feat(syscalls): add the EF IO write_log symbol (no-op) per zkvm-standards#27

executor/programs/rust/ef_io_demo/src/main.rs

@@ -1,15 +1,25 @@
-// Demo guest exercising the EF zkVM IO interface (`read_input` / `write_output`).
+// Demo guest exercising the EF zkVM IO interface (`read_input` / `write_output` /
+// `write_log`).
 //
 // Reads the private input via the EF zero-copy `read_input` shim, then emits it
 // back as the public output in TWO `write_output` calls (split in halves) to
 // exercise the multi-call concatenation requirement of the EF spec.
+//
+// Also makes one `write_log` call. That channel is a no-op on Lambda VM, so this
+// proves the symbol contract rather than any output: a portable guest calling
+// `write_log` links and runs. It is the only thing that does — the symbol is
+// dead-stripped from guests that never call it.
 use lambda_vm_syscalls as syscalls;
 
+/// Diagnostic text for the `write_log` call. Must be valid UTF-8 per the EF spec.
+const LOG_MESSAGE: &str = "ef_io_demo: echoing private input to public output\n";
+
 pub fn main() {
     let mut buf_ptr: *const u8 = core::ptr::null();
     let mut buf_size: usize = 0;
     unsafe {
         syscalls::ef_io::read_input(&mut buf_ptr, &mut buf_size);
+        syscalls::ef_io::write_log(LOG_MESSAGE.as_ptr(), LOG_MESSAGE.len());
     }
 
     if buf_size > 0 {

syscalls/src/ef_io.rs

@@ -1,18 +1,27 @@
 //! EF zkVM IO interface: <https://github.com/eth-act/zkvm-standards/blob/main/standards/io-interface/README.md>
 //!
-//! Two C-callable functions that match the EF standard so portable applications
-//! compile unchanged across zkVMs:
+//! Three C-callable functions that match the EF standard so portable
+//! applications compile unchanged across zkVMs:
 //!
 //! - `read_input`: returns a zero-copy pointer + size to the private input.
 //! - `write_output`: appends bytes to the public output. Multiple calls
 //!   concatenate.
+//! - `write_log`: diagnostic (`println`-style) UTF-8 text for the host. Not
+//!   part of the statement being proven. Specified by
+//!   <https://github.com/eth-act/zkvm-standards/pull/27>, which amends the
+//!   interface above from two functions to three; until it merges, the linked
+//!   README documents only the first two.
 //!
 //! On Lambda VM these map to:
 //! - `read_input` → memory-mapped private input region at `0xFF000000`
 //!   (4-byte LE length prefix at base, data at `+4`).
 //! - `write_output` → ECALL #64 (Commit). The trace builder maintains a
 //!   running commitment index in synthetic register `x254`, so multiple
 //!   ECALLs naturally concatenate at the proof level.
+//! - `write_log` → nothing at all. The standard lets proving environments that
+//!   don't expose diagnostics ignore these calls, and the `Print` ecall it
+//!   would otherwise use has no Ecall-bus receiver (see `write_log`'s doc
+//!   comment and `syscalls::print_string`).
 
 #[cfg(target_arch = "riscv64")]
 use core::arch::asm;
@@ -67,6 +76,26 @@ pub unsafe extern "C" fn write_output(output: *const u8, size: usize) {
     }
 }
 
+/// EF IO: emit `utf8_len` bytes of UTF-8 diagnostic text (`println`-style) for
+/// the host. Intentionally a no-op: it emits no ecall and reads no memory.
+///
+/// The natural lowering would be the `Print` ecall (a7=1), but that ecall has
+/// no receiver on the Ecall bus, so emitting it unbalances the LogUp argument
+/// and every proof using it fails to verify — the same reason
+/// `syscalls::print_string` is a no-op. Ignoring the call is conforming: per
+/// the spec the log text "does not contribute to the public output, is not part
+/// of the statement being proven", and calls "may be ignored by production
+/// proving environments that do not expose diagnostics". Like the rest of the
+/// interface it cannot fail; there is no error code.
+///
+/// # Safety
+///
+/// `utf8_bytes` must point to `utf8_len` readable bytes of guest memory. The
+/// requirement is nominal — the no-op never dereferences the pointer.
+#[cfg(target_arch = "riscv64")]
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn write_log(_utf8_bytes: *const u8, _utf8_len: usize) {}
+
 /// Host-side stub — Lambda VM's IO interface is only implemented for the
 /// `riscv64` guest target. Not exported with C linkage on host so the
 /// generic name doesn't collide with C dependencies in test builds.
@@ -82,3 +111,11 @@ pub fn read_input(_buf_ptr: *mut *const u8, _buf_size: *mut usize) {
 pub fn write_output(_output: *const u8, _size: usize) {
     unimplemented!("write_output is only implemented for riscv64 targets");
 }
+
+/// Host-side stub — Lambda VM's IO interface is only implemented for the
+/// `riscv64` guest target. Not exported with C linkage on host so the
+/// generic name doesn't collide with C dependencies in test builds.
+#[cfg(not(target_arch = "riscv64"))]
+pub fn write_log(_utf8_bytes: *const u8, _utf8_len: usize) {
+    unimplemented!("write_log is only implemented for riscv64 targets");
+}