bpf: Fix trampoline generation for fmod_ret programs
fmod_ret progs are emitted as:
start = __bpf_prog_enter();
call fmod_ret
*(u64 *)(rbp - 8) = rax
__bpf_prog_exit(, start);
test eax, eax
jne do_fexit
That 'test eax, eax' is working by accident. The compiler is free to use rax
inside __bpf_prog_exit() or inside functions that __bpf_prog_exit() is calling.
Which caused "test_progs -t modify_return" to sporadically fail depending on
compiler version and kconfig. Fix it by using 'cmp [rbp - 8], 0' instead of
'test eax, eax'.
Fixes: ae24082331
("bpf: Introduce BPF_MODIFY_RETURN")
Reported-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: KP Singh <kpsingh@google.com>
Link: https://lore.kernel.org/bpf/20200311003906.3643037-1-ast@kernel.org
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@ -1449,23 +1449,6 @@ static int emit_cond_near_jump(u8 **pprog, void *func, void *ip, u8 jmp_cond)
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return 0;
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}
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static int emit_mod_ret_check_imm8(u8 **pprog, int value)
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{
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u8 *prog = *pprog;
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int cnt = 0;
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if (!is_imm8(value))
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return -EINVAL;
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if (value == 0)
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EMIT2(0x85, add_2reg(0xC0, BPF_REG_0, BPF_REG_0));
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else
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EMIT3(0x83, add_1reg(0xF8, BPF_REG_0), value);
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*pprog = prog;
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return 0;
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}
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static int invoke_bpf(const struct btf_func_model *m, u8 **pprog,
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struct bpf_tramp_progs *tp, int stack_size)
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{
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@ -1485,7 +1468,7 @@ static int invoke_bpf_mod_ret(const struct btf_func_model *m, u8 **pprog,
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u8 **branches)
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{
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u8 *prog = *pprog;
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int i;
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int i, cnt = 0;
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/* The first fmod_ret program will receive a garbage return value.
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* Set this to 0 to avoid confusing the program.
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@ -1496,16 +1479,12 @@ static int invoke_bpf_mod_ret(const struct btf_func_model *m, u8 **pprog,
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if (invoke_bpf_prog(m, &prog, tp->progs[i], stack_size, true))
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return -EINVAL;
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/* Generate a branch:
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*
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* if (ret != 0)
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/* mod_ret prog stored return value into [rbp - 8]. Emit:
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* if (*(u64 *)(rbp - 8) != 0)
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* goto do_fexit;
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*
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* If needed this can be extended to any integer value which can
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* be passed by user-space when the program is loaded.
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*/
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if (emit_mod_ret_check_imm8(&prog, 0))
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return -EINVAL;
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/* cmp QWORD PTR [rbp - 0x8], 0x0 */
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EMIT4(0x48, 0x83, 0x7d, 0xf8); EMIT1(0x00);
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/* Save the location of the branch and Generate 6 nops
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* (4 bytes for an offset and 2 bytes for the jump) These nops
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