Good catch and thanks for fixing this!
On Wed, Mar 11, 2020 at 1:39 AM Alexei Starovoitov <ast@xxxxxxxxxx> wrote:
>
> 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@xxxxxx>
> Acked-by: Andrii Nakryiko <andriin@xxxxxx>
> Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
> ---
> arch/x86/net/bpf_jit_comp.c | 31 +++++--------------------------
> 1 file changed, 5 insertions(+), 26 deletions(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index b1fd000feb89..5ea7c2cf7ab4 100644
> --- 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
> --
> 2.23.0
>
Acked-by: KP Singh <kpsingh@xxxxxxxxxx>
