linux-kernel - [PATCH 5.6 166/166] bpf, test_verifier: switch bpf_get

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Message-Id: <20200422095106.153738046@linuxfoundation.org>
Date:   Wed, 22 Apr 2020 11:58:13 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        John Fastabend <john.fastabend@...il.com>
Subject: [PATCH 5.6 166/166] bpf, test_verifier: switch bpf_get_stacks 0 s> r8 test

From: Daniel Borkmann <daniel@...earbox.net>

[ no upstream commit ]

Switch the comparison, so that is_branch_taken() will recognize that below
branch is never taken:

  [...]
  17: [...] R1_w=inv0 [...] R8_w=inv(id=0,smin_value=-2147483648,smax_value=-1,umin_value=18446744071562067968,var_off=(0xffffffff80000000; 0x7fffffff)) [...]
  17: (67) r8 <<= 32
  18: [...] R8_w=inv(id=0,smax_value=-4294967296,umin_value=9223372036854775808,umax_value=18446744069414584320,var_off=(0x8000000000000000; 0x7fffffff00000000)) [...]
  18: (c7) r8 s>>= 32
  19: [...] R8_w=inv(id=0,smin_value=-2147483648,smax_value=-1,umin_value=18446744071562067968,var_off=(0xffffffff80000000; 0x7fffffff)) [...]
  19: (6d) if r1 s> r8 goto pc+16
  [...] R1_w=inv0 [...] R8_w=inv(id=0,smin_value=-2147483648,smax_value=-1,umin_value=18446744071562067968,var_off=(0xffffffff80000000; 0x7fffffff)) [...]
  [...]

Currently we check for is_branch_taken() only if either K is source, or source
is a scalar value that is const. For upstream it would be good to extend this
properly to check whether dst is const and src not.

For the sake of the test_verifier, it is probably not needed here:

  # ./test_verifier 101
  #101/p bpf_get_stack return R0 within range OK
  Summary: 1 PASSED, 0 SKIPPED, 0 FAILED

I haven't seen this issue in test_progs* though, they are passing fine:

  # ./test_progs-no_alu32 -t get_stack
  Switching to flavor 'no_alu32' subdirectory...
  #20 get_stack_raw_tp:OK
  Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

  # ./test_progs -t get_stack
  #20 get_stack_raw_tp:OK
  Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Acked-by: Alexei Starovoitov <ast@...nel.org>
Acked-by: John Fastabend <john.fastabend@...il.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 tools/testing/selftests/bpf/verifier/bpf_get_stack.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/tools/testing/selftests/bpf/verifier/bpf_get_stack.c
+++ b/tools/testing/selftests/bpf/verifier/bpf_get_stack.c
@@ -19,7 +19,7 @@
 	BPF_MOV64_REG(BPF_REG_8, BPF_REG_0),
 	BPF_ALU64_IMM(BPF_LSH, BPF_REG_8, 32),
 	BPF_ALU64_IMM(BPF_ARSH, BPF_REG_8, 32),
-	BPF_JMP_REG(BPF_JSGT, BPF_REG_1, BPF_REG_8, 16),
+	BPF_JMP_REG(BPF_JSLT, BPF_REG_8, BPF_REG_1, 16),
 	BPF_ALU64_REG(BPF_SUB, BPF_REG_9, BPF_REG_8),
 	BPF_MOV64_REG(BPF_REG_2, BPF_REG_7),
 	BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_8),