Always preload libm.so.6 in sidecar direct spawn

The portable (musl-built, glibc-compat) libdatadog_php.so has its libc
DT_NEEDED patched out and relies on the host's libc being in the global
scope. When the sidecar is direct-spawned, ld-linux execs the DSO itself,
so anything it needs beyond libc.so.6 (which ld-linux always brings in)
must be passed with --preload.

add_glibc_direct_preloads() only did that on glibc < 2.34, assuming 2.34
had merged all four DSOs into libc. It did not merge libm: exp/log/pow
(referenced by libdd-ddsketch, libdd-telemetry and datadog-ipc's shm
stats) still live in libm.so.6 on every glibc. On glibc >= 2.34 hosts the
spawned sidecar therefore died at load with

    libdatadog_php.so: symbol lookup error: undefined symbol: exp

which made every sidecar connection reset, so AppSec never started
("AppSec sidecar backend is unavailable"). This broke the FrankenPHP SSI
tests (dunglas/frankenphp:1.4-php8.4 is Debian 12, glibc 2.36): 27
FrankenphpClassicTests + 10 FrankenphpWorkerTests failures in the
"helper-rust integration coverage" job.

Preload libm.so.6 whenever the host is glibc, and keep libdl/libpthread/
librt for glibc < 2.34.

Verified with test8.4-release-zts-ssi (glibc 2.36, FrankenPHP): 27
failures before, 0 after; test8.3-release-ssi (glibc 2.31, the < 2.34
path) still green.

Author: cataphract

loader/dd_library_loader.c

@@ -39,8 +39,9 @@ static void *libdatadog_php_handle = NULL;
 
 #if !defined(__MUSL__)
 // A portable musl-built libdatadog_php.so has unversioned references to these
-// APIs. On glibc before 2.34 their providers are separate DSOs, so make them
-// global before loading libdatadog_php.so.
+// APIs. libm is always a separate DSO (2.34 merged libdl/libpthread/librt into
+// libc, but never libm), and before 2.34 so are the others, so make them global
+// before loading libdatadog_php.so.
 static const char *const libdatadog_php_glibc_dependencies[] = {
     "libdl.so.2",
     "libm.so.6",

sidecar/src/lib.rs

@@ -19,16 +19,31 @@ fn add_glibc_direct_preloads(config: &mut Config) {
         return;
     };
 
-    if !glibc_version().is_some_and(|version| version < (2, 34)) {
+    // On musl hosts the dynamic loader is libc itself (math included), so
+    // there is nothing to preload — and the glibc DSO names don't exist.
+    let Some(version) = glibc_version() else {
         return;
-    }
+    };
 
     // Direct spawn asks ld-linux to load the entrypoint DSO itself, before the
-    // sidecar can run any initialization code. Before 2.34, glibc provided
-    // these symbols in separate libraries, so load them explicitly.
-    direct_spawn
-        .preloads
-        .extend(["libdl.so.2", "libm.so.6", "libpthread.so.0", "librt.so.1"].map(String::from));
+    // sidecar can run any initialization code. The portable DSO has no
+    // DT_NEEDED entry for libc (it's built against musl and the dependency is
+    // patched out), so it relies on its undefined symbols being satisfied by
+    // whatever ld-linux has in the global scope.
+    //
+    // ld-linux always brings in libc.so.6, but libm was never merged into it
+    // (not even in 2.34, which only absorbed libdl/libpthread/librt/libutil/
+    // libanl). Math symbols — exp/log/pow, referenced by libdd-ddsketch,
+    // libdd-telemetry and datadog-ipc's shm stats — therefore always need
+    // libm.so.6 to be loaded explicitly.
+    direct_spawn.preloads.push("libm.so.6".to_string());
+
+    // Before 2.34 these were separate DSOs as well.
+    if version < (2, 34) {
+        direct_spawn
+            .preloads
+            .extend(["libdl.so.2", "libpthread.so.0", "librt.so.1"].map(String::from));
+    }
 }
 
 #[cfg(all(feature = "glibc-compat", target_os = "linux"))]