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Sprint 16: LC_FUNCTION_STARTS & LC_DATA_IN_CODE

Prerequisites

Sprint 14 — __LINKEDIT layout sequencing; Sprint 11 — atoms placed.

Goals

Emit the two small __LINKEDIT blobs used by debuggers, disassemblers, and the dynamic loader: LC_FUNCTION_STARTS (delta-encoded entry points) and LC_DATA_IN_CODE (markers for data embedded in __text).

Deliverables

1. LC_FUNCTION_STARTS

Format: a single stream of ULEB128 deltas. First ULEB = offset from the Mach-O header to the first function entry. Each subsequent ULEB = delta from the previous entry. A terminating 0 ends the stream. 8-byte aligned.

Source: every atom in __TEXT,__text plus .alt_entry chain members. Exclude atoms from __stubs and __stub_helper — ld doesn't list those.

2. LC_DATA_IN_CODE

Format: a packed array of:

struct data_in_code_entry {
    uint32 offset;    // from Mach-O header
    uint16 length;    // bytes
    uint16 kind;      // DICE_KIND_DATA=1, _JUMP_TABLE8=2, _JUMP_TABLE16=3,
                      //           _JUMP_TABLE32=4, _ABS_JUMP_TABLE32=5
}

Source: per-input LC_DATA_IN_CODE blocks. Remap each entry's offset from its input-section base to the final output VM address. Entries sorted by offset.

afs-as doesn't emit jump tables today, but we preserve whatever the input has so future C/Objective-C objects with jump tables survive linking.

3. Sorting determinism

Function starts: strictly ascending by VM address. Data-in-code: strictly ascending by output offset. Ties resolved by input command-line order.

4. Integration with __LINKEDIT layout (Sprint 14)

Both blobs get file offsets assigned after chained fixups / classic dyld-info but before the symbol table. Pointed at by their respective load commands with dataoff / datasize.

Testing Strategy

  • Differential: function starts list byte-identical between afs-ld and ld on every staging fixture.
  • Data-in-code: fixture with a jump table input; entries survive linking with correct remapped offsets.
  • Empty output: fixtures with no functions produce zero-byte LC_FUNCTION_STARTS (actually: ld still emits a terminator? check) and absent LC_DATA_IN_CODE when no input had data-in-code.

Definition of Done

  • LC_FUNCTION_STARTS parity with ld on every staging fixture.
  • LC_DATA_IN_CODE entries remapped correctly across linking.
  • Both blobs placed in the right __LINKEDIT slot per Sprint 14.
View source
1 # Sprint 16: LC_FUNCTION_STARTS & LC_DATA_IN_CODE
2
3 ## Prerequisites
4 Sprint 14 — `__LINKEDIT` layout sequencing; Sprint 11 — atoms placed.
5
6 ## Goals
7 Emit the two small `__LINKEDIT` blobs used by debuggers, disassemblers, and the dynamic loader: `LC_FUNCTION_STARTS` (delta-encoded entry points) and `LC_DATA_IN_CODE` (markers for data embedded in `__text`).
8
9 ## Deliverables
10
11 ### 1. LC_FUNCTION_STARTS
12
13 Format: a single stream of ULEB128 deltas. First ULEB = offset from the Mach-O header to the first function entry. Each subsequent ULEB = delta from the previous entry. A terminating `0` ends the stream. 8-byte aligned.
14
15 Source: every atom in `__TEXT,__text` plus `.alt_entry` chain members. Exclude atoms from `__stubs` and `__stub_helper` — ld doesn't list those.
16
17 ### 2. LC_DATA_IN_CODE
18
19 Format: a packed array of:
20 ```
21 struct data_in_code_entry {
22 uint32 offset; // from Mach-O header
23 uint16 length; // bytes
24 uint16 kind; // DICE_KIND_DATA=1, _JUMP_TABLE8=2, _JUMP_TABLE16=3,
25 // _JUMP_TABLE32=4, _ABS_JUMP_TABLE32=5
26 }
27 ```
28
29 Source: per-input `LC_DATA_IN_CODE` blocks. Remap each entry's offset from its input-section base to the final output VM address. Entries sorted by offset.
30
31 afs-as doesn't emit jump tables today, but we preserve whatever the input has so future C/Objective-C objects with jump tables survive linking.
32
33 ### 3. Sorting determinism
34
35 Function starts: strictly ascending by VM address. Data-in-code: strictly ascending by output offset. Ties resolved by input command-line order.
36
37 ### 4. Integration with `__LINKEDIT` layout (Sprint 14)
38
39 Both blobs get file offsets assigned after chained fixups / classic dyld-info but before the symbol table. Pointed at by their respective load commands with `dataoff / datasize`.
40
41 ## Testing Strategy
42
43 - Differential: function starts list byte-identical between afs-ld and `ld` on every staging fixture.
44 - Data-in-code: fixture with a jump table input; entries survive linking with correct remapped offsets.
45 - Empty output: fixtures with no functions produce zero-byte LC_FUNCTION_STARTS (actually: ld still emits a terminator? check) and absent LC_DATA_IN_CODE when no input had data-in-code.
46
47 ## Definition of Done
48
49 - LC_FUNCTION_STARTS parity with `ld` on every staging fixture.
50 - LC_DATA_IN_CODE entries remapped correctly across linking.
51 - Both blobs placed in the right `__LINKEDIT` slot per Sprint 14.