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

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On 10/3/24 1:47 PM, Kumar Kartikeya Dwivedi wrote:
On Thu, 3 Oct 2024 at 22:44, Yonghong Song <yonghong.song@xxxxxxxxx> wrote:

On 10/3/24 10:35 AM, Alexei Starovoitov wrote:
On Thu, Oct 3, 2024 at 6:40 AM Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> wrote:
On Thu, 3 Oct 2024 at 08:17, Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
On 10/1/24 6:26 PM, Alexei Starovoitov wrote:
On Tue, Oct 1, 2024 at 5:23 PM Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> 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?

I will help in debugging. Can you share the patch you applied locally
so I can reproduce?
Looks like the idea needs more thought.

in_task_stack() won't recognize the private stack,
so it will look like stack overflow and double fault.

do you have CONFIG_VMAP_STACK ?
Yes, my above test runs fine withCONFIG_VMAP_STACK. Let me guard private stack support with
CONFIG_VMAP_STACK for now. Not sure whether distributions enable
CONFIG_VMAP_STACK or not.

I think it is the default on most distributions (Debian, Ubuntu, Fedora, etc.).

Thanks for confirmation! Great CONFIG_VMAP_STACK is on by default for most distro's.





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