On 15/8/23 08:52, Eduard Zingerman wrote:
> On Mon, 2023-08-14 at 21:41 +0800, Leon Hwang wrote:
>> From commit ebf7d1f508a73871 ("bpf, x64: rework pro/epilogue and tailcall
>> handling in JIT"), the tailcall on x64 works better than before.
>>
>> From commit e411901c0b775a3a ("bpf: allow for tailcalls in BPF subprograms
>> for x64 JIT"), tailcall is able to run in BPF subprograms on x64.
>>
>> From commit 5b92a28aae4dd0f8 ("bpf: Support attaching tracing BPF program
>> to other BPF programs"), BPF program is able to trace other BPF programs.
>>
>> How about combining them all together?
>>
>> 1. FENTRY/FEXIT on a BPF subprogram.
>> 2. A tailcall runs in the BPF subprogram.
>> 3. The tailcall calls itself.
>>
>> As a result, a tailcall infinite loop comes up. And the loop halts the
>> machine.
>>
>> As we know, in tail call context, the tail_call_cnt propagates by stack
>> and RAX register between BPF subprograms. So do it in FENTRY/FEXIT
>> trampolines.
>
> Hi Leon,
>
> I'm not familiar with this part of the jit compiler, so decided that
> taking a look at your series might be a good learning point.
> I think I got the gist of it, but I don't understand where
> the initial value of RAX (== 0) is coming from in
> arch_prepare_bpf_trampoline(), could you please help me out?
>
> Also a nitpick:
> - in arch_prepare_bpf_trampoline() there is a comment detailing
> the stack layout, it probably should be updated to say that
> tail call count is stored as well;
> - before arch_prepare_bpf_trampoline() there is a comment with
> an example of generated assembly, should it be updated?
>
> Thanks,
> Eduard
>
a) Initial value of RAX is in emit_prologue().
if (!ebpf_from_cbpf) {
if (tail_call_reachable && !is_subprog)
/* When it's the entry of the whole
* tailcall context, zeroing the RAX
* means init tail_call_cnt.
*/
EMIT2(0x31, 0xC0); /* xor eax, eax */
else
// Keep the same asm layout.
EMIT2(0x66, 0x90); /* nop2 */
}
I'd like to add this comment to emit_prologue().
b) Good to update the stack layout. I'll do it.
c) Its comment will be updated also.
Thanks,
Leon
>>
>> Fixes: ebf7d1f508a7 ("bpf, x64: rework pro/epilogue and tailcall handling in JIT")
>> Fixes: e411901c0b77 ("bpf: allow for tailcalls in BPF subprograms for x64 JIT")
>> Signed-off-by: Leon Hwang <hffilwlqm@xxxxxxxxx>
>> ---
>> arch/x86/net/bpf_jit_comp.c | 23 +++++++++++++++++++----
>> include/linux/bpf.h | 6 ++++++
>> kernel/bpf/trampoline.c | 5 +++--
>> kernel/bpf/verifier.c | 9 +++++++--
>> 4 files changed, 35 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
>> index a5930042139d3..ca5366d97ad04 100644
>> --- a/arch/x86/net/bpf_jit_comp.c
>> +++ b/arch/x86/net/bpf_jit_comp.c
>> @@ -1018,6 +1018,10 @@ static void emit_shiftx(u8 **pprog, u32 dst_reg, u8 src_reg, bool is64, u8 op)
>>
>> #define INSN_SZ_DIFF (((addrs[i] - addrs[i - 1]) - (prog - temp)))
>>
>> +/* mov rax, qword ptr [rbp - rounded_stack_depth - 8] */
>> +#define RESTORE_TAIL_CALL_CNT(stack) \
>> + EMIT3_off32(0x48, 0x8B, 0x85, -round_up(stack, 8) - 8)
>> +
>> static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image,
>> int oldproglen, struct jit_context *ctx, bool jmp_padding)
>> {
>> @@ -1623,9 +1627,7 @@ st: if (is_imm8(insn->off))
>>
>> func = (u8 *) __bpf_call_base + imm32;
>> if (tail_call_reachable) {
>> - /* mov rax, qword ptr [rbp - rounded_stack_depth - 8] */
>> - EMIT3_off32(0x48, 0x8B, 0x85,
>> - -round_up(bpf_prog->aux->stack_depth, 8) - 8);
>> + RESTORE_TAIL_CALL_CNT(bpf_prog->aux->stack_depth);
>> if (!imm32)
>> return -EINVAL;
>> offs = 7 + x86_call_depth_emit_accounting(&prog, func);
>> @@ -2464,6 +2466,8 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
>> else
>> /* sub rsp, stack_size */
>> EMIT4(0x48, 0x83, 0xEC, stack_size);
>> + if (flags & BPF_TRAMP_F_TAIL_CALL_CTX)
>> + EMIT1(0x50); /* push rax */
>> /* mov QWORD PTR [rbp - rbx_off], rbx */
>> emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_6, -rbx_off);
>>
>> @@ -2516,6 +2520,12 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
>> restore_regs(m, &prog, regs_off);
>> save_args(m, &prog, arg_stack_off, true);
>>
>> + if (flags & BPF_TRAMP_F_TAIL_CALL_CTX)
>> + /* Before calling the original function, restore the
>> + * tail_call_cnt from stack.
>> + */
>> + RESTORE_TAIL_CALL_CNT(stack_size);
>> +
>> if (flags & BPF_TRAMP_F_ORIG_STACK) {
>> emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, 8);
>> EMIT2(0xff, 0xd0); /* call *rax */
>> @@ -2569,7 +2579,12 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
>> ret = -EINVAL;
>> goto cleanup;
>> }
>> - }
>> + } else if (flags & BPF_TRAMP_F_TAIL_CALL_CTX)
>> + /* Before running the original function, restore the
>> + * tail_call_cnt from stack.
>> + */
>> + RESTORE_TAIL_CALL_CNT(stack_size);
>> +
>> /* restore return value of orig_call or fentry prog back into RAX */
>> if (save_ret)
>> emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, -8);
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index cfabbcf47bdb8..55c72086034ef 100644
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -1028,6 +1028,11 @@ struct btf_func_model {
>> */
>> #define BPF_TRAMP_F_SHARE_IPMODIFY BIT(6)
>>
>> +/* Indicate that current trampoline is in a tail call context. Then, it has to
>> + * cache and restore tail_call_cnt to avoid infinite tail call loop.
>> + */
>> +#define BPF_TRAMP_F_TAIL_CALL_CTX BIT(7)
>> +
>> /* Each call __bpf_prog_enter + call bpf_func + call __bpf_prog_exit is ~50
>> * bytes on x86.
>> */
>> @@ -1147,6 +1152,7 @@ struct bpf_attach_target_info {
>> struct module *tgt_mod;
>> const char *tgt_name;
>> const struct btf_type *tgt_type;
>> + bool tail_call_ctx;
>> };
>>
>> #define BPF_DISPATCHER_MAX 48 /* Fits in 2048B */
>> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
>> index 78acf28d48732..0fae334e3f7b8 100644
>> --- a/kernel/bpf/trampoline.c
>> +++ b/kernel/bpf/trampoline.c
>> @@ -415,8 +415,8 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr, bool lock_direct_mut
>> goto out;
>> }
>>
>> - /* clear all bits except SHARE_IPMODIFY */
>> - tr->flags &= BPF_TRAMP_F_SHARE_IPMODIFY;
>> + /* clear all bits except SHARE_IPMODIFY and TAIL_CALL_CTX */
>> + tr->flags &= (BPF_TRAMP_F_SHARE_IPMODIFY | BPF_TRAMP_F_TAIL_CALL_CTX);
>>
>> if (tlinks[BPF_TRAMP_FEXIT].nr_links ||
>> tlinks[BPF_TRAMP_MODIFY_RETURN].nr_links) {
>> @@ -783,6 +783,7 @@ struct bpf_trampoline *bpf_trampoline_get(u64 key,
>>
>> memcpy(&tr->func.model, &tgt_info->fmodel, sizeof(tgt_info->fmodel));
>> tr->func.addr = (void *)tgt_info->tgt_addr;
>> + tr->flags = (tgt_info->tail_call_ctx ? BPF_TRAMP_F_TAIL_CALL_CTX : 0);
>> out:
>> mutex_unlock(&tr->mutex);
>> return tr;
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 4ccca1f6c9981..a78e5a2ae5c72 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -19400,10 +19400,15 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
>> return -EINVAL;
>> fallthrough;
>> case BPF_MODIFY_RETURN:
>> - case BPF_LSM_MAC:
>> - case BPF_LSM_CGROUP:
>> case BPF_TRACE_FENTRY:
>> case BPF_TRACE_FEXIT:
>> + if (tgt_prog && subprog > 0 &&
>> + tgt_prog->aux->func[subprog]->is_func &&
>> + tgt_prog->aux->tail_call_reachable)
>> + tgt_info->tail_call_ctx = true;
>> + fallthrough;
>> + case BPF_LSM_MAC:
>> + case BPF_LSM_CGROUP:
>> if (!btf_type_is_func(t)) {
>> bpf_log(log, "attach_btf_id %u is not a function\n",
>> btf_id);
>
