Re: [PATCH v5 bpf-next 1/3] bpf, x64: Fix tailcall hierarchy

Leon Hwang
2024-07-11T02:02:22Z

On 11/7/24 08:15, Eduard Zingerman wrote:
> On Mon, 2024-06-24 at 00:15 +0800, Leon Hwang wrote:
>> This patch fixes a tailcall issue caused by abusing the tailcall in
>> bpf2bpf feature.
>>
>> As we know, tail_call_cnt propagates by rax from caller to callee when
>> to call subprog in tailcall context. But, like the following example,
>> MAX_TAIL_CALL_CNT won't work because of missing tail_call_cnt
>> back-propagation from callee to caller.
>>
>> \#include <linux/bpf.h>
>> \#include <bpf/bpf_helpers.h>
>> \#include "bpf_legacy.h"
>>
>> struct {
>> __uint(type, BPF_MAP_TYPE_PROG_ARRAY);
>> __uint(max_entries, 1);
>> __uint(key_size, sizeof(__u32));
>> __uint(value_size, sizeof(__u32));
>> } jmp_table SEC(".maps");
>>
>> int count = 0;
>>
>> static __noinline
>> int subprog_tail1(struct __sk_buff *skb)
>> {
>> bpf_tail_call_static(skb, &jmp_table, 0);
>> return 0;
>> }
>>
>> static __noinline
>> int subprog_tail2(struct __sk_buff *skb)
>> {
>> bpf_tail_call_static(skb, &jmp_table, 0);
>> return 0;
>> }
>>
>> SEC("tc")
>> int entry(struct __sk_buff *skb)
>> {
>> volatile int ret = 1;
>>
>> count++;
>> subprog_tail1(skb);
>> subprog_tail2(skb);
>>
>> return ret;
>> }
>>
>> char __license[] SEC("license") = "GPL";
>>
>> At run time, the tail_call_cnt in entry() will be propagated to
>> subprog_tail1() and subprog_tail2(). But, when the tail_call_cnt in
>> subprog_tail1() updates when bpf_tail_call_static(), the tail_call_cnt
>> in entry() won't be updated at the same time. As a result, in entry(),
>> when tail_call_cnt in entry() is less than MAX_TAIL_CALL_CNT and
>> subprog_tail1() returns because of MAX_TAIL_CALL_CNT limit,
>> bpf_tail_call_static() in suprog_tail2() is able to run because the
>> tail_call_cnt in subprog_tail2() propagated from entry() is less than
>> MAX_TAIL_CALL_CNT.
>>
>> So, how many tailcalls are there for this case if no error happens?
>>
>> From top-down view, does it look like hierarchy layer and layer?
>>
>> With this view, there will be 2+4+8+...+2^33 = 2^34 - 2 = 17,179,869,182
>> tailcalls for this case.
>>
>> How about there are N subprog_tail() in entry()? There will be almost
>> N^34 tailcalls.
>>
>> Then, in this patch, it resolves this case on x86_64.
>>
>> In stead of propagating tail_call_cnt from caller to callee, it
>> propagates its pointer, tail_call_cnt_ptr, tcc_ptr for short.
>>
>> However, where does it store tail_call_cnt?
>>
>> It stores tail_call_cnt on the stack of main prog. When tail call
>> happens in subprog, it increments tail_call_cnt by tcc_ptr.
>>
>> Meanwhile, it stores tail_call_cnt_ptr on the stack of main prog, too.
>>
>> And, before jump to tail callee, it has to pop tail_call_cnt and
>> tail_call_cnt_ptr.
>>
>> Then, at the prologue of subprog, it must not make rax as
>> tail_call_cnt_ptr again. It has to reuse tail_call_cnt_ptr from caller.
>>
>> As a result, at run time, it has to recognize rax is tail_call_cnt or
>> tail_call_cnt_ptr at prologue by:
>>
>> 1. rax is tail_call_cnt if rax is <= MAX_TAIL_CALL_CNT.
>> 2. rax is tail_call_cnt_ptr if rax is > MAX_TAIL_CALL_CNT, because a
>> pointer won't be <= MAX_TAIL_CALL_CNT.
>>
>> Furthermore, when trampoline is the caller of bpf prog, which is
>> tail_call_reachable, it is required to propagate rax through trampoline.
>>
>> 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@gmail.com>
>> ---
>
> Hi Leon,
>
> Sorry for delayed response.
> I've looked through this patch and the changes make sense to me.
> One thing that helped to understand the gist of the changes,
> was dumping jited program using bpftool and annotating it with comments:
> https://gist.github.com/eddyz87/0d48da052e9d174b2bb84174295c4215
> Maybe consider adding something along these lines to the patch
> description?

Sure, I'll resend the patch with updated description along annotating
comments later.

Thanks,
Leon

>
> Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>
>
> …