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Date: Wed, 29 May 2024 09:38:27 +0200
From: Benjamin Tissoires <bentiss@kernel.org>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>, Jiri Kosina <jikos@kernel.org>, 
	Jonathan Corbet <corbet@lwn.net>, Alexei Starovoitov <ast@kernel.org>, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>, bpf <bpf@vger.kernel.org>, 
	"open list:HID CORE LAYER" <linux-input@vger.kernel.org>, "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
Subject: Re: [PATCH HID 03/13] HID: bpf: implement HID-BPF through
 bpf_struct_ops
Message-ID: <bbmrcmm2tsbxm3f7bdjbdprqt62hkrw3d7w4w4taxaxx24rlu5@iu5krfvhcaru>
References: <20240528-hid_bpf_struct_ops-v1-0-8c6663df27d8@kernel.org>
 <20240528-hid_bpf_struct_ops-v1-3-8c6663df27d8@kernel.org>
 <CAADnVQJbc4uVmhh+ABPMBGFjTYwQQp3wGLeTBYEUWti3R8V8aw@mail.gmail.com>
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In-Reply-To: <CAADnVQJbc4uVmhh+ABPMBGFjTYwQQp3wGLeTBYEUWti3R8V8aw@mail.gmail.com>

On May 28 2024, Alexei Starovoitov wrote:
> On Tue, May 28, 2024 at 6:15 AM Benjamin Tissoires <bentiss@kernel.org> wrote:
> >
> > We do this implementation in several steps to not have the CI failing:
> > - first (this patch), we add struct_ops while keeping the existing infra
> >   available
> > - then we change the selftests, the examples and the existing in-tree
> >   HID-BPF programs
> > - then we remove the existing trace points making old HID-BPF obsolete
> >
> > There are a few advantages of struct_ops over tracing:
> > - compatibility with sleepable programs (for hid_hw_raw_request() in
> >   a later patch)
> > - a lot simpler in the kernel: it's a simple rcu protected list
> > - we can add more parameters to the function called without much trouble
> > - the "attach" is now generic through BPF-core: the caller just needs to
> >   set hid_id and flags before calling __load().
> > - all the BPF tough part is not handled in BPF-core through generic
> >   processing
> > - hid_bpf_ctx is now only writable where it needs be
> >
> > Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>
> > ---
> >  drivers/hid/bpf/Makefile             |   2 +-
> >  drivers/hid/bpf/hid_bpf_dispatch.c   |  52 +++++++-
> >  drivers/hid/bpf/hid_bpf_dispatch.h   |   4 +
> >  drivers/hid/bpf/hid_bpf_jmp_table.c  |   3 +
> >  drivers/hid/bpf/hid_bpf_struct_ops.c | 246 +++++++++++++++++++++++++++++++++++
> >  include/linux/hid_bpf.h              |  64 ++++++++-
> >  6 files changed, 362 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/hid/bpf/Makefile b/drivers/hid/bpf/Makefile
> > index cf55120cf7d6..1cb3f31e9335 100644
> > --- a/drivers/hid/bpf/Makefile
> > +++ b/drivers/hid/bpf/Makefile
> > @@ -8,4 +8,4 @@ LIBBPF_INCLUDE = $(srctree)/tools/lib
> >  obj-$(CONFIG_HID_BPF) += hid_bpf.o
> >  CFLAGS_hid_bpf_dispatch.o += -I$(LIBBPF_INCLUDE)
> >  CFLAGS_hid_bpf_jmp_table.o += -I$(LIBBPF_INCLUDE)
> > -hid_bpf-objs += hid_bpf_dispatch.o hid_bpf_jmp_table.o
> > +hid_bpf-objs += hid_bpf_dispatch.o hid_bpf_jmp_table.o hid_bpf_struct_ops.o
> > diff --git a/drivers/hid/bpf/hid_bpf_dispatch.c b/drivers/hid/bpf/hid_bpf_dispatch.c
> > index c8bb79ce2354..7216c3c7713d 100644
> > --- a/drivers/hid/bpf/hid_bpf_dispatch.c
> > +++ b/drivers/hid/bpf/hid_bpf_dispatch.c
> > @@ -58,6 +58,7 @@ dispatch_hid_bpf_device_event(struct hid_device *hdev, enum hid_report_type type
> >                 },
> >                 .data = hdev->bpf.device_data,
> >         };
> > +       struct hid_bpf_ops *e;
> >         int ret;
> >
> >         if (type >= HID_REPORT_TYPES)
> > @@ -70,9 +71,25 @@ dispatch_hid_bpf_device_event(struct hid_device *hdev, enum hid_report_type type
> >         memset(ctx_kern.data, 0, hdev->bpf.allocated_data);
> >         memcpy(ctx_kern.data, data, *size);
> >
> > +       rcu_read_lock();
> > +       list_for_each_entry_rcu(e, &hdev->bpf.prog_list, list) {
> 
> In the typical case there will be only one prog per device, right?

90% of the time there is only one prog per device, but I need to have
more than one for the following reasons:
- when one prog is loaded, the output data gets modified, so to be able
  to access the raw data from the device I need to add a "tracing" program
  *before* the one that changes data to forward raw data to userspace
- we already have situation where for convenience multiple programs are
  attached to one device (see drivers/hid/bpf/progs/XPPen__ArtistPro16Gen2.bpf.c)
- once firewalling will be in place, we will likely have one program for
  the firewall and one for the device fix

> The for_each is future proofing?
> 
> > +               if (e->hid_device_event) {
> > +                       ret = e->hid_device_event(&ctx_kern.ctx, type);
> > +                       if (ret < 0) {
> > +                               rcu_read_unlock();
> > +                               return ERR_PTR(ret);
> > +                       }
> > +
> > +                       if (ret)
> > +                               ctx_kern.ctx.retval = ret;
> > +               }
> > +       }
> > +       rcu_read_unlock();
> > +
> >         ret = hid_bpf_prog_run(hdev, HID_BPF_PROG_TYPE_DEVICE_EVENT, &ctx_kern);
> >         if (ret < 0)
> >                 return ERR_PTR(ret);
> > +       ret = ctx_kern.ctx.retval;
> >
> >         if (ret) {
> >                 if (ret > ctx_kern.ctx.allocated_size)
> > @@ -122,7 +139,10 @@ u8 *call_hid_bpf_rdesc_fixup(struct hid_device *hdev, u8 *rdesc, unsigned int *s
> >
> >         memcpy(ctx_kern.data, rdesc, min_t(unsigned int, *size, HID_MAX_DESCRIPTOR_SIZE));
> >
> > -       ret = hid_bpf_prog_run(hdev, HID_BPF_PROG_TYPE_RDESC_FIXUP, &ctx_kern);
> > +       if (hdev->bpf.rdesc_ops)
> > +               ret = hdev->bpf.rdesc_ops->hid_rdesc_fixup(&ctx_kern.ctx);
> > +       else
> > +               ret = hid_bpf_prog_run(hdev, HID_BPF_PROG_TYPE_RDESC_FIXUP, &ctx_kern);
> 
> This is for backward compat?

yes

> I don't see it's being removed in the later patches.

see patch 8/13, all the tracing capabilities are gone.

I made a backward compatibility step here so I do not break the
selftests while changing the API.

> 
> >         if (ret < 0)
> >                 goto ignore_bpf;
> >
> > @@ -150,7 +170,7 @@ static int device_match_id(struct device *dev, const void *id)
> >         return hdev->id == *(int *)id;
> >  }
> >
> > -static struct hid_device *hid_get_device(unsigned int hid_id)
> > +struct hid_device *hid_get_device(unsigned int hid_id)
> >  {
> >         struct device *dev;
> >
> > @@ -164,7 +184,7 @@ static struct hid_device *hid_get_device(unsigned int hid_id)
> >         return to_hid_device(dev);
> >  }
> >
> > -static void hid_put_device(struct hid_device *hid)
> > +void hid_put_device(struct hid_device *hid)
> >  {
> >         put_device(&hid->dev);
> >  }
> > @@ -205,7 +225,7 @@ static int __hid_bpf_allocate_data(struct hid_device *hdev, u8 **data, u32 *size
> >         return 0;
> >  }
> >
> > -static int hid_bpf_allocate_event_data(struct hid_device *hdev)
> > +int hid_bpf_allocate_event_data(struct hid_device *hdev)
> >  {
> >         /* hdev->bpf.device_data is already allocated, abort */
> >         if (hdev->bpf.device_data)
> > @@ -592,14 +612,22 @@ static const struct btf_kfunc_id_set hid_bpf_syscall_kfunc_set = {
> >
> >  int hid_bpf_connect_device(struct hid_device *hdev)
> >  {
> > -       struct hid_bpf_prog_list *prog_list;
> > +       bool need_to_allocate = false;
> > +       struct hid_bpf_ops *e;
> >
> >         rcu_read_lock();
> > -       prog_list = rcu_dereference(hdev->bpf.progs[HID_BPF_PROG_TYPE_DEVICE_EVENT]);
> > +       list_for_each_entry_rcu(e, &hdev->bpf.prog_list, list) {
> > +               if (e->hid_device_event) {
> > +                       need_to_allocate = true;
> > +                       break;
> > +               }
> > +       }
> > +       if (rcu_dereference(hdev->bpf.progs[HID_BPF_PROG_TYPE_DEVICE_EVENT]))
> > +               need_to_allocate = true;
> >         rcu_read_unlock();
> >
> >         /* only allocate BPF data if there are programs attached */
> > -       if (!prog_list)
> > +       if (!need_to_allocate)
> >                 return 0;
> >
> >         return hid_bpf_allocate_event_data(hdev);
> > @@ -623,12 +651,15 @@ void hid_bpf_destroy_device(struct hid_device *hdev)
> >         hdev->bpf.destroyed = true;
> >
> >         __hid_bpf_destroy_device(hdev);
> > +       __hid_bpf_ops_destroy_device(hdev);
> >  }
> >  EXPORT_SYMBOL_GPL(hid_bpf_destroy_device);
> >
> >  void hid_bpf_device_init(struct hid_device *hdev)
> >  {
> >         spin_lock_init(&hdev->bpf.progs_lock);
> > +       INIT_LIST_HEAD(&hdev->bpf.prog_list);
> > +       mutex_init(&hdev->bpf.prog_list_lock);
> >  }
> >  EXPORT_SYMBOL_GPL(hid_bpf_device_init);
> >
> > @@ -662,6 +693,13 @@ static int __init hid_bpf_init(void)
> >                 return 0;
> >         }
> >
> > +       /* register struct_ops kfuncs after we are sure we can load our preloaded bpf program */
> > +       err = register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS, &hid_bpf_kfunc_set);
> > +       if (err) {
> > +               pr_warn("error while setting HID BPF tracing kfuncs: %d", err);
> > +               return 0;
> > +       }
> > +
> >         /* register syscalls after we are sure we can load our preloaded bpf program */
> >         err = register_btf_kfunc_id_set(BPF_PROG_TYPE_SYSCALL, &hid_bpf_syscall_kfunc_set);
> >         if (err) {
> > diff --git a/drivers/hid/bpf/hid_bpf_dispatch.h b/drivers/hid/bpf/hid_bpf_dispatch.h
> > index fbe0639d09f2..e52c43d81650 100644
> > --- a/drivers/hid/bpf/hid_bpf_dispatch.h
> > +++ b/drivers/hid/bpf/hid_bpf_dispatch.h
> > @@ -10,12 +10,16 @@ struct hid_bpf_ctx_kern {
> >         u8 *data;
> >  };
> >
> > +struct hid_device *hid_get_device(unsigned int hid_id);
> > +void hid_put_device(struct hid_device *hid);
> > +int hid_bpf_allocate_event_data(struct hid_device *hdev);
> >  int hid_bpf_preload_skel(void);
> >  void hid_bpf_free_links_and_skel(void);
> >  int hid_bpf_get_prog_attach_type(struct bpf_prog *prog);
> >  int __hid_bpf_attach_prog(struct hid_device *hdev, enum hid_bpf_prog_type prog_type, int prog_fd,
> >                           struct bpf_prog *prog, __u32 flags);
> >  void __hid_bpf_destroy_device(struct hid_device *hdev);
> > +void __hid_bpf_ops_destroy_device(struct hid_device *hdev);
> >  int hid_bpf_prog_run(struct hid_device *hdev, enum hid_bpf_prog_type type,
> >                      struct hid_bpf_ctx_kern *ctx_kern);
> >  int hid_bpf_reconnect(struct hid_device *hdev);
> > diff --git a/drivers/hid/bpf/hid_bpf_jmp_table.c b/drivers/hid/bpf/hid_bpf_jmp_table.c
> > index aa8e1c79cdf5..8a54ba447718 100644
> > --- a/drivers/hid/bpf/hid_bpf_jmp_table.c
> > +++ b/drivers/hid/bpf/hid_bpf_jmp_table.c
> > @@ -81,6 +81,9 @@ static int hid_bpf_program_count(struct hid_device *hdev,
> >         if (type >= HID_BPF_PROG_TYPE_MAX)
> >                 return -EINVAL;
> >
> > +       if (type == HID_BPF_PROG_TYPE_RDESC_FIXUP && hdev->bpf.rdesc_ops)
> > +               n += 1;
> > +
> >         FOR_ENTRIES(i, jmp_table.tail, jmp_table.head) {
> >                 struct hid_bpf_prog_entry *entry = &jmp_table.entries[i];
> >
> > diff --git a/drivers/hid/bpf/hid_bpf_struct_ops.c b/drivers/hid/bpf/hid_bpf_struct_ops.c
> > new file mode 100644
> > index 000000000000..be514a98e55b
> > --- /dev/null
> > +++ b/drivers/hid/bpf/hid_bpf_struct_ops.c
> > @@ -0,0 +1,246 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +
> > +/*
> > + *  HID-BPF support for Linux
> > + *
> > + *  Copyright (c) 2024 Benjamin Tissoires
> > + */
> > +
> > +#include <linux/bitops.h>
> > +#include <linux/bpf_verifier.h>
> > +#include <linux/bpf.h>
> > +#include <linux/btf.h>
> > +#include <linux/btf_ids.h>
> > +#include <linux/filter.h>
> > +#include <linux/hid.h>
> > +#include <linux/hid_bpf.h>
> > +#include <linux/init.h>
> > +#include <linux/module.h>
> > +#include <linux/workqueue.h>
> > +#include "hid_bpf_dispatch.h"
> > +
> > +static struct btf *hid_bpf_ops_btf;
> > +
> > +static int hid_bpf_ops_init(struct btf *btf)
> > +{
> > +       hid_bpf_ops_btf = btf;
> > +       return 0;
> > +}
> > +
> > +static bool hid_bpf_ops_is_valid_access(int off, int size,
> > +                                         enum bpf_access_type type,
> > +                                         const struct bpf_prog *prog,
> > +                                         struct bpf_insn_access_aux *info)
> > +{
> > +       return bpf_tracing_btf_ctx_access(off, size, type, prog, info);
> > +}
> > +
> > +static int hid_bpf_ops_check_member(const struct btf_type *t,
> > +                                     const struct btf_member *member,
> > +                                     const struct bpf_prog *prog)
> > +{
> > +       u32 moff = __btf_member_bit_offset(t, member) / 8;
> > +
> 
> this is good...
> 
> > +       switch (moff) {
> > +       case offsetof(struct hid_bpf_ops, hid_rdesc_fixup):
> > +               break;
> > +       default:
> > +               if (prog->sleepable)
> > +                       return -EINVAL;
> > +       }
> > +
> > +       return 0;
> > +}
> > +
> > +static int hid_bpf_ops_btf_struct_access(struct bpf_verifier_log *log,
> > +                                          const struct bpf_reg_state *reg,
> > +                                          int off, int size)
> > +{
> > +       const struct btf_type *state;
> > +       const struct btf_type *t;
> > +       s32 type_id;
> > +
> > +       type_id = btf_find_by_name_kind(reg->btf, "hid_bpf_ctx",
> > +                                       BTF_KIND_STRUCT);
> > +       if (type_id < 0)
> > +               return -EINVAL;
> > +
> > +       t = btf_type_by_id(reg->btf, reg->btf_id);
> > +       state = btf_type_by_id(reg->btf, type_id);
> > +       if (t != state) {
> > +               bpf_log(log, "only access to hid_bpf_ctx is supported\n");
> > +               return -EACCES;
> > +       }
> > +
> > +       /* out-of-bound access in hid_bpf_ctx */
> > +       if (off + size > sizeof(struct hid_bpf_ctx)) {
> > +               bpf_log(log, "write access at off %d with size %d\n", off, size);
> > +               return -EACCES;
> > +       }
> > +
> > +       if (off < offsetof(struct hid_bpf_ctx, retval)) {
> > +               bpf_log(log,
> > +                       "write access at off %d with size %d on read-only part of hid_bpf_ctx\n",
> > +                       off, size);
> > +               return -EACCES;
> > +       }
> > +
> > +       return NOT_INIT;
> > +}
> > +
> > +static const struct bpf_verifier_ops hid_bpf_verifier_ops = {
> > +       .is_valid_access = hid_bpf_ops_is_valid_access,
> > +       .btf_struct_access = hid_bpf_ops_btf_struct_access,
> > +};
> > +
> > +static int hid_bpf_ops_init_member(const struct btf_type *t,
> > +                                const struct btf_member *member,
> > +                                void *kdata, const void *udata)
> > +{
> > +       u32 flags;
> > +
> > +       switch (member->offset) {
> > +       case offsetof(struct hid_bpf_ops, hid_id) * 8:
> 
> but here you're open coding it and adding ugly * 8
> Just do:
>         moff = __btf_member_bit_offset(t, member) / 8;
>         switch (moff) {
>         case offsetof(struct tcp_congestion_ops, flags):
> 
> the way bpf_tcp_ca.c is doing?
> Open code of ->offset is asking for trouble.

heh, I just monkey-copied tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c

Will change it :)

> 
> > +               /* For hid_id and flags fields, this function has to copy it
> > +                * and return 1 to indicate that the data has been handled by
> > +                * the struct_ops type, or the verifier will reject the map if
> > +                * the value of those fields is not zero.
> > +                */
> > +               ((struct hid_bpf_ops *)kdata)->hid_id = ((struct hid_bpf_ops *)udata)->hid_id;
> > +               return 1;
> > +       case offsetof(struct hid_bpf_ops, flags) * 8:
> > +               flags = ((struct hid_bpf_ops *)udata)->flags;
> > +               if (flags & ~HID_BPF_FLAG_MASK)
> > +                       return -EINVAL;
> > +               ((struct hid_bpf_ops *)kdata)->flags = flags;
> > +               return 1;
> > +       }
> > +       return 0;
> > +}
> > +
> > +static int hid_bpf_reg(void *kdata)
> > +{
> > +       struct hid_bpf_ops *ops = kdata;
> > +       struct hid_device *hdev;
> > +       int count, err = 0;
> > +
> > +       hdev = hid_get_device(ops->hid_id);
> > +       if (IS_ERR(hdev))
> > +               return PTR_ERR(hdev);
> > +
> > +       ops->hdev = hdev;
> > +
> > +       mutex_lock(&hdev->bpf.prog_list_lock);
> > +
> > +       count = list_count_nodes(&hdev->bpf.prog_list);
> > +       if (count >= HID_BPF_MAX_PROGS_PER_DEV) {
> > +               err = -E2BIG;
> > +               goto out_unlock;
> > +       }
> > +
> > +       if (ops->hid_rdesc_fixup) {
> > +               if (hdev->bpf.rdesc_ops) {
> > +                       err = -EINVAL;
> > +                       goto out_unlock;
> > +               }
> > +
> > +               hdev->bpf.rdesc_ops = ops;
> > +       }
> > +
> > +       if (ops->hid_device_event) {
> > +               err = hid_bpf_allocate_event_data(hdev);
> > +               if (err)
> > +                       goto out_unlock;
> > +       }
> > +
> > +       if (ops->flags & HID_BPF_FLAG_INSERT_HEAD)
> > +               list_add_rcu(&ops->list, &hdev->bpf.prog_list);
> > +       else
> > +               list_add_tail_rcu(&ops->list, &hdev->bpf.prog_list);
> 
> Looks like future proofing, but I feel it's too little to materialize.

again, the case already happens, so I need a list (or equivalent of) :)

> Take a look at include/linux/bpf_mprog.h
> I suspect it might be useful here too.

From the quick look of it this is interesting, but for plain struct
bpf_prog, right?

So I would have to adapt this include for struct_ops?

> 
> > +
> > +out_unlock:
> > +       mutex_unlock(&hdev->bpf.prog_list_lock);
> > +
> > +       if (err) {
> > +               if (hdev->bpf.rdesc_ops == ops)
> > +                       hdev->bpf.rdesc_ops = NULL;
> > +               hid_put_device(hdev);
> > +       } else if (ops->hid_rdesc_fixup) {
> > +               hid_bpf_reconnect(hdev);
> > +       }
> > +
> > +       return err;
> > +}
> > +
> > +static void hid_bpf_unreg(void *kdata)
> > +{
> > +       struct hid_bpf_ops *ops = kdata;
> > +       struct hid_device *hdev;
> > +       bool reconnect = false;
> > +
> > +       hdev = ops->hdev;
> > +
> > +       /* check if __hid_bpf_ops_destroy_device() has been called */
> > +       if (!hdev)
> > +               return;
> > +
> > +       mutex_lock(&hdev->bpf.prog_list_lock);
> > +
> > +       list_del_rcu(&ops->list);
> > +
> > +       reconnect = hdev->bpf.rdesc_ops == ops;
> > +       if (reconnect)
> > +               hdev->bpf.rdesc_ops = NULL;
> > +
> > +       mutex_unlock(&hdev->bpf.prog_list_lock);
> > +
> > +       if (reconnect)
> > +               hid_bpf_reconnect(hdev);
> > +
> > +       hid_put_device(hdev);
> > +}
> > +
> > +static int __hid_bpf_device_event(struct hid_bpf_ctx *ctx, enum hid_report_type type)
> > +{
> > +       return 0;
> > +}
> > +
> > +static int __hid_bpf_rdesc_fixup(struct hid_bpf_ctx *ctx)
> > +{
> > +       return 0;
> > +}
> > +
> > +static struct hid_bpf_ops __bpf_hid_bpf_ops = {
> > +       .hid_device_event = __hid_bpf_device_event,
> > +       .hid_rdesc_fixup = __hid_bpf_rdesc_fixup,
> > +};
> > +
> > +static struct bpf_struct_ops bpf_hid_bpf_ops = {
> > +       .verifier_ops = &hid_bpf_verifier_ops,
> > +       .init = hid_bpf_ops_init,
> > +       .check_member = hid_bpf_ops_check_member,
> > +       .init_member = hid_bpf_ops_init_member,
> > +       .reg = hid_bpf_reg,
> > +       .unreg = hid_bpf_unreg,
> > +       .name = "hid_bpf_ops",
> > +       .cfi_stubs = &__bpf_hid_bpf_ops,
> > +       .owner = THIS_MODULE,
> > +};
> > +
> > +void __hid_bpf_ops_destroy_device(struct hid_device *hdev)
> > +{
> > +       struct hid_bpf_ops *e;
> > +
> > +       rcu_read_lock();
> > +       list_for_each_entry_rcu(e, &hdev->bpf.prog_list, list) {
> > +               hid_put_device(hdev);
> > +               e->hdev = NULL;
> > +       }
> > +       rcu_read_unlock();
> > +}
> > +
> > +static int __init hid_bpf_struct_ops_init(void)
> > +{
> > +       return register_bpf_struct_ops(&bpf_hid_bpf_ops, hid_bpf_ops);
> > +}
> > +late_initcall(hid_bpf_struct_ops_init);
> > diff --git a/include/linux/hid_bpf.h b/include/linux/hid_bpf.h
> > index a66103618e6e..96495e977204 100644
> > --- a/include/linux/hid_bpf.h
> > +++ b/include/linux/hid_bpf.h
> > @@ -65,11 +65,12 @@ struct hid_bpf_ctx {
> >   * @HID_BPF_FLAG_INSERT_HEAD: insert the given program before any other program
> >   *                            currently attached to the device. This doesn't
> >   *                            guarantee that this program will always be first
> > - * @HID_BPF_FLAG_MAX: sentinel value, not to be used by the callers
> >   */
> >  enum hid_bpf_attach_flags {
> >         HID_BPF_FLAG_NONE = 0,
> >         HID_BPF_FLAG_INSERT_HEAD = _BITUL(0),
> > +
> > +       /* private: internal use only */
> >         HID_BPF_FLAG_MAX,
> >  };
> >
> > @@ -112,6 +113,63 @@ struct hid_ops {
> >
> >  extern struct hid_ops *hid_ops;
> >
> > +/**
> > + * struct hid_bpf_ops - A BPF struct_ops of callbacks allowing to attach HID-BPF
> > + *                     programs to a HID device
> > + * @hid_id: the HID uniq ID to attach to. This is writeable before ``load()``, and
> > + *         cannot be changed after
> > + * @flags: &enum hid_bpf_attach_flags to assign flags before ``load()``.
> > + *        Writeable only before ``load()``
> > + */
> > +struct hid_bpf_ops {
> > +       /* hid_id needs to stay first so we can easily change it
> > +        * from userspace.
> > +        */
> 
> hmm.
> patch 5 just does:
> +       skel->struct_ops.mouse_invert->hid_id = hid_id;
> 
> I don't see a reason why it needs to be first.

See the selftests (and my external loader, udev-hid-bpf):

		map = bpf_object__find_map_by_name(*self->skel->skeleton->obj,
							  programs[i].name + 4);
		ASSERT_OK_PTR(map) TH_LOG("can not find struct_ops by name '%s'",
					  programs[i].name + 4);

		/* hid_id is the first field of struct hid_bpf_ops */
		ops_hid_id = bpf_map__initial_value(map, NULL);
		ASSERT_OK_PTR(ops_hid_id) TH_LOG("unable to retrieve struct_ops data");

		*ops_hid_id = self->hid_id;

Basically, I want to have to deal with .bpf.o and not the skeletons for
the generic case. I want to be able to drop a file in /lib/firmware/hid
and have udev-hid-bpf load it without having to recompile udev-hid-bpf.

I couldn't seem to be able to access the struct_ops BTF information from
libbpf, so I decided to have hid_id and flags first and consider this to
be ABI while I figure out how to get to the real offsets :)

> 
> > +       int                     hid_id;
> > +       u32                     flags;
> > +
> > +       /* private: internal use only */
> > +       struct list_head        list;
> > +
> > +       /* public: rest is public */
> > +
> > +/* fast path fields are put first to fill one cache line */
> > +
> > +       /**
> > +        * @hid_device_event: called whenever an event is coming in from the device
> > +        *
> > +        * It has the following arguments:
> > +        *
> > +        * ``ctx``: The HID-BPF context as &struct hid_bpf_ctx
> > +        *
> > +        * Return: %0 on success and keep processing; a positive
> > +        * value to change the incoming size buffer; a negative
> > +        * error code to interrupt the processing of this event
> > +        *
> > +        * Context: Interrupt context.
> > +        */
> > +       int (*hid_device_event)(struct hid_bpf_ctx *ctx, enum hid_report_type report_type);
> > +
> > +/* control/slow paths put last */
> > +
> > +       /**
> > +        * @hid_rdesc_fixup: called when the probe function parses the report descriptor
> > +        * of the HID device
> > +        *
> > +        * It has the following arguments:
> > +        *
> > +        * ``ctx``: The HID-BPF context as &struct hid_bpf_ctx
> > +        *
> > +        * Return: %0 on success and keep processing; a positive
> > +        * value to change the incoming size buffer; a negative
> > +        * error code to interrupt the processing of this device
> > +        */
> > +       int (*hid_rdesc_fixup)(struct hid_bpf_ctx *ctx);
> 
> It's fine since you want to keep the prog mostly unchanged,
> but since the whole api is improving maybe it's time
> to do something about hid_bpf_ctx in all callbacks?
> Pass into prog what prog needs to see.
> In the old api all progs had to have a common hid_bpf_ctx.
> With struct-ops it can be fine tuned.

I agree, but I had a few cons:
- right now, I can recompile the same bpf.c with just a -D in the
  cflags. Both types of program are compatible, which allows to write
  the file once and recompile it for both pre/post struct_ops.
  This will be very useful while HID-BPF struct_ops is not in mainline
  distributions
- I need to have access to a few things: the struct hid_device, and the
  incoming buffer. And I don't think I can teach the verifier about the
  incoming buffer size without having to store it into a dedicated
  struct, which results into the same hid_bpf_ctx...
- I like how hid_bpf_allocate_context() works with the same struct that
  can be later used with the same functions hid_bpf_input_event() or
  such (arguably I could return the struct hid_device directly)

I'll try to give it more thoughts :)

> 
> Overall it looks great.
> hid_id did the trick :)

Thanks!

I agree, relying on struct_ops allowed to do a heavy cleanup in my code,
and TBH, even if I had only one callback, I think the gain is clear,
because it's much straightforward and you can also authorize some fine
grain write on your own structs (like changing the name of the
hid_device, or its flags, which is not part of this series, but which
will likely come later).

Cheers,
Benjamin