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

arch/x86/net/bpf_jit_comp.c

@@ -1449,23 +1449,6 @@ static int emit_cond_near_jump(u8 **pprog, void *func, void *ip, u8 jmp_cond)
 	return 0;
 }
 
-static int emit_mod_ret_check_imm8(u8 **pprog, int value)
-{
-	u8 *prog = *pprog;
-	int cnt = 0;
-
-	if (!is_imm8(value))
-		return -EINVAL;
-
-	if (value == 0)
-		EMIT2(0x85, add_2reg(0xC0, BPF_REG_0, BPF_REG_0));
-	else
-		EMIT3(0x83, add_1reg(0xF8, BPF_REG_0), value);
-
-	*pprog = prog;
-	return 0;
-}
-
 static int invoke_bpf(const struct btf_func_model *m, u8 **pprog,
 		      struct bpf_tramp_progs *tp, int stack_size)
 {
@@ -1485,7 +1468,7 @@ static int invoke_bpf_mod_ret(const struct btf_func_model *m, u8 **pprog,
 			      u8 **branches)
 {
 	u8 *prog = *pprog;
-	int i;
+	int i, cnt = 0;
 
 	/* The first fmod_ret program will receive a garbage return value.
 	 * Set this to 0 to avoid confusing the program.
@@ -1496,16 +1479,12 @@ static int invoke_bpf_mod_ret(const struct btf_func_model *m, u8 **pprog,
 		if (invoke_bpf_prog(m, &prog, tp->progs[i], stack_size, true))
 			return -EINVAL;
 
-		/* Generate a branch:
-		 *
-		 * if (ret !=  0)
+		/* mod_ret prog stored return value into [rbp - 8]. Emit:
+		 * if (*(u64 *)(rbp - 8) !=  0)
 		 *	goto do_fexit;
-		 *
-		 * If needed this can be extended to any integer value which can
-		 * be passed by user-space when the program is loaded.
 		 */
-		if (emit_mod_ret_check_imm8(&prog, 0))
-			return -EINVAL;
+		/* cmp QWORD PTR [rbp - 0x8], 0x0 */
+		EMIT4(0x48, 0x83, 0x7d, 0xf8); EMIT1(0x00);
 
 		/* Save the location of the branch and Generate 6 nops
 		 * (4 bytes for an offset and 2 bytes for the jump) These nops