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[BPF] handle unsigned icmp ops in BPFAdjustOpt pass 

When investigating an issue with bcc tool inject.py, I found
a verifier failure with latest clang. The portion of code
can be illustrated as below:
  struct pid_struct {
    u64 curr_call;
    u64 conds_met;
    u64 stack[2];
  };
  struct pid_struct *bpf_map_lookup_elem();
  int foo() {
    struct pid_struct *p = bpf_map_lookup_elem();
    if (!p) return 0;
    p->curr_call--;
    if (p->conds_met < 1 || p->conds_met >= 3)
        return 0;
    if (p->stack[p->conds_met - 1] == p->curr_call)
        p->conds_met--;
    ...
  }

The verifier failure looks like:
  ...
  8: (79) r1 = *(u64 *)(r0 +0)
   R0_w=map_value(id=0,off=0,ks=4,vs=32,imm=0) R10=fp0 fp-8=mmmm????
  9: (07) r1 += -1
  10: (7b) *(u64 *)(r0 +0) = r1
   R0_w=map_value(id=0,off=0,ks=4,vs=32,imm=0) R1_w=inv(id=0) R10=fp0 fp-8=mmmm????
  11: (79) r2 = *(u64 *)(r0 +8)
   R0_w=map_value(id=0,off=0,ks=4,vs=32,imm=0) R1_w=inv(id=0) R10=fp0 fp-8=mmmm????
  12: (bf) r3 = r2
  13: (07) r3 += -3
  14: (b7) r4 = -2
  15: (2d) if r4 > r3 goto pc+13
   R0=map_value(id=0,off=0,ks=4,vs=32,imm=0) R1=inv(id=0) R2=inv(id=2)
   R3=inv(id=0,umin_value=18446744073709551614,var_off=(0xffffffff00000000; 0xffffffff))
   R4=inv-2 R10=fp0 fp-8=mmmm????
  16: (07) r2 += -1
  17: (bf) r3 = r2
  18: (67) r3 <<= 3
  19: (bf) r4 = r0
  20: (0f) r4 += r3
  math between map_value pointer and register with unbounded min value is not allowed

Here the compiler optimized "p->conds_met < 1 || p->conds_met >= 3" to
  r2 = p->conds_met
  r3 = r2
  r3 += -3
  r4 = -2
  if (r3 < r4) return 0
  r2 += -1
  r3 = r2
  ...
In the above, r3 is initially equal to r2, but is modified used by the comparison.
But later on r2 is used again. This caused verification failure.

BPF backend has a pass, AdjustOpt, to prevent such transformation, but only
focused on signed integers since typical bpf helper returns signed integers.
To fix this case, let us handle unsigned integers as well.

Differential Revision: https://reviews.llvm.org/D121937
This commit is contained in:
Yonghong Song 2022-03-16 22:09:59 -07:00
parent 6c4931e7d0
commit 2e94d8e67a
2 changed files with 77 additions and 0 deletions
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6
llvm/lib/Target/BPF/BPFAdjustOpt.cpp
View File

@ -264,6 +264,12 @@ bool BPFAdjustOptImpl::serializeICMPCrossBB(BasicBlock &BB) {
} else if (Cond1Op == ICmpInst::ICMP_SLT || Cond1Op == ICmpInst::ICMP_SLE) {
if (Cond2Op != ICmpInst::ICMP_SGT && Cond2Op != ICmpInst::ICMP_SGE)
return false;
} else if (Cond1Op == ICmpInst::ICMP_ULT || Cond1Op == ICmpInst::ICMP_ULE) {
if (Cond2Op != ICmpInst::ICMP_UGT && Cond2Op != ICmpInst::ICMP_UGE)
return false;
} else if (Cond1Op == ICmpInst::ICMP_UGT || Cond1Op == ICmpInst::ICMP_UGE) {
if (Cond2Op != ICmpInst::ICMP_ULT && Cond2Op != ICmpInst::ICMP_ULE)
return false;
} else {
return false;
}

71
llvm/test/CodeGen/BPF/adjust-opt-icmp6.ll Normal file
View File

@ -0,0 +1,71 @@
; RUN: opt -O2 -S -mtriple=bpf-pc-linux %s -o %t1
; RUN: llc %t1 -o - | FileCheck -check-prefixes=CHECK,CHECK-V1 %s
; RUN: opt -O2 -S -mtriple=bpf-pc-linux %s -o %t1
; RUN: llc %t1 -mcpu=v3 -o - | FileCheck -check-prefixes=CHECK,CHECK-V3 %s
;
; Source:
; unsigned bar(unsigned);
; unsigned int test(unsigned *p) {
; if (*p <= 1 || *p >= 7)
; return 0;
; return bar(*p);
; }
; Compilation flag:
; clang -target bpf -O2 -S -emit-llvm -Xclang -disable-llvm-passes test.c
; Function Attrs: nounwind
define dso_local i32 @test(i32* noundef %p) #0 {
entry:
%retval = alloca i32, align 4
%p.addr = alloca i32*, align 8
store i32* %p, i32** %p.addr, align 8, !tbaa !3
%0 = load i32*, i32** %p.addr, align 8, !tbaa !3
%1 = load i32, i32* %0, align 4, !tbaa !7
%cmp = icmp ule i32 %1, 1
br i1 %cmp, label %if.then, label %lor.lhs.false
lor.lhs.false: ; preds = %entry
%2 = load i32*, i32** %p.addr, align 8, !tbaa !3
%3 = load i32, i32* %2, align 4, !tbaa !7
%cmp1 = icmp uge i32 %3, 7
br i1 %cmp1, label %if.then, label %if.end
if.then: ; preds = %lor.lhs.false, %entry
store i32 0, i32* %retval, align 4
br label %return
if.end: ; preds = %lor.lhs.false
%4 = load i32*, i32** %p.addr, align 8, !tbaa !3
%5 = load i32, i32* %4, align 4, !tbaa !7
%call = call i32 @bar(i32 noundef %5)
store i32 %call, i32* %retval, align 4
br label %return
return: ; preds = %if.end, %if.then
%6 = load i32, i32* %retval, align 4
ret i32 %6
}
; CHECK-LABEL: test
; CHECK-V1: if r[[#]] > r[[#]] goto
; CHECK-V1: if r[[#]] > 6 goto
; CHECK-V3: if w[[#]] < 2 goto
; CHECK-V3: if w[[#]] > 6 goto
declare dso_local i32 @bar(i32 noundef) #1
attributes #0 = { nounwind "frame-pointer"="all" "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" }
attributes #1 = { "frame-pointer"="all" "no-trapping-math"="true" "stack-protector-buffer-size"="8" }
!llvm.module.flags = !{!0, !1}
!llvm.ident = !{!2}
!0 = !{i32 1, !"wchar_size", i32 4}
!1 = !{i32 7, !"frame-pointer", i32 2}
!2 = !{!"clang version 15.0.0 (https://github.com/llvm/llvm-project.git 2a25e1af85f3138f70888c4c3f359c6a09e3cfe5)"}
!3 = !{!4, !4, i64 0}
!4 = !{!"any pointer", !5, i64 0}
!5 = !{!"omnipotent char", !6, i64 0}
!6 = !{!"Simple C/C++ TBAA"}
!7 = !{!8, !8, i64 0}
!8 = !{!"int", !5, i64 0}