What the developer wrote
Authored by Marko Bencun
Shrink secp256k1 callbacks
Override libsecp256k1's default callbacks with tiny local stubs.
The vendored callbacks print an error and then call abort(). In the
firmware image those failure-only paths pull in a large chunk of newlib
stdio and signal handling even though the callbacks are only used for
fatal illegal-argument and internal-consistency failures and must never
return.
Saved: 2672 bytes
The final linked image drops the abort/stdio path retained only by those
callbacks, including the public symbols abort, raise, _raise_r,
_kill_r, _getpid_r, fprintf, printf, and _vfprintf_r. It also drops the
private stdio helpers behind them, such as __sfputc_r, __sfputs_r,
__swsetup_r, __swbuf_r, _fflush_r, _fwalk_sglue, __swhatbuf_r,
__swrite, __sread, __sseek, __sclose, __sinit, and __smakebuf_r.
Meanwhile secp256k1_default_illegal_callback_fn and
secp256k1_default_error_callback_fn shrink from 28 bytes each to 2
bytes each.
This keeps the relevant behavior intact: both callbacks still trap the
firmware in a non-returning fatal path, but without dragging in unused
diagnostic formatting code.
What changed, and why it matters
This commit is a size-optimization cleanup, not a security fix. It replaces libsecp256k1's default fatal-error callbacks with smaller stubs so the firmware binary no longer pulls in unused standard-library printing and abort code. The behavior stays the same: on an internal or illegal-argument failure, the device still hangs forever instead of returning.
No security action required. Treat as a normal firmware optimization commit. If desired, verify that the infinite-loop stubs still satisfy the project's watchdog/reset expectations for unrecoverable failures.
Security signals we found
No security signal present: change is a binary-size optimization
Fatal callback behavior preserved (infinite loop, no return)
No new attack surface introduced
No input validation, memory safety, or cryptographic logic changed
Evidence from the diff
The change adds USE_EXTERNAL_DEFAULT_CALLBACKS=1 and supplies default_callbacks.c with two non-returning functions: secp256k1_default_illegal_callback_fn and secp256k1_default_error_callback_fn. The new stubs ignore arguments and loop forever, matching the original non-returning contract while removing newlib stdio/signal dependencies (abort, raise, fprintf, printf, etc.). This is a code-size reduction of 2672 bytes with no functional change to the failure path.
Changed components
src/rust/bitbox-secp256k1/build.rssrc/rust/bitbox-secp256k1/src/default_callbacks.cVendored libsecp256k1-zkp default error/illegal callbacksInspect captured patch +21 / −0
diff --git a/src/rust/bitbox-secp256k1/build.rs b/src/rust/bitbox-secp256k1/build.rs
index 2552cfd..ac3c453 100644
--- a/src/rust/bitbox-secp256k1/build.rs
+++ b/src/rust/bitbox-secp256k1/build.rs
@@ -4,23 +4,27 @@ use std::path::PathBuf;
fn main() {
let secp_dir = PathBuf::from("depend/secp256k1-zkp");
+ let callbacks = PathBuf::from("src/default_callbacks.c");
println!(
"cargo::rerun-if-changed={}",
secp_dir.join("include").display()
);
println!("cargo::rerun-if-changed={}", secp_dir.join("src").display());
+ println!("cargo::rerun-if-changed={}", callbacks.display());
let mut build = cc::Build::new();
build
.file(secp_dir.join("src/secp256k1.c"))
.file(secp_dir.join("src/precomputed_ecmult.c"))
.file(secp_dir.join("src/precomputed_ecmult_gen.c"))
+ .file(&callbacks)
.include(secp_dir.join("include"))
// Suppress all warnings in this dependency, we don't have control over them.
.flag_if_supported("-w")
.define("ECMULT_WINDOW_SIZE", Some("2"))
.define("ECMULT_GEN_PREC_BITS", Some("2"))
+ .define("USE_EXTERNAL_DEFAULT_CALLBACKS", Some("1"))
.define("ENABLE_MODULE_RECOVERY", Some("1")) // needed only in Rust unit tests.
.define("ENABLE_MODULE_EXTRAKEYS", Some("1"))
.define("ENABLE_MODULE_SCHNORRSIG", Some("1"))
diff --git a/src/rust/bitbox-secp256k1/src/default_callbacks.c b/src/rust/bitbox-secp256k1/src/default_callbacks.c
new file mode 100644
index 0000000..161e731
--- /dev/null
+++ b/src/rust/bitbox-secp256k1/src/default_callbacks.c
@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: Apache-2.0
+
+void secp256k1_default_illegal_callback_fn(const char* str, void* data)
+{
+ (void)str;
+ (void)data;
+ while (1) {
+ }
+}
+
+void secp256k1_default_error_callback_fn(const char* str, void* data)
+{
+ (void)str;
+ (void)data;
+ while (1) {
+ }
+}
Why this scored 14/100
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