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6.60.12 BPF Built-in Functions

The following built-in functions are available for eBPF targets.

Built-in Function: unsigned long long __builtin_bpf_load_byte (unsigned long long offset)

Load a byte from the struct sk_buff packet data pointed by the register %r6 and return it.

Built-in Function: unsigned long long __builtin_bpf_load_half (unsigned long long offset)

Load 16-bits from the struct sk_buff packet data pointed by the register %r6 and return it.

Built-in Function: unsigned long long __builtin_bpf_load_word (unsigned long long offset)

Load 32-bits from the struct sk_buff packet data pointed by the register %r6 and return it.

Built-in Function: void * __builtin_preserve_access_index (expr)

BPF Compile Once-Run Everywhere (CO-RE) support. Instruct GCC to generate CO-RE relocation records for any accesses to aggregate data structures (struct, union, array types) in expr. This builtin is otherwise transparent, the return value is whatever expr evaluates to. It is also overloaded: expr may be of any type (not necessarily a pointer), the return type is the same. Has no effect if -mco-re is not in effect (either specified or implied).

Built-in Function: unsigned int __builtin_preserve_field_info (expr, unsigned int kind)

BPF Compile Once-Run Everywhere (CO-RE) support. This builtin is used to extract information to aid in struct/union relocations. expr is an access to a field of a struct or union. Depending on kind, different information is returned to the program. A CO-RE relocation for the access in expr with kind kind is recorded if -mco-re is in effect.

The following values are supported for kind:

FIELD_BYTE_OFFSET = 0

The returned value is the offset, in bytes, of the field from the beginning of the containing structure. For bitfields, the byte offset of the containing word.

FIELD_BYTE_SIZE = 1

The returned value is the size, in bytes, of the field. For bitfields, the size in bytes of the containing word.

FIELD_EXISTENCE = 2

The returned value is 1 if the field exists, 0 otherwise. Always 1 at compile time.

FIELD_SIGNEDNESS = 3

The returned value is 1 if the field is signed, 0 otherwise.

FIELD_LSHIFT_U64 = 4
FIELD_RSHIFT_U64 = 5

The returned value is the number of bits of left- or right-shifting respectively needed in order to recover the original value of the field, after it has been loaded by a read of FIELD_BYTE_SIZE bytes into an unsigned 64-bit value. Primarily useful for reading bitfield values from structures which may change between kernel versions.

Note that the return value is a constant which is known at compile-time. If the field has a variable offset then FIELD_BYTE_OFFSET, FIELD_LSHIFT_U64 and FIELD_RSHIFT_U64 are not supported. Similarly, if the field has a variable size then FIELD_BYTE_SIZE, FIELD_LSHIFT_U64 and FIELD_RSHIFT_U64 are not supported.

For example, __builtin_preserve_field_info can be used to reliably extract bitfield values from a structure which may change between kernel versions:

struct S
{
  short a;
  int x:7;
  int y:5;
};

int
read_y (struct S *arg)
{
  unsigned long long val;
  unsigned int offset = __builtin_preserve_field_info (arg->y, FIELD_BYTE_OFFSET);
  unsigned int size = __builtin_presrve_field_info (arg->y, FIELD_BYTE_SIZE);

  /* Read size bytes from arg + offset into val.  */
  bpf_probe_read (&val, size, arg + offset);

  val <<= __builtin_preserve_field_info (arg->y, FIELD_LSHIFT_U64);

  if (__builtin_preserve_field_info (arg->y, FIELD_SIGNEDNESS))
    val = ((long long) val >> __builtin_preserve_field_info (arg->y, FIELD_RSHIFT_U64));
  else
    val >>= __builtin_preserve_field_info (arg->y, FIELD_RSHIFT_U64);

  return val;
}


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