SchOS/ldls/btf.c

// SPDX-License-Identifier: Apache-2.0
//
// Reading BTF. See ldls/btf.h.
//
// Implemented from Documentation/bpf/btf.rst. The format is small enough to
// hold in your head: a header, a run of variable-length type records, and a
// string table. The only fiddly part is that a record's length depends on its
// kind, so iterating means knowing how much trails each one -- which is the
// table in trailing_bytes() below and nothing more.

#include "btf.h"

static void
btf_fail(struct btf *b, const char *msg, const char *what)
{
  int i = 0, j = 0;
  while (msg[i] && i < (int)sizeof(b->err) - 1) { b->err[i] = msg[i]; i++; }
  if (what)
    while (what[j] && i < (int)sizeof(b->err) - 1) b->err[i++] = what[j++];
  b->err[i] = 0;
}

static int
btf_streq(const char *a, const char *c)
{
  while (*a && *a == *c) { a++; c++; }
  return *a == 0 && *c == 0;
}

const char *btf_err(const struct btf *b) { return b->err[0] ? b->err : "no error"; }

// How many bytes follow a type record, given its kind and vlen.
static unsigned int
trailing_bytes(unsigned int info)
{
  unsigned int kind = BTF_INFO_KIND(info);
  unsigned int vlen = BTF_INFO_VLEN(info);

  switch (kind) {
  case BTF_KIND_INT:        return 4;
  case BTF_KIND_ARRAY:      return 12;          /* type, index_type, nelems */
  case BTF_KIND_STRUCT:
  case BTF_KIND_UNION:      return vlen * 12;   /* struct btf_member */
  case BTF_KIND_ENUM:       return vlen * 8;
  case BTF_KIND_ENUM64:     return vlen * 12;
  case BTF_KIND_FUNC_PROTO: return vlen * 8;    /* struct btf_param */
  case BTF_KIND_VAR:        return 4;
  case BTF_KIND_DATASEC:    return vlen * 12;
  case BTF_KIND_DECL_TAG:   return 4;
  default:                  return 0;           /* PTR, TYPEDEF, CONST, ... */
  }
}

long
btf_count(const void *image, unsigned long long len)
{
  const struct btf_header *h = (const struct btf_header *)image;
  const unsigned char *types;
  unsigned int at = 0;
  long n = 0;

  if (len < sizeof(struct btf_header))
    return -1;
  if (h->magic != BTF_MAGIC || h->version != 1)
    return -1;
  if ((unsigned long long)h->hdr_len + h->type_off + h->type_len > len)
    return -1;

  types = (const unsigned char *)image + h->hdr_len + h->type_off;

  while (at + sizeof(struct btf_type) <= h->type_len) {
    const struct btf_type *t = (const struct btf_type *)(types + at);
    unsigned int step = sizeof(struct btf_type) + trailing_bytes(t->info);
    if (step == 0 || at + step > h->type_len)
      break;
    at += step;
    n++;
    if (n > BTF_MAXTYPES)
      return -1;
  }
  return n;
}

int
btf_open(struct btf *b, const void *image, unsigned long long len,
         unsigned int *off, unsigned int noff)
{
  const struct btf_header *h = (const struct btf_header *)image;
  unsigned int at = 0, id = 1;
  int i;

  for (i = 0; i < (int)sizeof(b->err); i++)
    b->err[i] = 0;
  b->img = (const unsigned char *)image;
  b->len = len;
  b->off = off;
  b->ntypes = 0;

  if (len < sizeof(struct btf_header))
    { btf_fail(b, "too short to be BTF", 0); return -1; }
  if (h->magic != BTF_MAGIC)
    { btf_fail(b, "not BTF: wrong magic", 0); return -1; }
  if (h->version != 1)
    { btf_fail(b, "BTF version is not 1", 0); return -1; }
  if ((unsigned long long)h->hdr_len + h->type_off + h->type_len > len ||
      (unsigned long long)h->hdr_len + h->str_off + h->str_len > len)
    { btf_fail(b, "BTF sections run off the end of the file", 0); return -1; }

  b->h         = h;
  b->types     = b->img + h->hdr_len + h->type_off;
  b->types_len = h->type_len;
  b->str       = (const char *)(b->img + h->hdr_len + h->str_off);
  b->str_len   = h->str_len;

  // Index every type. Ids are assigned in order starting at 1; id 0 is void
  // and is not stored.
  while (at + sizeof(struct btf_type) <= b->types_len) {
    const struct btf_type *t = (const struct btf_type *)(b->types + at);
    unsigned int step = sizeof(struct btf_type) + trailing_bytes(t->info);

    if (at + step > b->types_len)
      break;
    if (id >= noff)
      { btf_fail(b, "the caller's type index is too small", 0); return -1; }
    off[id++] = at;
    at += step;
  }
  b->ntypes = id - 1;
  return 0;
}

static const struct btf_type *
btf_at(struct btf *b, int id)
{
  if (id <= 0 || (unsigned int)id > b->ntypes)
    return 0;
  return (const struct btf_type *)(b->types + b->off[id]);
}

const char *
btf_name_of(struct btf *b, int id)
{
  const struct btf_type *t = btf_at(b, id);
  if (!t || t->name_off >= b->str_len)
    return "";
  return b->str + t->name_off;
}

int
btf_kind_of(struct btf *b, int id)
{
  const struct btf_type *t = btf_at(b, id);
  return t ? (int)BTF_INFO_KIND(t->info) : -1;
}

int
btf_find(struct btf *b, const char *name, int kind)
{
  unsigned int id;

  for (id = 1; id <= b->ntypes; id++) {
    const struct btf_type *t = (const struct btf_type *)(b->types + b->off[id]);
    if ((int)BTF_INFO_KIND(t->info) != kind)
      continue;
    if (t->name_off >= b->str_len)
      continue;
    if (btf_streq(b->str + t->name_off, name))
      return (int)id;
  }
  return 0;
}

// The value of a named enumerator, wherever it lives.
//
// Not every constant a driver depends on is a struct offset. `netif_carrier_ok`
// is an inline that tests bit `__LINK_STATE_NOCARRIER` of `net_device.state`,
// so a host that wants to say "the link is down" has to set the right bit --
// and getting it wrong means a driver that believes the carrier is already up
// and skips everything it would have done on link-up. e1000 skips enabling its
// transmitter, which is not an error anywhere.
//
// The enum is anonymous, so there is no type to look up by name; the search is
// over enumerators rather than over types. Which is the same rule 3½ bargain as
// the offsets: the number is the installed kernel's, not ours.
//
// Returns 0 and sets *val on success, -1 if no enumerator has that name.
int
btf_enum_value(struct btf *b, const char *name, long long *val)
{
  unsigned int id;

  for (id = 1; id <= b->ntypes; id++) {
    const struct btf_type *t = (const struct btf_type *)(b->types + b->off[id]);
    int kind = (int)BTF_INFO_KIND(t->info);
    unsigned int n = BTF_INFO_VLEN(t->info), i;

    if (kind == BTF_KIND_ENUM) {
      const unsigned int *e = (const unsigned int *)(t + 1);
      for (i = 0; i < n; i++) {
        unsigned int noff = e[i * 2];
        if (noff < b->str_len && btf_streq(b->str + noff, name)) {
          *val = (int)e[i * 2 + 1];
          return 0;
        }
      }
    } else if (kind == BTF_KIND_ENUM64) {
      const unsigned int *e = (const unsigned int *)(t + 1);
      for (i = 0; i < n; i++) {
        unsigned int noff = e[i * 3];
        if (noff < b->str_len && btf_streq(b->str + noff, name)) {
          *val = (long long)e[i * 3 + 1] |
                 ((long long)e[i * 3 + 2] << 32);
          return 0;
        }
      }
    }
  }
  btf_fail(b, "no enumerator named ", name);
  return -1;
}

// Follow the things that wrap a type without changing its layout.
static int
btf_strip(struct btf *b, int id)
{
  int guard = 0;

  while (id > 0 && guard++ < 32) {
    const struct btf_type *t = btf_at(b, id);
    if (!t)
      return 0;
    switch (BTF_INFO_KIND(t->info)) {
    case BTF_KIND_TYPEDEF:
    case BTF_KIND_CONST:
    case BTF_KIND_VOLATILE:
    case BTF_KIND_RESTRICT:
    case BTF_KIND_TYPE_TAG:
      id = (int)t->u.type;
      continue;
    default:
      return id;
    }
  }
  return 0;
}

unsigned int
btf_type_size(struct btf *b, int id)
{
  const struct btf_type *t;

  id = btf_strip(b, id);
  t  = btf_at(b, id);
  if (!t)
    return 0;

  switch (BTF_INFO_KIND(t->info)) {
  case BTF_KIND_INT:
  case BTF_KIND_STRUCT:
  case BTF_KIND_UNION:
  case BTF_KIND_ENUM:
  case BTF_KIND_ENUM64:
  case BTF_KIND_FLOAT:
    return t->u.size;
  case BTF_KIND_PTR:
    return 8;                    // x86-64; this reader is built for one ABI
  case BTF_KIND_ARRAY: {
    const unsigned int *a = (const unsigned int *)(t + 1);
    return btf_type_size(b, (int)a[0]) * a[2];
  }
  default:
    return 0;
  }
}

int
btf_member_off(struct btf *b, int struct_id, const char *member,
               unsigned int *off_bytes, unsigned int *size_bytes)
{
  const struct btf_type *t;
  const struct btf_member *m;
  unsigned int i, vlen, kflag;

  struct_id = btf_strip(b, struct_id);
  t = btf_at(b, struct_id);
  if (!t)
    { btf_fail(b, "no such type", 0); return -1; }
  if (BTF_INFO_KIND(t->info) != BTF_KIND_STRUCT &&
      BTF_INFO_KIND(t->info) != BTF_KIND_UNION)
    { btf_fail(b, "not a struct or union", 0); return -1; }

  vlen  = BTF_INFO_VLEN(t->info);
  kflag = BTF_INFO_KFLAG(t->info);
  m     = (const struct btf_member *)(t + 1);

  for (i = 0; i < vlen; i++, m++) {
    const char *nm = (m->name_off < b->str_len) ? b->str + m->name_off : "";
    unsigned int bit;

    // An unnamed member is an anonymous struct or union, and what looks like
    // a field of the outer type is really a field of that. Recurse, adding
    // its own offset -- without this, every kernel struct with an anonymous
    // union in it reports its later fields as missing.
    if (nm[0] == 0) {
      unsigned int sub_off, sub_size;
      unsigned int here = kflag ? (m->offset & 0xffffff) : m->offset;
      if (btf_member_off(b, (int)m->type, member, &sub_off, &sub_size) == 0) {
        *off_bytes  = here / 8 + sub_off;
        *size_bytes = sub_size;
        return 0;
      }
      continue;
    }

    if (!btf_streq(nm, member))
      continue;

    if (kflag && (m->offset >> 24) != 0)
      { btf_fail(b, "that member is a bitfield: ", member); return -1; }

    bit = kflag ? (m->offset & 0xffffff) : m->offset;
    if (bit % 8)
      { btf_fail(b, "that member is not byte-aligned: ", member); return -1; }

    *off_bytes  = bit / 8;
    *size_bytes = btf_type_size(b, (int)m->type);
    return 0;
  }

  btf_fail(b, "no such member: ", member);
  return -1;
}