1 files changed, 49 insertions, 1 deletions
diff --git a/third_party/abseil-cpp/absl/random/internal/seed_material.cc b/third_party/abseil-cpp/absl/random/internal/seed_material.cc
index 4d38a57419..c03cad8502 100644
--- a/third_party/abseil-cpp/absl/random/internal/seed_material.cc
+++ b/third_party/abseil-cpp/absl/random/internal/seed_material.cc
@@ -28,6 +28,7 @@
 #include <cstdlib>
 #include <cstring>
 
+#include "absl/base/dynamic_annotations.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/strings/ascii.h"
 #include "absl/strings/escaping.h"
@@ -50,6 +51,18 @@
 
 #endif
 
+#if defined(__GLIBC__) && \
+    (__GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 25))
+// glibc >= 2.25 has getentropy()
+#define ABSL_RANDOM_USE_GET_ENTROPY 1
+#endif
+
+#if defined(__EMSCRIPTEN__)
+#include <sys/random.h>
+// Emscripten has getentropy, but it resides in a different header.
+#define ABSL_RANDOM_USE_GET_ENTROPY 1
+#endif
+
 #if defined(ABSL_RANDOM_USE_BCRYPT)
 #include <bcrypt.h>
 
@@ -122,8 +135,32 @@ bool ReadSeedMaterialFromOSEntropyImpl(absl::Span<uint32_t> values) {
 
 #else
 
+#if defined(ABSL_RANDOM_USE_GET_ENTROPY)
+// On *nix, use getentropy() if supported. Note that libc may support
+// getentropy(), but the kernel may not, in which case this function will return
+// false.
+bool ReadSeedMaterialFromGetEntropy(absl::Span<uint32_t> values) {
+  auto buffer = reinterpret_cast<uint8_t*>(values.data());
+  size_t buffer_size = sizeof(uint32_t) * values.size();
+  while (buffer_size > 0) {
+    // getentropy() has a maximum permitted length of 256.
+    size_t to_read = std::min<size_t>(buffer_size, 256);
+    int result = getentropy(buffer, to_read);
+    if (result < 0) {
+      return false;
+    }
+    // https://github.com/google/sanitizers/issues/1173
+    // MemorySanitizer can't see through getentropy().
+    ABSL_ANNOTATE_MEMORY_IS_INITIALIZED(buffer, to_read);
+    buffer += to_read;
+    buffer_size -= to_read;
+  }
+  return true;
+}
+#endif  // defined(ABSL_RANDOM_GETENTROPY)
+
 // On *nix, read entropy from /dev/urandom.
-bool ReadSeedMaterialFromOSEntropyImpl(absl::Span<uint32_t> values) {
+bool ReadSeedMaterialFromDevURandom(absl::Span<uint32_t> values) {
   const char kEntropyFile[] = "/dev/urandom";
 
   auto buffer = reinterpret_cast<uint8_t*>(values.data());
@@ -150,6 +187,17 @@ bool ReadSeedMaterialFromOSEntropyImpl(absl::Span<uint32_t> values) {
   return success;
 }
 
+bool ReadSeedMaterialFromOSEntropyImpl(absl::Span<uint32_t> values) {
+#if defined(ABSL_RANDOM_USE_GET_ENTROPY)
+  if (ReadSeedMaterialFromGetEntropy(values)) {
+    return true;
+  }
+#endif
+  // Libc may support getentropy, but the kernel may not, so we still have
+  // to fallback to ReadSeedMaterialFromDevURandom().
+  return ReadSeedMaterialFromDevURandom(values);
+}
+
 #endif
 
 }  // namespace