Subject: Re: [PATCH v3 bpf] libbpf: detect supported kernel BTF features and sanitize BTF

From: Stanislav Fomichev <[email-masked]>
Date: Fri, 10 May 2019 15:00:23 -0700

On 05/10, Andrii Nakryiko wrote:
> On Fri, May 10, 2019 at 2:36 PM Stanislav Fomichev <[email-masked]> wrote:
> >
> > On 05/10, Andrii Nakryiko wrote:
> > > Depending on used versions of libbpf, Clang, and kernel, it's possible to
> > > have valid BPF object files with valid BTF information, that still won't
> > > load successfully due to Clang emitting newer BTF features (e.g.,
> > > BTF_KIND_FUNC, .BTF.ext's line_info/func_info, BTF_KIND_DATASEC, etc), that
> > > are not yet supported by older kernel.
> > >
> > > This patch adds detection of BTF features and sanitizes BPF object's BTF
> > > by substituting various supported BTF kinds, which have compatible layout:
> > >   - BTF_KIND_FUNC -> BTF_KIND_TYPEDEF
> > >   - BTF_KIND_FUNC_PROTO -> BTF_KIND_ENUM
> > >   - BTF_KIND_VAR -> BTF_KIND_INT
> > >   - BTF_KIND_DATASEC -> BTF_KIND_STRUCT
> > >
> > > Replacement is done in such a way as to preserve as much information as
> > > possible (names, sizes, etc) where possible without violating kernel's
> > > validation rules.
> > >
> > > v2->v3:
> > >   - remove duplicate #defines from libbpf_util.h
> > >
> > > v1->v2:
> > >   - add internal libbpf_internal.h w/ common stuff
> > How is libbpf_internal.h different from libbpf_util.h? libbpf_util.h
> > looks pretty "internal" to me. Maybe use that one instead?
> 
> It's not anymore. It's included from xsk.h, which is not internal, so
> libbpf_util.h was recently exposed as public as well.
But I still don't see any LIBBPF_API exported functions in libbpf_util.h.
It looks like the usage is still mostly (only?) internal. The barrier
stuff is for internal usage as well.

Also, why do think your new probe helper should be internal? I guess
that at some point bpftool might use it to probe and dump BTF features
as well.

I also see us copying around all the BTF_XXX macros, I brought this up for
some selftest patches and now we have a single place for BTX_XXX macros
in selftests (tools/testing/selftests/test_bpf.h).
Maybe they should belong to libbpf instead?

> 
> >
> > >   - switch SK storage BTF to use new libbpf__probe_raw_btf()
> > >
> > > Reported-by: Alexei Starovoitov <[email-masked]>
> > > Signed-off-by: Andrii Nakryiko <[email-masked]>
> > > ---
> > >  tools/lib/bpf/libbpf.c          | 130 +++++++++++++++++++++++++++++++-
> > >  tools/lib/bpf/libbpf_internal.h |  27 +++++++
> > >  tools/lib/bpf/libbpf_probes.c   |  73 ++++++++++--------
> > >  3 files changed, 197 insertions(+), 33 deletions(-)
> > >  create mode 100644 tools/lib/bpf/libbpf_internal.h
> > >
> > > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > > index 11a65db4b93f..7e3b79d7c25f 100644
> > > --- a/tools/lib/bpf/libbpf.c
> > > +++ b/tools/lib/bpf/libbpf.c
> > > @@ -44,6 +44,7 @@
> > >  #include "btf.h"
> > >  #include "str_error.h"
> > >  #include "libbpf_util.h"
> > > +#include "libbpf_internal.h"
> > >
> > >  #ifndef EM_BPF
> > >  #define EM_BPF 247
> > > @@ -128,6 +129,10 @@ struct bpf_capabilities {
> > >       __u32 name:1;
> > >       /* v5.2: kernel support for global data sections. */
> > >       __u32 global_data:1;
> > > +     /* BTF_KIND_FUNC and BTF_KIND_FUNC_PROTO support */
> > > +     __u32 btf_func:1;
> > > +     /* BTF_KIND_VAR and BTF_KIND_DATASEC support */
> > > +     __u32 btf_datasec:1;
> > >  };
> > >
> > >  /*
> > > @@ -1021,6 +1026,74 @@ static bool section_have_execinstr(struct bpf_object *obj, int idx)
> > >       return false;
> > >  }
> > >
> > > +static void bpf_object__sanitize_btf(struct bpf_object *obj)
> > > +{
> > > +     bool has_datasec = obj->caps.btf_datasec;
> > > +     bool has_func = obj->caps.btf_func;
> > > +     struct btf *btf = obj->btf;
> > > +     struct btf_type *t;
> > > +     int i, j, vlen;
> > > +     __u16 kind;
> > > +
> > > +     if (!obj->btf || (has_func && has_datasec))
> > > +             return;
> > > +
> > > +     for (i = 1; i <= btf__get_nr_types(btf); i++) {
> > > +             t = (struct btf_type *)btf__type_by_id(btf, i);
> > > +             kind = BTF_INFO_KIND(t->info);
> > > +
> > > +             if (!has_datasec && kind == BTF_KIND_VAR) {
> > > +                     /* replace VAR with INT */
> > > +                     t->info = BTF_INFO_ENC(BTF_KIND_INT, 0, 0);
> > > +                     t->size = sizeof(int);
> > > +                     *(int *)(t+1) = BTF_INT_ENC(0, 0, 32);
> > > +             } else if (!has_datasec && kind == BTF_KIND_DATASEC) {
> > > +                     /* replace DATASEC with STRUCT */
> > > +                     struct btf_var_secinfo *v = (void *)(t + 1);
> > > +                     struct btf_member *m = (void *)(t + 1);
> > > +                     struct btf_type *vt;
> > > +                     char *name;
> > > +
> > > +                     name = (char *)btf__name_by_offset(btf, t->name_off);
> > > +                     while (*name) {
> > > +                             if (*name == '.')
> > > +                                     *name = '_';
> > > +                             name++;
> > > +                     }
> > > +
> > > +                     vlen = BTF_INFO_VLEN(t->info);
> > > +                     t->info = BTF_INFO_ENC(BTF_KIND_STRUCT, 0, vlen);
> > > +                     for (j = 0; j < vlen; j++, v++, m++) {
> > > +                             /* order of field assignments is important */
> > > +                             m->offset = v->offset * 8;
> > > +                             m->type = v->type;
> > > +                             /* preserve variable name as member name */
> > > +                             vt = (void *)btf__type_by_id(btf, v->type);
> > > +                             m->name_off = vt->name_off;
> > > +                     }
> > > +             } else if (!has_func && kind == BTF_KIND_FUNC_PROTO) {
> > > +                     /* replace FUNC_PROTO with ENUM */
> > > +                     vlen = BTF_INFO_VLEN(t->info);
> > > +                     t->info = BTF_INFO_ENC(BTF_KIND_ENUM, 0, vlen);
> > > +                     t->size = sizeof(__u32); /* kernel enforced */
> > > +             } else if (!has_func && kind == BTF_KIND_FUNC) {
> > > +                     /* replace FUNC with TYPEDEF */
> > > +                     t->info = BTF_INFO_ENC(BTF_KIND_TYPEDEF, 0, 0);
> > > +             }
> > > +     }
> > > +}
> > > +
> > > +static void bpf_object__sanitize_btf_ext(struct bpf_object *obj)
> > > +{
> > > +     if (!obj->btf_ext)
> > > +             return;
> > > +
> > > +     if (!obj->caps.btf_func) {
> > > +             btf_ext__free(obj->btf_ext);
> > > +             obj->btf_ext = NULL;
> > > +     }
> > > +}
> > > +
> > >  static int bpf_object__elf_collect(struct bpf_object *obj, int flags)
> > >  {
> > >       Elf *elf = obj->efile.elf;
> > > @@ -1164,8 +1237,10 @@ static int bpf_object__elf_collect(struct bpf_object *obj, int flags)
> > >                       obj->btf = NULL;
> > >               } else {
> > >                       err = btf__finalize_data(obj, obj->btf);
> > > -                     if (!err)
> > > +                     if (!err) {
> > > +                             bpf_object__sanitize_btf(obj);
> > >                               err = btf__load(obj->btf);
> > > +                     }
> > >                       if (err) {
> > >                               pr_warning("Error finalizing and loading %s into kernel: %d. Ignored and continue.\n",
> > >                                          BTF_ELF_SEC, err);
> > > @@ -1187,6 +1262,8 @@ static int bpf_object__elf_collect(struct bpf_object *obj, int flags)
> > >                                          BTF_EXT_ELF_SEC,
> > >                                          PTR_ERR(obj->btf_ext));
> > >                               obj->btf_ext = NULL;
> > > +                     } else {
> > > +                             bpf_object__sanitize_btf_ext(obj);
> > >                       }
> > >               }
> > >       }
> > > @@ -1556,12 +1633,63 @@ bpf_object__probe_global_data(struct bpf_object *obj)
> > >       return 0;
> > >  }
> > >
> > > +static int bpf_object__probe_btf_func(struct bpf_object *obj)
> > > +{
> > > +     const char strs[] = "\0int\0x\0a";
> > > +     /* void x(int a) {} */
> > > +     __u32 types[] = {
> > > +             /* int */
> > > +             BTF_TYPE_INT_ENC(1, BTF_INT_SIGNED, 0, 32, 4),  /* [1] */
> > > +             /* FUNC_PROTO */                                /* [2] */
> > > +             BTF_TYPE_ENC(0, BTF_INFO_ENC(BTF_KIND_FUNC_PROTO, 0, 1), 0),
> > > +             BTF_PARAM_ENC(7, 1),
> > > +             /* FUNC x */                                    /* [3] */
> > > +             BTF_TYPE_ENC(5, BTF_INFO_ENC(BTF_KIND_FUNC, 0, 0), 2),
> > > +     };
> > > +     int res;
> > > +
> > > +     res = libbpf__probe_raw_btf((char *)types, sizeof(types),
> > > +                                 strs, sizeof(strs));
> > > +     if (res < 0)
> > > +             return res;
> > > +     if (res > 0)
> > > +             obj->caps.btf_func = 1;
> > > +     return 0;
> > > +}
> > > +
> > > +static int bpf_object__probe_btf_datasec(struct bpf_object *obj)
> > > +{
> > > +     const char strs[] = "\0x\0.data";
> > > +     /* static int a; */
> > > +     __u32 types[] = {
> > > +             /* int */
> > > +             BTF_TYPE_INT_ENC(0, BTF_INT_SIGNED, 0, 32, 4),  /* [1] */
> > > +             /* VAR x */                                     /* [2] */
> > > +             BTF_TYPE_ENC(1, BTF_INFO_ENC(BTF_KIND_VAR, 0, 0), 1),
> > > +             BTF_VAR_STATIC,
> > > +             /* DATASEC val */                               /* [3] */
> > > +             BTF_TYPE_ENC(3, BTF_INFO_ENC(BTF_KIND_DATASEC, 0, 1), 4),
> > > +             BTF_VAR_SECINFO_ENC(2, 0, 4),
> > > +     };
> > > +     int res;
> > > +
> > > +     res = libbpf__probe_raw_btf((char *)types, sizeof(types),
> > > +                                 strs, sizeof(strs));
> > > +     if (res < 0)
> > > +             return res;
> > > +     if (res > 0)
> > > +             obj->caps.btf_datasec = 1;
> > > +     return 0;
> > > +}
> > > +
> > >  static int
> > >  bpf_object__probe_caps(struct bpf_object *obj)
> > >  {
> > >       int (*probe_fn[])(struct bpf_object *obj) = {
> > >               bpf_object__probe_name,
> > >               bpf_object__probe_global_data,
> > > +             bpf_object__probe_btf_func,
> > > +             bpf_object__probe_btf_datasec,
> > >       };
> > >       int i, ret;
> > >
> > > diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
> > > new file mode 100644
> > > index 000000000000..789e435b5900
> > > --- /dev/null
> > > +++ b/tools/lib/bpf/libbpf_internal.h
> > > @@ -0,0 +1,27 @@
> > > +/* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
> > > +
> > > +/*
> > > + * Internal libbpf helpers.
> > > + *
> > > + * Copyright (c) 2019 Facebook
> > > + */
> > > +
> > > +#ifndef __LIBBPF_LIBBPF_INTERNAL_H
> > > +#define __LIBBPF_LIBBPF_INTERNAL_H
> > > +
> > > +#define BTF_INFO_ENC(kind, kind_flag, vlen) \
> > > +     ((!!(kind_flag) << 31) | ((kind) << 24) | ((vlen) & BTF_MAX_VLEN))
> > > +#define BTF_TYPE_ENC(name, info, size_or_type) (name), (info), (size_or_type)
> > > +#define BTF_INT_ENC(encoding, bits_offset, nr_bits) \
> > > +     ((encoding) << 24 | (bits_offset) << 16 | (nr_bits))
> > > +#define BTF_TYPE_INT_ENC(name, encoding, bits_offset, bits, sz) \
> > > +     BTF_TYPE_ENC(name, BTF_INFO_ENC(BTF_KIND_INT, 0, 0), sz), \
> > > +     BTF_INT_ENC(encoding, bits_offset, bits)
> > > +#define BTF_MEMBER_ENC(name, type, bits_offset) (name), (type), (bits_offset)
> > > +#define BTF_PARAM_ENC(name, type) (name), (type)
> > > +#define BTF_VAR_SECINFO_ENC(type, offset, size) (type), (offset), (size)
> > > +
> > > +int libbpf__probe_raw_btf(const char *raw_types, size_t types_len,
> > > +                       const char *str_sec, size_t str_len);
> > > +
> > > +#endif /* __LIBBPF_LIBBPF_INTERNAL_H */
> > > diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c
> > > index a2c64a9ce1a6..5e2aa83f637a 100644
> > > --- a/tools/lib/bpf/libbpf_probes.c
> > > +++ b/tools/lib/bpf/libbpf_probes.c
> > > @@ -15,6 +15,7 @@
> > >
> > >  #include "bpf.h"
> > >  #include "libbpf.h"
> > > +#include "libbpf_internal.h"
> > >
> > >  static bool grep(const char *buffer, const char *pattern)
> > >  {
> > > @@ -132,21 +133,43 @@ bool bpf_probe_prog_type(enum bpf_prog_type prog_type, __u32 ifindex)
> > >       return errno != EINVAL && errno != EOPNOTSUPP;
> > >  }
> > >
> > > -static int load_btf(void)
> > > +int libbpf__probe_raw_btf(const char *raw_types, size_t types_len,
> > > +                       const char *str_sec, size_t str_len)
> > >  {
> > > -#define BTF_INFO_ENC(kind, kind_flag, vlen) \
> > > -     ((!!(kind_flag) << 31) | ((kind) << 24) | ((vlen) & BTF_MAX_VLEN))
> > > -#define BTF_TYPE_ENC(name, info, size_or_type) \
> > > -     (name), (info), (size_or_type)
> > > -#define BTF_INT_ENC(encoding, bits_offset, nr_bits) \
> > > -     ((encoding) << 24 | (bits_offset) << 16 | (nr_bits))
> > > -#define BTF_TYPE_INT_ENC(name, encoding, bits_offset, bits, sz) \
> > > -     BTF_TYPE_ENC(name, BTF_INFO_ENC(BTF_KIND_INT, 0, 0), sz), \
> > > -     BTF_INT_ENC(encoding, bits_offset, bits)
> > > -#define BTF_MEMBER_ENC(name, type, bits_offset) \
> > > -     (name), (type), (bits_offset)
> > > -
> > > -     const char btf_str_sec[] = "\0bpf_spin_lock\0val\0cnt\0l";
> > > +     struct btf_header hdr = {
> > > +             .magic = BTF_MAGIC,
> > > +             .version = BTF_VERSION,
> > > +             .hdr_len = sizeof(struct btf_header),
> > > +             .type_len = types_len,
> > > +             .str_off = types_len,
> > > +             .str_len = str_len,
> > > +     };
> > > +     int btf_fd, btf_len;
> > > +     __u8 *raw_btf;
> > > +
> > > +     btf_len = hdr.hdr_len + hdr.type_len + hdr.str_len;
> > > +     raw_btf = malloc(btf_len);
> > > +     if (!raw_btf)
> > > +             return -ENOMEM;
> > > +
> > > +     memcpy(raw_btf, &hdr, sizeof(hdr));
> > > +     memcpy(raw_btf + hdr.hdr_len, raw_types, hdr.type_len);
> > > +     memcpy(raw_btf + hdr.hdr_len + hdr.type_len, str_sec, hdr.str_len);
> > > +
> > > +     btf_fd = bpf_load_btf(raw_btf, btf_len, NULL, 0, false);
> > > +     if (btf_fd < 0) {
> > > +             free(raw_btf);
> > > +             return 0;
> > > +     }
> > > +
> > > +     close(btf_fd);
> > > +     free(raw_btf);
> > > +     return 1;
> > > +}
> > > +
> > > +static int load_sk_storage_btf(void)
> > > +{
> > > +     const char strs[] = "\0bpf_spin_lock\0val\0cnt\0l";
> > >       /* struct bpf_spin_lock {
> > >        *   int val;
> > >        * };
> > > @@ -155,7 +178,7 @@ static int load_btf(void)
> > >        *   struct bpf_spin_lock l;
> > >        * };
> > >        */
> > > -     __u32 btf_raw_types[] = {
> > > +     __u32 types[] = {
> > >               /* int */
> > >               BTF_TYPE_INT_ENC(0, BTF_INT_SIGNED, 0, 32, 4),  /* [1] */
> > >               /* struct bpf_spin_lock */                      /* [2] */
> > > @@ -166,23 +189,9 @@ static int load_btf(void)
> > >               BTF_MEMBER_ENC(19, 1, 0), /* int cnt; */
> > >               BTF_MEMBER_ENC(23, 2, 32),/* struct bpf_spin_lock l; */
> > >       };
> > > -     struct btf_header btf_hdr = {
> > > -             .magic = BTF_MAGIC,
> > > -             .version = BTF_VERSION,
> > > -             .hdr_len = sizeof(struct btf_header),
> > > -             .type_len = sizeof(btf_raw_types),
> > > -             .str_off = sizeof(btf_raw_types),
> > > -             .str_len = sizeof(btf_str_sec),
> > > -     };
> > > -     __u8 raw_btf[sizeof(struct btf_header) + sizeof(btf_raw_types) +
> > > -                  sizeof(btf_str_sec)];
> > > -
> > > -     memcpy(raw_btf, &btf_hdr, sizeof(btf_hdr));
> > > -     memcpy(raw_btf + sizeof(btf_hdr), btf_raw_types, sizeof(btf_raw_types));
> > > -     memcpy(raw_btf + sizeof(btf_hdr) + sizeof(btf_raw_types),
> > > -            btf_str_sec, sizeof(btf_str_sec));
> > >
> > > -     return bpf_load_btf(raw_btf, sizeof(raw_btf), 0, 0, 0);
> > > +     return libbpf__probe_raw_btf((char *)types, sizeof(types),
> > > +                                  strs, sizeof(strs));
> > >  }
> > >
> > >  bool bpf_probe_map_type(enum bpf_map_type map_type, __u32 ifindex)
> > > @@ -222,7 +231,7 @@ bool bpf_probe_map_type(enum bpf_map_type map_type, __u32 ifindex)
> > >               value_size = 8;
> > >               max_entries = 0;
> > >               map_flags = BPF_F_NO_PREALLOC;
> > > -             btf_fd = load_btf();
> > > +             btf_fd = load_sk_storage_btf();
> > >               if (btf_fd < 0)
> > >                       return false;
> > >               break;
> > > --
> > > 2.17.1
> > >