Re: yet another approach Was: [PATCH bpf-next v3 4/5] bpf, x86: Add jit support for private stack

[Yonghong Song <yonghong.song@linux.dev>]

On 10/1/24 6:26 PM, Alexei Starovoitov wrote:
> On Tue, Oct 1, 2024 at 5:23 PM Kumar Kartikeya Dwivedi <memxor@gmail.com> wrote:
>> Makes sense, though will we have cases where hierarchical scheduling
>> attaches the same prog at different points of the hierarchy?
> I'm not sure anyone was asking for such a use case.
>
>> Then the
>> limit of 4 may not be enough (e.g. say with cgroup nested levels > 4).
> Well, 4 was the number from TJ.
>
> Anyway the proposed pseudo code:
>
> __bpf_prog_enter_recur_limited()
> {
>    cnt = this_cpu_inc_return(*(prog->active));
>    if (cnt > 4) {
>       inc_miss
>       return 0;
>    }
>   // pass cnt into bpf prog somehow, like %rdx ?
>   // or re-read prog->active from prog
> }
>
>
> then in the prologue emit:
>
> push rbp
> mov rbp, rsp
> if %rdx == 1
>     // main prog is called for the first time
>     mov rsp, pcpu_priv_stack_top
> else
>     // 2+nd time main prog is called or 1+ time subprog
>    sub rsp, stack_size
>    if rsp < pcpu_priv_stack_bottom
>      goto exit  // stack is too small, exit
> fi

I have tried to implement this approach (not handling
recursion yet) based on the above approach. It works
okay with nested bpf subprogs like
    main prog  // set rsp = pcpu_priv_stack_top
      subprog1 // some stack
        subprog2 // some stack

The pcpu_priv_stack is allocated like
   priv_stack_ptr = __alloc_percpu_gfp(1024 * 16, 8, GFP_KERNEL);

But whenever the prog called an external function,
e.g. a helper in this case, I will get a double fault.
An example could be
    main prog  // set rsp = pcpu_priv_stack_top
      subprog1 // some stack
        subprog2 // some stack
      call bpf_seq_printf
(I modified bpf_iter_ipv6_route.c bpf prog for the above
purpose.)
I added some printk statements from the beginning of bpf_seq_printf and
nothing printed out either and of course traps still happens.

I tried another example without subprog and the mainprog calls
a helper and the same double traps happens below too.

The error log looks like

[   54.024955] traps: PANIC: double fault, error_code: 0x0
[   54.024969] Oops: double fault: 0000 [#1] PREEMPT SMP KASAN PTI
[   54.024977] CPU: 3 UID: 0 PID: 1946 Comm: test_progs Tainted: G           OE      6.11.0-10577-gf25c172fd840-dirty #968
[   54.024982] Tainted: [O]=OOT_MODULE, [E]=UNSIGNED_MODULE
[   54.024983] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
[   54.024986] RIP: 0010:error_entry+0x1e/0x140
[   54.024996] Code: ff ff 90 90 90 90 90 90 90 90 90 90 56 48 8b 74 24 08 48 89 7c 24 08 52 51 50 41 50 41 51 41 52 41 53 53 55 41 54 41 55 41 56 <41> 57 56 31 f6 31 d1
[   54.024999] RSP: 0018:ffffe8ffff580000 EFLAGS: 00010806
[   54.025002] RAX: f3f3f300f1f1f1f1 RBX: fffff91fffeb0044 RCX: ffffffff84201701
[   54.025005] RDX: fffff91fffeb0044 RSI: ffffffff8420128d RDI: ffffe8ffff580178
[   54.025007] RBP: ffffe8ffff580140 R08: 0000000000000000 R09: 0000000000000000
[   54.025009] R10: 0000000000000000 R11: 0000000000000000 R12: dffffc0000000000
[   54.025010] R13: 1ffffd1fffeb0014 R14: 0000000000000003 R15: ffffe8ffff580178
[   54.025012] FS:  00007fd076525d00(0000) GS:ffff8881f7180000(0000) knlGS:0000000000000000
[   54.025015] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   54.025017] CR2: ffffe8ffff57fff8 CR3: 000000010cd80002 CR4: 0000000000370ef0
[   54.025021] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   54.025022] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   54.025024] Call Trace:
[   54.025026]  <#DF>
[   54.025028]  ? __die_body+0xaf/0xc0
[   54.025032]  ? die+0x2f/0x50
[   54.025036]  ? exc_double_fault+0x73/0x80
[   54.025040]  ? asm_exc_double_fault+0x23/0x30
[   54.025044]  ? common_interrupt_return+0xb1/0xcc
[   54.025048]  ? asm_exc_page_fault+0xd/0x30
[   54.025051]  ? error_entry+0x1e/0x140
[   54.025055]  </#DF>
[   54.025056] Modules linked in: bpf_testmod(OE)
[   54.025061] ---[ end trace 0000000000000000 ]---

Maybe somebody could give a hint why I got a double fault
when calling external functions (outside of bpf programs)
with allocated stack?

>
> Since stack bottom/top are known at JIT time we can
> generate reliable stack overflow checks.
> Much better than guard pages and -fstack-protector.
> The prog can alloc percpu
> (stack size of main prog + subprogs + extra) * 4
> and it likely will be enough.
> If not, the stack protection will gently exit the prog
> when the stack is too deep.
> kfunc won't have such a check, so we need a buffer zone.
> Can have a guard page too, but feels like overkill.