <non_const>' which will
go through both pathes and generate the following verificaiton states:
...
89: (07) r2 += 1 ; R2_w=5
90: (79) r8 = *(u64 *)(r10 -48) ; R8_w=scalar() R10=fp0
91: (79) r1 = *(u64 *)(r10 -56) ; R1_w=scalar(umax=5,var_off=(0x0; 0x7)) R10=fp0
92: (ad) if r2 < r1 goto pc+41 ; R0_w=scalar() R1_w=scalar(umin=6,umax=5,var_off=(0x4; 0x3))
R2_w=5 R6_w=scalar(id=385) R7_w=0 R8_w=scalar() R9_w=scalar(umax=21474836475,var_off=(0x0; 0x7ffffffff))
R10=fp0 fp-8=mmmmmmmm fp-16=mmmmmmmm fp-24=mmmm???? fp-32= fp-40_w=4 fp-48=mmmmmmmm fp-56= fp-64=mmmmmmmm
...
89: (07) r2 += 1 ; R2_w=6
90: (79) r8 = *(u64 *)(r10 -48) ; R8_w=scalar() R10=fp0
91: (79) r1 = *(u64 *)(r10 -56) ; R1_w=scalar(umax=5,var_off=(0x0; 0x7)) R10=fp0
92: (ad) if r2 < r1 goto pc+41 ; R0_w=scalar() R1_w=scalar(umin=7,umax=5,var_off=(0x4; 0x3))
R2_w=6 R6=scalar(id=388) R7=0 R8_w=scalar() R9_w=scalar(umax=25769803770,var_off=(0x0; 0x7ffffffff))
R10=fp0 fp-8=mmmmmmmm fp-16=mmmmmmmm fp-24=mmmm???? fp-32= fp-40=5 fp-48=mmmmmmmm fp-56= fp-64=mmmmmmmm
...
89: (07) r2 += 1 ; R2_w=4088
90: (79) r8 = *(u64 *)(r10 -48) ; R8_w=scalar() R10=fp0
91: (79) r1 = *(u64 *)(r10 -56) ; R1_w=scalar(umax=5,var_off=(0x0; 0x7)) R10=fp0
92: (ad) if r2 < r1 goto pc+41 ; R0=scalar() R1=scalar(umin=4089,umax=5,var_off=(0x0; 0x7))
R2=4088 R6=scalar(id=12634) R7=0 R8=scalar() R9=scalar(umax=17557826301960,var_off=(0x0; 0xfffffffffff))
R10=fp0 fp-8=mmmmmmmm fp-16=mmmmmmmm fp-24=mmmm???? fp-32= fp-40=4087 fp-48=mmmmmmmm fp-56= fp-64=mmmmmmmm
Patch 3 fixed the above issue by handling '<const> <cond_op> <non_const>' properly.
During developing selftests for Patch 3, I found some issues with bound deduction with
BPF_EQ/BPF_NE and fixed the issue in Patch 1.
After the above issue is fixed, the second issue shows up.
...
67: (07) r1 += -16 ; R1_w=fp-16
; bpf_probe_read_kernel(&sgp, sizeof(sgp), sgs + i);
68: (b4) w2 = 8 ; R2_w=8
69: (85) call bpf_probe_read_kernel#113 ; R0_w=scalar() fp-16=mmmmmmmm
; return sgp;
70: (79) r6 = *(u64 *)(r10 -16) ; R6=scalar() R10=fp0
; for (n = 0, sgp = get_sgp(sgs, i); sgp && (n < SG_MAX);
71: (15) if r6 == 0x0 goto pc-49 ; R6=scalar()
72: (b4) w1 = 0 ; R1_w=0
73: (05) goto pc-46
; for (i = 0; (i < VIRTIO_MAX_SGS) && (i < out_sgs); i++) {
28: (bc) w7 = w1 ; R1_w=0 R7_w=0
; bpf_probe_read_kernel(&len, sizeof(len), &sgp->length);
...
23: (79) r3 = *(u64 *)(r10 -40) ; R3_w=2 R10=fp0
; for (i = 0; (i < VIRTIO_MAX_SGS) && (i < out_sgs); i++) {
24: (07) r3 += 1 ; R3_w=3
; for (i = 0; (i < VIRTIO_MAX_SGS) && (i < out_sgs); i++) {
25: (79) r1 = *(u64 *)(r10 -56) ; R1_w=scalar(umax=5,var_off=(0x0; 0x7)) R10=fp0
26: (ad) if r3 < r1 goto pc+34 61: R0=scalar() R1_w=scalar(umin=4,umax=5,var_off=(0x4; 0x1)) R3_w=3 R6=scalar(id=1658)
R7=0 R8=scalar(id=1653) R9=scalar(umax=4294967295,var_off=(0x0; 0xffffffff)) R10=fp0 fp-8=mmmmmmmm fp-16=mmmmmmmm
fp-24=mmmm???? fp-32= fp-40=2 fp-56= fp-64=mmmmmmmm
; if (sg_is_chain(&sg))
61: (7b) *(u64 *)(r10 -40) = r3 ; R3_w=3 R10=fp0 fp-40_w=3
...
67: (07) r1 += -16 ; R1_w=fp-16
; bpf_probe_read_kernel(&sgp, sizeof(sgp), sgs + i);
68: (b4) w2 = 8 ; R2_w=8
69: (85) call bpf_probe_read_kernel#113 ; R0_w=scalar() fp-16=mmmmmmmm
; return sgp;
70: (79) r6 = *(u64 *)(r10 -16)
; for (n = 0, sgp = get_sgp(sgs, i); sgp && (n < SG_MAX);
infinite loop detected at insn 71
verification time 90800 usec
stack depth 64
processed 25017 insns (limit 1000000) max_states_per_insn 20 total_states 491 peak_states 169 mark_read 12
-- END PROG LOAD LOG --
libbpf: prog 'trace_virtqueue_add_sgs': failed to load: -22
Further analysis found the index variable 'i' is spilled but since it is not marked as precise.
This is more tricky as identifying induction variable is not easy in verifier. Although a heuristic
is possible, let us leave it for now.
[1] https://reviews.llvm.org/D143726
[2] Commit
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.