The mocks

The mock/ folder holds the small, app-owned host-side glue. Everything generic — crypto, NBGL, OS, syscalls, SDK libraries — is provided by the framework and linked through secure_sdk; do not reimplement it here.

Required symbols

mock/mocks.h declares the symbols the harness and framework read:

#pragma once
#include <stddef.h>
#include <stdint.h>
#include "fuzz_defs.h"
#include "exceptions.h"
 
extern try_context_t fuzz_exit_jump_ctx;   /* longjmp target used to unwind os_exit */
 
#define FUZZ_CTRL_SIZE 16
extern uint8_t        fuzz_ctrl[FUZZ_CTRL_SIZE];  /* control bytes (lane, command…) */
extern const uint8_t *fuzz_tail_ptr;              /* APDU tail pointer */
extern size_t         fuzz_tail_len;              /* APDU tail length */

mock/mocks.c defines them:

#include "mocks.h"
 
uint8_t        fuzz_ctrl[FUZZ_CTRL_SIZE];
const uint8_t *fuzz_tail_ptr = NULL;
size_t         fuzz_tail_len = 0;

Note

fuzz_app_cleanup() is not required here. The framework ships a weak no-op default, so define it (in the harness or here) only if your target needs per-iteration teardown.

Recommended stubs

Most apps also add two small no-ops in mocks.c:

#include <stdarg.h>
 
/* PRINTF is removed from the macro set for fuzz builds (see the Build macros
   page) so app sources compile it as a real call. This keeps it a no-op. */
int PRINTF(const char *format, ...) { (void) format; return 0; }
 
/* Zeroing BSS would erase the prefix state Absolution restores each iteration. */
void os_explicit_zero_BSS_segment(void) {}

Add any other app-specific stub the fuzz build needs the same way (a hardware peripheral the host cannot provide, for instance). Keep them minimal and side-effect free.

mock/scenario_layout.h

This header is tooling-owned. It records the prefix layout for the current build:

#pragma once
#define SCEN_PREFIX_SIZE 64
#define SCEN_CTRL_OFF    0
#define SCEN_CTRL_LEN    16

ledger_fuzz_setup() bootstraps it with the sentinel values above on a fresh checkout, and scripts/update-scenario-layout.py rewrites the numbers after every build. Commit the bootstrap version so a clone configures cleanly, but do not stage the locally rewritten numbers — the harness reads the fresh values at build time. Advanced apps may append extra SCEN_* offsets via [layout].extra_args (see The fuzz manifest).

Overriding a shared mock

If an app must supply its own version of a symbol the shared mocks already provide, list the replacement source in the manifest's [mocks].override_sources (see The fuzz manifest) rather than editing the SDK. This is rarely needed.