selftests/bpf: Fix test_varlen verification failure with latest llvm

selftests/bpf: Fix test_varlen verification failure with latest llvm

With latest llvm15, test_varlen failed with the following verifier log:

  17: (85) call bpf_probe_read_kernel_str#115   ; R0_w=scalar(smin=-4095,smax=256)
  18: (bf) r1 = r0                      ; R0_w=scalar(id=1,smin=-4095,smax=256) R1_w=scalar(id=1,smin=-4095,smax=256)
  19: (67) r1 <<= 32                    ; R1_w=scalar(smax=1099511627776,umax=18446744069414584320,var_off=(0x0; 0xffffffff00000000),s32_min=0,s32_max=0,u32_max=)
  20: (bf) r2 = r1                      ; R1_w=scalar(id=2,smax=1099511627776,umax=18446744069414584320,var_off=(0x0; 0xffffffff00000000),s32_min=0,s32_max=0,u32)
  21: (c7) r2 s>>= 32                   ; R2=scalar(smin=-2147483648,smax=256)
  ; if (len >= 0) {
  22: (c5) if r2 s< 0x0 goto pc+7       ; R2=scalar(umax=256,var_off=(0x0; 0x1ff))
  ; payload4_len1 = len;
  23: (18) r2 = 0xffffc90000167418      ; R2_w=map_value(off=1048,ks=4,vs=1572,imm=0)
  25: (63) *(u32 *)(r2 +0) = r0         ; R0=scalar(id=1,smin=-4095,smax=256) R2_w=map_value(off=1048,ks=4,vs=1572,imm=0)
  26: (77) r1 >>= 32                    ; R1_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff))
  ; payload += len;
  27: (18) r6 = 0xffffc90000167424      ; R6_w=map_value(off=1060,ks=4,vs=1572,imm=0)
  29: (0f) r6 += r1                     ; R1_w=Pscalar(umax=4294967295,var_off=(0x0; 0xffffffff)) R6_w=map_value(off=1060,ks=4,vs=1572,umax=4294967295,var_off=(0)
  ; len = bpf_probe_read_kernel_str(payload, MAX_LEN, &buf_in2[0]);
  30: (bf) r1 = r6                      ; R1_w=map_value(off=1060,ks=4,vs=1572,umax=4294967295,var_off=(0x0; 0xffffffff)) R6_w=map_value(off=1060,ks=4,vs=1572,um)
  31: (b7) r2 = 256                     ; R2_w=256
  32: (18) r3 = 0xffffc90000164100      ; R3_w=map_value(off=256,ks=4,vs=1056,imm=0)
  34: (85) call bpf_probe_read_kernel_str#115
  R1 unbounded memory access, make sure to bounds check any such access
  processed 27 insns (limit 1000000) max_states_per_insn 0 total_states 2 peak_states 2 mark_read 1
  -- END PROG LOAD LOG --
  libbpf: failed to load program 'handler32_signed'

The failure is due to
  20: (bf) r2 = r1                      ; R1_w=scalar(id=2,smax=1099511627776,umax=18446744069414584320,var_off=(0x0; 0xffffffff00000000),s32_min=0,s32_max=0,u32)
  21: (c7) r2 s>>= 32                   ; R2=scalar(smin=-2147483648,smax=256)
  22: (c5) if r2 s< 0x0 goto pc+7       ; R2=scalar(umax=256,var_off=(0x0; 0x1ff))
  26: (77) r1 >>= 32                    ; R1_w=scalar(umax=4294967295,var_off=(0x0; 0xffffffff))
  29: (0f) r6 += r1                     ; R1_w=Pscalar(umax=4294967295,var_off=(0x0; 0xffffffff)) R6_w=map_value(off=1060,ks=4,vs=1572,umax=4294967295,var_off=(0)
where r1 has conservative value range compared to r2 and r1 is used later.

In llvm, commit [1] triggered the above code generation and caused
verification failure.

It may take a while for llvm to address this issue. In the main time,
let us change the variable 'len' type to 'long' and adjust condition properly.
Tested with llvm14 and latest llvm15, both worked fine.

 [1] https://reviews.llvm.org/D126647

Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220613233449.2860753-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>