netdev - Re: [PATCH bpf-next v2] bpf: move rcu lock management out of BPF_PROG

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Date:   Mon, 18 Apr 2022 22:18:16 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Stanislav Fomichev <sdf@...gle.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>
Subject: Re: [PATCH bpf-next v2] bpf: move rcu lock management out of
 BPF_PROG_RUN routines

On Mon, Apr 18, 2022 at 9:50 AM <sdf@...gle.com> wrote:
>
> On 04/16, Alexei Starovoitov wrote:
> > On Thu, Apr 14, 2022 at 9:12 AM Stanislav Fomichev <sdf@...gle.com> wrote:
> > > +static int
> > > +bpf_prog_run_array_cg_flags(const struct cgroup_bpf *cgrp,
> > > +                           enum cgroup_bpf_attach_type atype,
> > > +                           const void *ctx, bpf_prog_run_fn run_prog,
> > > +                           int retval, u32 *ret_flags)
> > > +{
> > > +       const struct bpf_prog_array_item *item;
> > > +       const struct bpf_prog *prog;
> > > +       const struct bpf_prog_array *array;
> > > +       struct bpf_run_ctx *old_run_ctx;
> > > +       struct bpf_cg_run_ctx run_ctx;
> > > +       u32 func_ret;
> > > +
> > > +       run_ctx.retval = retval;
> > > +       migrate_disable();
> > > +       rcu_read_lock();
> > > +       array = rcu_dereference(cgrp->effective[atype]);
> > > +       item = &array->items[0];
> > > +       old_run_ctx = bpf_set_run_ctx(&run_ctx.run_ctx);
> > > +       while ((prog = READ_ONCE(item->prog))) {
> > > +               run_ctx.prog_item = item;
> > > +               func_ret = run_prog(prog, ctx);
> > ...
> > > +       ret = bpf_prog_run_array_cg(&cgrp->bpf, CGROUP_GETSOCKOPT,
> > >                                     &ctx, bpf_prog_run, retval);
>
> > Did you check the asm that bpf_prog_run gets inlined
> > after being passed as a pointer to a function?
> > Crossing fingers... I suspect not every compiler can do that :(
> > De-virtualization optimization used to be tricky.
>
> No, I didn't, but looking at it right now, both gcc and clang
> seem to be doing inlining all way up to bpf_dispatcher_nop_func.
>
> clang:
>
>    0000000000001750 <__cgroup_bpf_run_filter_sock_addr>:
>    __cgroup_bpf_run_filter_sock_addr():
>    ./kernel/bpf/cgroup.c:1226
>    int __cgroup_bpf_run_filter_sock_addr(struct sock *sk,
>                                       struct sockaddr *uaddr,
>                                       enum cgroup_bpf_attach_type atype,
>                                       void *t_ctx,
>                                       u32 *flags)
>    {
>
>    ...
>
>    ./include/linux/filter.h:628
>                 ret = dfunc(ctx, prog->insnsi, prog->bpf_func);
>        1980:    49 8d 75 48             lea    0x48(%r13),%rsi
>    bpf_dispatcher_nop_func():
>    ./include/linux/bpf.h:804
>         return bpf_func(ctx, insnsi);
>        1984:    4c 89 f7                mov    %r14,%rdi
>        1987:    41 ff 55 30             call   *0x30(%r13)
>        198b:    89 c3                   mov    %eax,%ebx
>
> gcc (w/retpoline):
>
>    0000000000001110 <__cgroup_bpf_run_filter_sock_addr>:
>    __cgroup_bpf_run_filter_sock_addr():
>    kernel/bpf/cgroup.c:1226
>    {
>
>    ...
>
>    ./include/linux/filter.h:628
>                 ret = dfunc(ctx, prog->insnsi, prog->bpf_func);
>        11c5:    49 8d 75 48             lea    0x48(%r13),%rsi
>    bpf_dispatcher_nop_func():
>    ./include/linux/bpf.h:804
>        11c9:    48 8d 7c 24 10          lea    0x10(%rsp),%rdi
>        11ce:    e8 00 00 00 00          call   11d3
> <__cgroup_bpf_run_filter_sock_addr+0xc3>
>                         11cf: R_X86_64_PLT32    __x86_indirect_thunk_rax-0x4
>        11d3:    89 c3                   mov    %eax,%ebx

Hmm. I'm not sure how you've got this asm.
Here is what I see with gcc 8 and gcc 10:
bpf_prog_run_array_cg:
...
        movq    %rcx, %r12      # run_prog, run_prog
...
# ../kernel/bpf/cgroup.c:77:            run_ctx.prog_item = item;
        movq    %rbx, (%rsp)    # item, run_ctx.prog_item
# ../kernel/bpf/cgroup.c:78:            if (!run_prog(prog, ctx) &&
!IS_ERR_VALUE((long)run_ctx.retval))
        movq    %rbp, %rsi      # ctx,
        call    *%r12   # run_prog

__cgroup_bpf_run_filter_sk:
        movq    $bpf_prog_run, %rcx     #,
# ../kernel/bpf/cgroup.c:1202:  return
bpf_prog_run_array_cg(&cgrp->bpf, atype, sk, bpf_prog_run, 0);
        leaq    1520(%rax), %rdi        #, tmp92
# ../kernel/bpf/cgroup.c:1202:  return
bpf_prog_run_array_cg(&cgrp->bpf, atype, sk, bpf_prog_run, 0);
        jmp     bpf_prog_run_array_cg   #

This is without kasan, lockdep and all debug configs are off.

So the generated code is pretty bad as I predicted :(

So I'm afraid this approach is no go.