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Date: Tue, 19 Apr 2022 09:20:03 -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 Tue, Apr 19, 2022 at 8:42 AM <sdf@...gle.com> wrote:
>
> On 04/18, Alexei Starovoitov wrote:
> > 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.
>
> I've retested again and it still unrolls it for me on gcc 11 :-/
> Anyway, I guess we have two options:
>
> 1. Go back to defines.
> 2. Don't pass a ptr to func, but pass an enum which indicates whether
> to use bpf_prog_run or __bpf_prog_run_save_cb. Seems like in this
> case the compiler shouldn't have any trouble unwrapping it?
>
> I'll prototype and send (2). If it won't work out we can always get back
> to (1).
Going back to defines is probably not necessary.
Could you try moving bpf_prog_run_array_cg*() back to .h
and use static __always_inline ?
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