Subject: Re: [PATCH bpf-next v2] bpf: move rcu lock management out of BPF_PROG_RUN routines

From: [email-masked]
Date: Mon, 18 Apr 2022 09:50:45 -0700

On 04/16, Alexei Starovoitov wrote:
> On Thu, Apr 14, 2022 at 9:12 AM Stanislav Fomichev <[email-masked]> 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