Integral structs in GNU poke
[20-07-2020]

by Jose E. Marchesi

This weekend I finally implemented support for the so-called "integral structs" in poke. This expands the expressivity power of Poke structs to cover cases where data is stored in composited integral containers, i.e. where data is structured within stored integers.

Basically, when we structure data using Poke structs, arrays and the like, we often use the same structure than a C programmer would use. For example, to model ELF RELA structures, which are defined in C like:

typedef struct
{
  Elf64_Addr   r_offset;  /* Address */
  Elf64_Xword  r_info;    /* Relocation type and symbol index */
  Elf64_Sxword r_addend;  /* Addend */
} Elf64_Rela;
    

we could use something like this in Poke:

deftype Elf64_Rela =
  struct
  {
    Elf64_Addr r_offset;
    Elf64_Xword r_info;
    Elf64_Sxword r_addend;
  };
    

Here the Poke struct type is pretty equivalent to the C incarnation. In both cases the fields are always stored in the given order, regardless of endianness or any other consideration.

However, there are situations where stored integral values are to be interpreted as composite data. This is the case of the r_info field above, which is a 64-bit unsigned integer (Elf64_Xword) which is itself composed by several fields, depicted here:

 63                                          0
+----------------------+----------------------+
|       r_sym          |      r_type          |
+----------------------+----------------------+
MSB                                         LSB
    

In order to support this kind of composition of integers, C programmers usually resort to either bit masking (most often) or to the often obscure and undefined behaviour-prone C bit fields. In the case of ELF, the GNU implementations define a few macros to access these "sub-fields":

#define ELF64_R_SYM(i)         ((i) >> 32)
#define ELF64_R_TYPE(i)        ((i) & 0xffffffff)
#define ELF64_R_INFO(sym,type) ((((Elf64_Xword) (sym)) << 32) + (type))
    

Where ELF64_R_SYM and ELF64_R_TYPE are used to extract the fields from an r_info, and ELF64_R_INFO is used to compose it. This is typical of C data structures.

We could of course mimic the C implementation in Poke:

defun Elf64_R_Sym = (Elf64_Xword i) uint<32>:
   { return i .>> 32; }
defun Elf64_R_Type = (Elf64_Xword i) uint<32>:
   { return i & 0xffff_ffff; }
defun Elf64_R_Info = (uint<32> sym, uint<32> type) Elf64_Xword:
   { return sym as Elf64_Xword <<. 32 + type; }
     

However, this approach has a huge disadvantage: since we are not able to encode the logic of these "sub-fields" in proper Poke fields, they become second class citizens, with all that implies: no constraints on their own, can't be auto-completed, can't be assigned individually, etc etc.

But starting today we can use "integral structs"! These are structs that are defined exactly like your garden variety Poke structs, with a small addition:

deftype Elf64_RelInfo =
  struct uint<64>
  {
    uint<32> r_sym;
    uint<32> r_type;
  };
    

Note the uint<64> addition after `struct'. This can be any integer type (signed or unsigned). The fields of an integral struct should be integral themselves (this includes both integers and offsets) and the total size occupied by the fields should be the same size than the one declared in the struct's integer type. This is checked and enforced by the compiler.

The Elf64 RELA in Poke can then be encoded like:

deftype Elf64_Rela =
  struct
  {
    Elf64_Addr r_offset;
    struct Elf64_Xword
    {
      uint<32> r_sym;
      uint<32> r_type;
    } r_info;
    Elf64_Sxword r_addend;
  };
    

When an integral struct is mapped from some IO space, the total number of bytes occupied by the struct is read as a single integer value, and then the values of the fields are extracted from it. A similar process is using when writing. That is what makes it different with respect a normal Poke struct.

Consider for example we have the following sequence of bytes in our IO space (like a file):

0x10 0x20 0x30 0x40  0x50 0x60 0x70 0x80

Let's see what happens when we map the integral struct above, in both big and little endian:

(poke) .set endian big
(poke) Elf64_RelInfo @ 0#B
Elf64_RelInfo {
  r_sym=0x10203040U,
  r_type=0x50607080U
}
(poke) .set endian little (poke) Elf64_RelInfo @ 0#B Elf64_RelInfo {
  r_sym=0x80706050U,
  r_type=0x40302010U
}

For comparison, this is what happens when we do the same with an "equivalent" (not really) non-integral struct operating on the same data:

deftype Elf64_RelInfoBogus =
  struct
  {
    uint<32> r_sym;
    uint<32> r_type;
  };

We would get:

(poke) .set endian big
(poke) Elf64_RelInfoBogus @ 0#B
Elf64_RelInfoBogus {
  r_sym=0x10203040U,
  r_type=0x50607080U
}
(poke) .set endian little (poke) Elf64_RelInfoBogus @ 0#B Elf64_RelInfoBogus {
  r_sym=0x40302010U,
  r_type=0x80706050U
}

In this case, and unlike with integral structs, the endianness impacts the bytes of the individual fields, not of the whole struct.

As you can see, integral structs can be used to denote a lot of commonly found idioms in data structures and this includes a lot of what is sometimes denoted in C bit field. However, one should be cautious when "translating" C structures to Poke, especially when the C programmer has not been careful and incurres in sometimes obscure implementation-defined behavior. An integral struct is not always the right abstraction to use when we see a C bit field!

As an example of the above, consider the following C struct:

struct regs
{
  __u8 dst_reg:4;
  __u8 src_reg:4;
};
    

Certain virtual architecture uses that data layout to store registers in instructions (no comment.) Thing is, in bit fields like the above with sub-byte field sizes, the ordering of the fields is not clearly defined, and ultimately what order to use is up to the compiler, i.e. to lore and tradition. As it happens, GCC encodes src_reg in the most significant nibble of the byte and dst_reg in the least significant nibble of the byte when compiling for a little-endian target, and the other way around when compiling for a little-endian target. (I may have had that wrong, this always confuses me.)

How could we encode the C struct regs in Poke? Let's see.

A normal Poke struct clearly won't do it:

deftype RegsBogus1 =
  struct
  {
    uint<4> src;
    uint<4> dst;
  };

The reason being, the ordering of src and dst does not change when you switch endianness (since this is Poke, we can in fact talk about real ordering of bits)... remember, poke is WYPIWIG (what you poke is what you get) ;)

What about an integral struct?

deftype RegsBogus2 =
  struct uint<8>
  {
    uint<4> src;
    uint<4> dst;
  };
    

This won't work either. In fact, the net effect of the normal decoding of the normal struct type RegsBogus1 and the map-an-integer-and-extract-fields decoding of the integral struct RegsBogus2 is in this case totally equivalent.

A solution is to use a normal struct, and field labels:

deftype RegsBogus =
  struct
  {
    defvar little_p = (get_endian == ENDIAN_LITTLE);

    uint<8> src @ !little_p * 4#b;
    uint<8> dst @ little_p * 4#b;
  };
    

At this point, you may be wondering: is there anything particular in a field defined in an integral struct? The answer is: no, not at all. These are regular, first-class fields. Likewise, integral structs are perfectly regular structs. And of course, since this is poke, you can have integral structs of say, 11 bits, or 3 bits, map them at offsets not aligned to bytes, and all the typical poke-atrocities that we enjoy so much.

However, there exist a few restrictions, some of them fundamental, the others to be lifted eventually:

BONUS

My intention is to add support for casts from/to integral structs to/from integers, so we can do things like:

rel.r_info as uint<64>

And also automatic promotions in arithmetic operators, like:

rel.r_info + 20 * rel.r_info.r_type

Happy poking friends!

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