cpu_guard test: probe a libc symbol no sanitizer interposes

cargo-careful has been red on every master merge since #355. Under
-Zcareful-sanitizer (ASan) `malloc` is ASan's interceptor, linked into the
executable, so the can-stay-silent test's premise (`malloc` lives in libc)
is false there. The guard code is unchanged; the test now probes glibc's
`gnu_get_libc_version`, which no sanitizer replaces.

Verified: plain build 14/14 cpu_guard tests; `cargo +nightly careful test
-Zcareful-sanitizer` passes; reverting to `malloc` fails there exactly as
CI does (cpu_guard.rs:712). Scratch reproduction with -Zsanitizer=address:
malloc in main image true, gnu_get_libc_version false.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011BMJnDBAfzcuQh5HeT39Rh

Author: claude

.claude/blackboard.md

@@ -1,3 +1,9 @@
+## 2026-10-13 — `cargo-careful` red since #355: the libc probe was ASan's `malloc`
+
+- **Cause:** `cpu_guard::tests::an_address_in_libc_is_not_in_the_main_executable` used `malloc` as its libc address. CI's `cargo-careful` job runs `-Zcareful-sanitizer` (AddressSanitizer), which interposes `malloc` with an interceptor linked into the executable, so the address really is in the main image there. Red on every master merge from #355 (where the test landed) through #361; green at #354.
+- **Change:** the test probes glibc's `gnu_get_libc_version`, which no sanitizer replaces. The guard itself is unchanged.
+- **Measured here:** scratch crate with `-Zsanitizer=address`: `malloc` in main image `true`, `gnu_get_libc_version` `false`, own function `true` (plain build: false/false/true). `cargo +nightly careful test -Zcareful-sanitizer`: new test passes; with `malloc` restored it fails at `cpu_guard.rs:712`, as in CI. Plain build: 14/14 `cpu_guard` tests.
+
 ## 2026-10-13 — CI compiles both production tiers: x86-64-v4 and Sapphire Rapids
 
 - **Change:** `tier4-avx512-check` in `ci.yaml` is now a two-entry matrix, `x86-64-v4` (`.cargo/config-v4.toml`) and `sapphirerapids` (`.cargo/config-avx512.toml`). Each entry runs both `cargo check` feature sets and the asm-only codegen witness. Neither is executed: GitHub's runners are mostly v3. `config-avx512.toml` gains `-Dwarnings`, matching `config-v4.toml`, so both entries gate warnings and the witness script (which word-splits `CARGO_ARGS`) needs no extra quoted `--config`. The job sets `CARGO_PROFILE_{DEV,TEST}_DEBUG=0`: with `--target`, build scripts take the profile's debug level.

src/cpu_guard.rs

@@ -692,11 +692,16 @@ mod tests {
         assert!(guard_is_in_main_executable());
     }
 
-    /// Can-stay-silent half: an address in a loaded system library (libc's
-    /// `malloc`) is not in the main executable. Without this the lookup
-    /// could be `true` everywhere and the library case would never happen.
-    /// Assumes libc is a shared object (the default dynamic linux-gnu
-    /// build): with `+crt-static`, `malloc` is inside the executable.
+    /// Can-stay-silent half: an address in a loaded system library (glibc's
+    /// `gnu_get_libc_version`) is not in the main executable. Without this
+    /// the lookup could be `true` everywhere and the library case would never
+    /// happen. Assumes libc is a shared object (the default dynamic linux-gnu
+    /// build): with `+crt-static`, libc is inside the executable.
+    ///
+    /// Not `malloc`: a sanitizer interposes it with its own copy linked into
+    /// the executable, so under `cargo careful -Zcareful-sanitizer` (ASan)
+    /// `malloc` really is in the main image. No sanitizer replaces
+    /// `gnu_get_libc_version`.
     #[cfg(all(
         not(miri),
         target_arch = "x86_64",
@@ -707,9 +712,10 @@ mod tests {
     #[test]
     fn an_address_in_libc_is_not_in_the_main_executable() {
         extern "C" {
-            fn malloc(size: usize) -> *mut core::ffi::c_void;
+            fn gnu_get_libc_version() -> *const core::ffi::c_char;
         }
-        assert!(!main_image::contains(malloc as unsafe extern "C" fn(usize) -> *mut core::ffi::c_void as usize));
+        let in_libc = gnu_get_libc_version as unsafe extern "C" fn() -> *const core::ffi::c_char as usize;
+        assert!(!main_image::contains(in_libc));
     }
 
     #[test]