Fix building on old libc systems without getentropy (#329)

Signed-off-by: Matthias Wahl <mwahl@wayfair.com>

CMakeLists.txt

@@ -34,6 +34,22 @@ size_t malloc_usable_size(const void* ptr) { return 0; }
 int main() { return 0; }
 " CONST_QUALIFIED_MALLOC_USABLE_SIZE)
 
+# older libcs might not have getentropy, e.g. it appeared in gliobc 2.25
+# so we need to fallback if we cannot compile this
+CHECK_C_SOURCE_COMPILES("
+#if __has_include(<unistd.h>)
+#  include <unistd.h>
+#endif
+#if __has_include(<sys/random.h>)
+#include <sys/random.h>
+#endif
+int main() {
+  int entropy = 0;
+  int res = getentropy(&entropy, sizeof(entropy));
+  return res;
+}
+" SNMALLOC_PLATFORM_HAS_GETENTROPY)
+
 if (NOT SNMALLOC_CI_BUILD)
   option(USE_POSIX_COMMIT_CHECKS "Instrument Posix PAL to check for access to unused blocks of memory." Off)
 else ()
@@ -160,6 +176,12 @@ if(USE_POSIX_COMMIT_CHECKS)
   target_compile_definitions(snmalloc_lib INTERFACE -DUSE_POSIX_COMMIT_CHECKS)
 endif()
 
+if(SNMALLOC_PLATFORM_HAS_GETENTROPY)
+  target_compile_definitions(snmalloc_lib INTERFACE -DSNMALLOC_PLATFORM_HAS_GETENTROPY=Entropy)
+else()
+  target_compile_definitions(snmalloc_lib INTERFACE -DSNMALLOC_PLATFORM_HAS_GETENTROPY=0)
+endif()
+
 if(CONST_QUALIFIED_MALLOC_USABLE_SIZE)
   target_compile_definitions(snmalloc_lib INTERFACE -DMALLOC_USABLE_SIZE_QUALIFIER=const)
 endif()

azure-pipelines.yml

@@ -342,6 +342,6 @@ jobs:
 
   - script: |
       set -eo pipefail
-      clang-tidy-9  src/override/malloc.cc  -header-filter="`pwd`/*" -warnings-as-errors='*' -export-fixes=tidy.fail -- -std=c++17 -mcx16
+      clang-tidy-9  src/override/malloc.cc  -header-filter="`pwd`/*" -warnings-as-errors='*' -export-fixes=tidy.fail -- -std=c++17 -mcx16 -DSNMALLOC_PLATFORM_HAS_GETENTROPY=0
 
     displayName: 'Clang-Tidy'

src/pal/pal_haiku.h

@@ -1,17 +1,6 @@
 #pragma once
 
 #if defined(__HAIKU__)
-#  include <cassert>
-
-namespace snmalloc
-{
-  template<typename... Args>
-  int getentropy(Args...)
-  {
-    assert(0 && "Unreachable path");
-    return -1;
-  }
-}
 
 #  include "pal_posix.h"
 
@@ -30,11 +19,8 @@ namespace snmalloc
      * Bitmap of PalFeatures flags indicating the optional features that this
      * PAL supports.
      *
-     * Disable the POSIX default implementation of Entropy as not supported on
-     * Haiku.
      */
-    static constexpr uint64_t pal_features =
-      PALPOSIX::pal_features & ~(Entropy);
+    static constexpr uint64_t pal_features = PALPOSIX::pal_features;
 
     /**
      * Haiku requires an explicit no-reserve flag in `mmap` to guarantee lazy

src/pal/pal_netbsd.h

@@ -1,18 +1,6 @@
 #pragma once
 
 #ifdef __NetBSD__
-#  include <cassert>
-
-namespace snmalloc
-{
-  template<typename... Args>
-  int getentropy(Args...)
-  {
-    assert(0 && "Unreachable path");
-    return -1;
-  }
-}
-
 #  include "pal_bsd_aligned.h"
 
 namespace snmalloc
@@ -35,12 +23,10 @@ namespace snmalloc
      * field is declared explicitly to remind anyone modifying this class to
      * add new features that they should add any required feature flags.
      *
-     * We disable the default implementation of randomness on NetBSD as it
-     * does not have the getentropy call. This will currently fallback to
-     * C++ libraries std::random_device.
+     * As NetBSD does not have the getentropy call, get_entropy64 will
+     * currently fallback to C++ libraries std::random_device.
      */
-    static constexpr uint64_t pal_features =
-      PALBSD_Aligned::pal_features & ~Entropy;
+    static constexpr uint64_t pal_features = PALBSD_Aligned::pal_features;
   };
 } // namespace snmalloc
 #endif

src/pal/pal_posix.h

@@ -19,6 +19,12 @@
 #  include <unistd.h>
 #endif
 
+// default value for builds not using CMake
+#ifndef SNMALLOC_PLATFORM_HAS_GETENTROPY
+#  define SNMALLOC_PLATFORM_HAS_GETENTROPY 0
+#  warning snmalloc not using entropy source. To silence this warning please define SNMALLOC_PLATFORM_HAS_GETENTROPY to 0, or to Entropy if your platform implements getentropy().
+#endif
+
 extern "C" int puts(const char* str);
 
 namespace snmalloc
@@ -117,9 +123,11 @@ namespace snmalloc
      * Bitmap of PalFeatures flags indicating the optional features that this
      * PAL supports.
      *
-     * POSIX systems are assumed to support lazy commit.
+     * POSIX systems are assumed to support lazy commit. The build system checks
+     * getentropy is available, only then this PAL supports Entropy.
      */
-    static constexpr uint64_t pal_features = LazyCommit | Entropy;
+    static constexpr uint64_t pal_features =
+      LazyCommit | SNMALLOC_PLATFORM_HAS_GETENTROPY;
 
     static constexpr size_t page_size = Aal::smallest_page_size;
 
@@ -290,11 +298,14 @@ namespace snmalloc
       }
       else
       {
+#ifdef SNMALLOC_PLATFORM_HAS_GETENTROPY
         uint64_t result;
         if (getentropy(&result, sizeof(result)) != 0)
           error("Failed to get system randomness");
         return result;
+#endif
       }
+      error("Entropy requested on platform that does not provide entropy");
     }
   };
 } // namespace snmalloc