SchOS/ldls/btf.h

// SPDX-License-Identifier: Apache-2.0
//
// Reading BTF: struct layouts and function prototypes, from the kernel that
// the driver was built for.
//
// docs/architecture/ldls.md, rule 3½ -- the thing that makes rule 3 comfortable.
//
// The shim has to agree with a compiled driver about where `pci_driver.probe`
// sits, and a compiled driver is not negotiable about it. Writing those
// offsets into our source would mean a field-for-field match with Linux's
// headers, which *looks* like transcription however it was arrived at.
//
// It does not have to be in our source. A kernel built with
// CONFIG_DEBUG_INFO_BTF publishes `/sys/kernel/btf/vmlinux`: every struct
// layout and every function prototype, as generated binary type data
// describing the kernel that is installed. LDLS reads it at load time, on the
// user's machine, from the user's own kernel.
//
// Two things follow. The offsets are never ours to have copied, and they are
// right for the kernel actually present rather than one we pinned by guessing
// -- which is most of what rule 4 was worried about.
//
// The format is described in Documentation/bpf/btf.rst, which is a published
// specification; implementing it from that is the case ../docs/licensing.md §5
// covers. Freestanding, like the rest of ldls/.

// --- the format -------------------------------------------------------------

#define BTF_MAGIC 0xeB9F

struct btf_header {
  unsigned short magic;
  unsigned char  version;
  unsigned char  flags;
  unsigned int   hdr_len;
  // Both offsets are from the end of the header, not from the start of it.
  unsigned int   type_off;
  unsigned int   type_len;
  unsigned int   str_off;
  unsigned int   str_len;
};

struct btf_type {
  unsigned int name_off;
  // vlen in bits 0-15, kind in bits 24-28, kind_flag in bit 31.
  unsigned int info;
  union {
    unsigned int size;          // STRUCT, UNION, ENUM, INT, DATASEC, FLOAT
    unsigned int type;          // PTR, TYPEDEF, CONST, FUNC, VAR, ...
  } u;
};

struct btf_member {
  unsigned int name_off;
  unsigned int type;
  // With kind_flag set this is a bitfield: size in bits 24-31, bit offset in
  // bits 0-23. Without it, a plain bit offset.
  unsigned int offset;
};

struct btf_param {
  unsigned int name_off;
  unsigned int type;
};

#define BTF_KIND_INT        1
#define BTF_KIND_PTR        2
#define BTF_KIND_ARRAY      3
#define BTF_KIND_STRUCT     4
#define BTF_KIND_UNION      5
#define BTF_KIND_ENUM       6
#define BTF_KIND_FWD        7
#define BTF_KIND_TYPEDEF    8
#define BTF_KIND_VOLATILE   9
#define BTF_KIND_CONST     10
#define BTF_KIND_RESTRICT  11
#define BTF_KIND_FUNC      12
#define BTF_KIND_FUNC_PROTO 13
#define BTF_KIND_VAR       14
#define BTF_KIND_DATASEC   15
#define BTF_KIND_FLOAT     16
#define BTF_KIND_DECL_TAG  17
#define BTF_KIND_TYPE_TAG  18
#define BTF_KIND_ENUM64    19

#define BTF_INFO_KIND(i)  (((i) >> 24) & 0x1f)
#define BTF_INFO_VLEN(i)  ((i) & 0xffff)
#define BTF_INFO_KFLAG(i) (((i) >> 31) & 1)

// --- the reader ---------------------------------------------------------------

#define BTF_MAXTYPES 262144     // type ids we will index; vmlinux is ~150k

struct btf {
  const unsigned char *img;
  unsigned long long   len;
  const struct btf_header *h;
  const unsigned char *types;   // start of the type section
  unsigned int         types_len;
  const char          *str;
  unsigned int         str_len;

  // Type id -> byte offset into the type section. Id 0 is void and has no
  // entry, so this is 1-based and index 0 is unused.
  unsigned int *off;            // the caller's, ntypes + 1 entries
  unsigned int  ntypes;

  char err[128];
};

// How many type ids this BTF holds, so the caller can size the index. -1 if
// the image is not BTF at all.
long btf_count(const void *image, unsigned long long len);

// Index it. `off` must have btf_count()+1 entries.
int btf_open(struct btf *b, const void *image, unsigned long long len,
             unsigned int *off, unsigned int noff);

// The type id of a named type of a given kind, or 0.
int btf_find(struct btf *b, const char *name, int kind);

// Byte offset and size of a named member of a struct or union.
//
// Returns 0 on success. Bitfields are refused rather than rounded: a driver
// that reads one needs the bit position, and a byte offset would be a
// plausible-looking wrong answer.
int btf_member_off(struct btf *b, int struct_id, const char *member,
                   unsigned int *off_bytes, unsigned int *size_bytes);

// Size in bytes of a type, following typedefs and qualifiers. 0 if unknown.
unsigned int btf_type_size(struct btf *b, int id);

const char *btf_name_of(struct btf *b, int id);
int         btf_kind_of(struct btf *b, int id);
const char *btf_err(const struct btf *b);