On 2024/9/14 03:28, Alexei Starovoitov wrote:
> On Sun, Sep 1, 2024 at 6:41 AM Leon Hwang <leon.hwang@xxxxxxxxx> wrote:
>>
>> @@ -573,10 +575,13 @@ int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type t,
>>
>> /*
>> * See emit_prologue(), for IBT builds the trampoline hook is preceded
>> - * with an ENDBR instruction.
>> + * with an ENDBR instruction and 3 bytes tail_call_cnt initialization
>> + * instruction.
>> */
>> if (is_endbr(*(u32 *)ip))
>> ip += ENDBR_INSN_SIZE;
>> + if (is_bpf_text_address((long)ip))
>> + ip += X86_POKE_EXTRA;
>
> This is a foot gun.
> bpf_arch_text_poke() is used not only at the beginning of the function.
> So unconditional ip += 3 is not just puzzling with 'what is this for',
> but dangerous and wasteful...
>
>> @@ -2923,6 +2930,8 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im
>> */
>> if (is_endbr(*(u32 *)orig_call))
>> orig_call += ENDBR_INSN_SIZE;
>> + if (is_bpf_text_address((long)orig_call))
>> + orig_call += X86_POKE_EXTRA;
>> orig_call += X86_PATCH_SIZE;
>> }
>
> ..this bit needs to be hacked too...
>
>> @@ -3025,6 +3034,7 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im
>> /* remember return value in a stack for bpf prog to access */
>> emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -8);
>> im->ip_after_call = image + (prog - (u8 *)rw_image);
>> + emit_nops(&prog, X86_POKE_EXTRA);
>> emit_nops(&prog, X86_PATCH_SIZE);
>
> And this is just pure waste of kernel code and cpu run-time.
>
> You're adding 3 byte nop for no reason at all.
>
> See commit e21aa341785c ("bpf: Fix fexit trampoline.")
> that added:
> int err = bpf_arch_text_poke(im->ip_after_call, BPF_MOD_JUMP,
> NULL, im->ip_epilogue);
> logic that is patching bpf trampoline in the middle of it.
> (not at the start).
>
> Because of unconditional +=3 in bpf_arch_text_poke() every trampoline
> will have to waste nop3 ?
> No.
>
> Please fix freplace and tail call combination without
> this kind of unacceptable shortcuts.
>
> I very much prefer to stop hacking into JITs and trampolines because
> tailcalls and freplace don't work well together.
>
> We cannot completely disable that combination because libxdp
> is using freplace to populate chain of progs the main prog is calling
> and these freplace progs might be doing tailcall,
> but we can still prevent such infinite loop that you describe in commit log:
> entry_tc -> subprog_tc -> entry_freplace -> subprog_tail --tailcall-> entry_tc
> in the verifier without resorting to JIT hacks.
>
> pw-bot: cr
IIUC, it's going to prevent this niche case at update/attach time, that
a prog cannot be updated to prog_array map and be extended by freplace
prog at the same time.
More specific explanation:
1. If a prog has been updated to prog_array, it and its subprog cannot
be extended by freplace prog.
2. If a prog or its subprog has been extended by freplace prog, it
cannot be updated to prog_array map.
Then, I do a POC[0] to prevent this niche case. And the POC does not
break any selftest.
[0] https://github.com/kernel-patches/bpf/pull/7732
Here's the diff of the POC.
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 0c3893c471711..b864b37e67c17 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1483,6 +1483,8 @@ struct bpf_prog_aux {
bool xdp_has_frags;
bool exception_cb;
bool exception_boundary;
+ bool is_extended; /* true if extended by freplace program */
+ atomic_t tail_callee_cnt;
struct bpf_arena *arena;
/* BTF_KIND_FUNC_PROTO for valid attach_btf_id */
const struct btf_type *attach_func_proto;
diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
index 79660e3fca4c1..be41240d4fb3a 100644
--- a/kernel/bpf/arraymap.c
+++ b/kernel/bpf/arraymap.c
@@ -951,18 +951,27 @@ static void *prog_fd_array_get_ptr(struct bpf_map
*map,
if (IS_ERR(prog))
return prog;
- if (!bpf_prog_map_compatible(map, prog)) {
+ if (!bpf_prog_map_compatible(map, prog) || prog->aux->is_extended) {
+ /* Extended prog can not be tail callee. It's to prevent a
+ * potential infinite loop like:
+ * tail callee prog entry -> tail callee prog subprog ->
+ * freplace prog entry --tailcall-> tail callee prog entry.
+ */
bpf_prog_put(prog);
return ERR_PTR(-EINVAL);
}
+ atomic_inc(&prog->aux->tail_callee_cnt);
return prog;
}
static void prog_fd_array_put_ptr(struct bpf_map *map, void *ptr, bool
need_defer)
{
+ struct bpf_prog *prog = ptr;
+
/* bpf_prog is freed after one RCU or tasks trace grace period */
- bpf_prog_put(ptr);
+ atomic_dec(&prog->aux->tail_callee_cnt);
+ bpf_prog_put(prog);
}
static u32 prog_fd_array_sys_lookup_elem(void *ptr)
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 8a4117f6d7610..be829016d8182 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3292,8 +3292,11 @@ static void bpf_tracing_link_release(struct
bpf_link *link)
bpf_trampoline_put(tr_link->trampoline);
/* tgt_prog is NULL if target is a kernel function */
- if (tr_link->tgt_prog)
+ if (tr_link->tgt_prog) {
+ if (link->prog->type == BPF_PROG_TYPE_EXT)
+ tr_link->tgt_prog->aux->is_extended = false;
bpf_prog_put(tr_link->tgt_prog);
+ }
}
static void bpf_tracing_link_dealloc(struct bpf_link *link)
@@ -3498,6 +3501,18 @@ static int bpf_tracing_prog_attach(struct
bpf_prog *prog,
tgt_prog = prog->aux->dst_prog;
}
+ if (prog->type == BPF_PROG_TYPE_EXT &&
+ atomic_read(&tgt_prog->aux->tail_callee_cnt)) {
+ /* Program extensions can not extend target prog when the target
+ * prog has been updated to any prog_array map as tail callee.
+ * It's to prevent a potential infinite loop like:
+ * tgt prog entry -> tgt prog subprog -> freplace prog entry
+ * --tailcall-> tgt prog entry.
+ */
+ err = -EINVAL;
+ goto out_unlock;
+ }
+
err = bpf_link_prime(&link->link.link, &link_primer);
if (err)
goto out_unlock;
@@ -3523,6 +3538,8 @@ static int bpf_tracing_prog_attach(struct bpf_prog
*prog,
if (prog->aux->dst_trampoline && tr != prog->aux->dst_trampoline)
/* we allocated a new trampoline, so free the old one */
bpf_trampoline_put(prog->aux->dst_trampoline);
+ if (prog->type == BPF_PROG_TYPE_EXT)
+ tgt_prog->aux->is_extended = true;
prog->aux->dst_prog = NULL;
prog->aux->dst_trampoline = NULL;
Thanks,
Leon
