kit

link_image_id.c (2060B)

---

 /* link_image_id_compute: format-agnostic 16-byte identity hash for a
  * resolved LinkImage.
  *
  * A single SHA-256 stream is fed each segment's vaddr, file_size, and
  * post-shift bytes, so the digest changes if either content or layout
  * shifts.  The 16-byte image id is the first half of the 32-byte digest.
  * The SHA-256 core uses the AArch64 crypto extension where the host has
  * it (the same hardware path that hashes the Mach-O code signature), so
  * the per-byte fold is far cheaper than the old twin-FNV mix — this was
  * the residual top frame on pathologically large links.  Determinism (no
  * time / random component) is intentional — reproducible builds.
  *
  * Wrapped per format:
  *   - ELF      .note.gnu.build-id (link_emit_elf)
  *   - Mach-O   LC_UUID payload    (link_emit_macho, Phase 3)
  *   - COFF/PE  debug directory    (deferred)
  *
  * Lived in link_elf.c through Phase 0; lifted out so the Mach-O writer
  * sees the same bytes. */
 
 #include "core/core.h"
 #include "core/sha256.h"
 #include "link/link_internal.h"
 
 /* sha256_update takes a u32 length; feed large segments in chunks so a
  * >4 GiB segment can never truncate the stream. */
 static void image_id_update(Sha256* s, const u8* data, size_t n) {
   while (n) {
     u32 chunk = n > 0x40000000u ? 0x40000000u : (u32)n;
     sha256_update(s, data, chunk);
     data += chunk;
     n -= chunk;
   }
 }
 
 void link_image_id_compute(const LinkImage* img, u8 out[LINK_IMAGE_ID_BYTES]) {
   Sha256 s;
   sha256_init(&s);
   u32 i;
   for (i = 0; i < img->nsegments; ++i) {
     const LinkSegment* seg = &img->segments[i];
     u64 vaddr = seg->vaddr;
     u64 fsz = seg->file_size;
     sha256_update(&s, (const u8*)&vaddr, (u32)sizeof vaddr);
     sha256_update(&s, (const u8*)&fsz, (u32)sizeof fsz);
     if (img->segment_bytes[i] && fsz)
       image_id_update(&s, img->segment_bytes[i], (size_t)fsz);
   }
   u8 digest[SHA256_DIGEST_LEN];
   sha256_final(&s, digest);
   /* Image id is the first 16 bytes of the 256-bit digest. */
   for (i = 0; i < LINK_IMAGE_ID_BYTES; ++i) out[i] = digest[i];
 }