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: ae24082331d9 ("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

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index b1fd000..5ea7c2c 100644 (file)
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/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