libbinder: move session ID RNG code to Utils.cpp

This moves the code that reads from /dev/urandom from RpcServer.cpp
into Utils.cpp so other operating systems can provide their own
implementations by replacing Utils.cpp.

Test: atest binderRpcTest
Bug: 224644083
Change-Id: I2923b25537c07060b830b0d8378df8c969bbd02f

3 files changed, 21 insertions, 5 deletions

diff --git a/libs/binder/RpcServer.cpp b/libs/binder/RpcServer.cpp
index eb39bd9bb2..d63c3f160e 100644
--- a/libs/binder/RpcServer.cpp
+++ b/libs/binder/RpcServer.cpp
@@ -24,7 +24,6 @@
 #include <thread>
 #include <vector>
 
-#include <android-base/file.h>
 #include <android-base/hex.h>
 #include <android-base/scopeguard.h>
 #include <binder/Parcel.h>
@@ -37,6 +36,7 @@
 #include "RpcSocketAddress.h"
 #include "RpcState.h"
 #include "RpcWireFormat.h"
+#include "Utils.h"
 
 namespace android {
 
@@ -381,10 +381,9 @@ void RpcServer::establishConnection(sp<RpcServer>&& server, base::unique_fd clie
                     return;
                 }
 
-                base::unique_fd fd(TEMP_FAILURE_RETRY(
-                        open("/dev/urandom", O_RDONLY | O_CLOEXEC | O_NOFOLLOW)));
-                if (!base::ReadFully(fd, sessionId.data(), sessionId.size())) {
-                    ALOGE("Could not read from /dev/urandom to create session ID");
+                auto status = getRandomBytes(sessionId.data(), sessionId.size());
+                if (status != OK) {
+                    ALOGE("Failed to read random session ID: %s", strerror(-status));
                     return;
                 }
             } while (server->mSessions.end() != server->mSessions.find(sessionId));
diff --git a/libs/binder/Utils.cpp b/libs/binder/Utils.cpp
index d2a5be1102..b0289a7196 100644
--- a/libs/binder/Utils.cpp
+++ b/libs/binder/Utils.cpp
@@ -16,6 +16,7 @@
 
 #include "Utils.h"
 
+#include <android-base/file.h>
 #include <string.h>
 
 using android::base::ErrnoError;
@@ -38,4 +39,17 @@ Result<void> setNonBlocking(android::base::borrowed_fd fd) {
     return {};
 }
 
+status_t getRandomBytes(uint8_t* data, size_t size) {
+    int ret = TEMP_FAILURE_RETRY(open("/dev/urandom", O_RDONLY | O_CLOEXEC | O_NOFOLLOW));
+    if (ret == -1) {
+        return -errno;
+    }
+
+    base::unique_fd fd(ret);
+    if (!base::ReadFully(fd, data, size)) {
+        return -errno;
+    }
+    return OK;
+}
+
 } // namespace android
diff --git a/libs/binder/Utils.h b/libs/binder/Utils.h
index ff2fad8834..150d520492 100644
--- a/libs/binder/Utils.h
+++ b/libs/binder/Utils.h
@@ -20,6 +20,7 @@
 #include <android-base/result.h>
 #include <android-base/unique_fd.h>
 #include <log/log.h>
+#include <utils/Errors.h>
 
 #define TEST_AND_RETURN(value, expr)            \
     do {                                        \
@@ -36,4 +37,6 @@ void zeroMemory(uint8_t* data, size_t size);
 
 android::base::Result<void> setNonBlocking(android::base::borrowed_fd fd);
 
+status_t getRandomBytes(uint8_t* data, size_t size);
+
 }   // namespace android