[libcxx-commits] [PATCH] D116045: [libc++][ABI BREAK] Start using `arc4random()` to implement `std::random_device` on Apple

[Louis Dionne via Phabricator via libcxx-commits]

ldionne updated this revision to Diff 395740.
ldionne retitled this revision from "[libc++] Use arc4random instead of /dev/urandom on Apple platforms" to "[libc++][ABI BREAK] Start using `arc4random()` to implement `std::random_device` on Apple".
ldionne edited the summary of this revision.
ldionne added a comment.

Update with ABI break mechanism and documentation


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D116045/new/

https://reviews.llvm.org/D116045

Files:
  libcxx/docs/ReleaseNotes.rst
  libcxx/include/__config
  libcxx/include/__random/random_device.h


Index: libcxx/include/__random/random_device.h
===================================================================
--- libcxx/include/__random/random_device.h
+++ libcxx/include/__random/random_device.h
@@ -55,7 +55,7 @@
 //      Use rand_s(), for use on Windows.
 //      When this option is used, the token passed to `std::random_device`'s
 //      constructor *must* be "/dev/urandom" -- anything else is an error.
-#if defined(__OpenBSD__)
+#if defined(__OpenBSD__) || defined(__APPLE__)
 #  define _LIBCPP_USING_ARC4_RANDOM
 #elif defined(__Fuchsia__) || defined(__wasi__)
 #  define _LIBCPP_USING_GETENTROPY
@@ -72,6 +72,18 @@
 #ifdef _LIBCPP_USING_DEV_RANDOM
     int __f_;
 #endif
+
+#ifndef _LIBCPP_ABI_NO_RANDOM_DEVICE_COMPATIBILITY_LAYOUT
+    // Apple platforms used to use the `_LIBCPP_USING_DEV_RANDOM` code path, and now
+    // use `arc4random()` as of this comment. In order to avoid breaking the ABI, we
+    // retain the same layout as before.
+#   if defined(__APPLE__)
+    int __padding_; // padding to fake the `__f_` field above
+#   endif
+
+    // ... vendors can add workarounds here if they switch to a different representation ...
+#endif
+
 public:
     // types
     typedef unsigned result_type;
Index: libcxx/include/__config
===================================================================
--- libcxx/include/__config
+++ libcxx/include/__config
@@ -107,6 +107,12 @@
 #  define _LIBCPP_ABI_ENABLE_UNIQUE_PTR_TRIVIAL_ABI
 // Enable clang::trivial_abi on std::shared_ptr and std::weak_ptr
 #  define _LIBCPP_ABI_ENABLE_SHARED_PTR_TRIVIAL_ABI
+// std::random_device holds some state when it uses an implementation that gets
+// entropy from a file. When switching from this implementation to another one
+// on a platform that has already shipped std::random_device, one needs to
+// retain the same object layout to remain ABI compatible. This switch removes
+// these workarounds for platforms that don't care about ABI compatibility.
+#  define _LIBCPP_ABI_NO_RANDOM_DEVICE_COMPATIBILITY_LAYOUT
 #elif _LIBCPP_ABI_VERSION == 1
 #  if !defined(_LIBCPP_OBJECT_FORMAT_COFF)
 // Enable compiling copies of now inline methods into the dylib to support
Index: libcxx/docs/ReleaseNotes.rst
===================================================================
--- libcxx/docs/ReleaseNotes.rst
+++ libcxx/docs/ReleaseNotes.rst
@@ -114,6 +114,13 @@
   comment `here <https://reviews.llvm.org/D109459>`_ if you are broken by this change
   and need to define the macro.
 
+- On Apple platforms, ``std::random_device`` is now implemented on top of ``arc4random()``
+  instead of reading from ``/dev/urandom``. Using any implementation-defined token different
+  from ``"/dev/urandom"`` when constructing a ``std::random_device`` will now be an error.
+  This behavior is preferred to ignoring the custom token in order to avoid surprising changes
+  in behavior when a custom token was expected to result in the corresponding file being used
+  for entropy, but would now be ignored.
+
 Build System Changes
 --------------------
 


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