[libc] implement unistd/getentropy (#122692)

SchrodingerZhu · web-flow · commit defd0d966d5e · 2025-01-15T18:27:05.000+08:00
Implement GNU extension getentropy. This function is used by many
programs to acquire entropy without handling the loop of getrandom.
diff --git a/libc/config/linux/aarch64/entrypoints.txt b/libc/config/linux/aarch64/entrypoints.txt
@@ -322,6 +322,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.unistd.fsync
     libc.src.unistd.ftruncate
     libc.src.unistd.getcwd
+    libc.src.unistd.getentropy
     libc.src.unistd.geteuid
     libc.src.unistd.getpid
     libc.src.unistd.getppid
diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
@@ -321,6 +321,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.unistd.fsync
     libc.src.unistd.ftruncate
     libc.src.unistd.getcwd
+    libc.src.unistd.getentropy
     libc.src.unistd.geteuid
     libc.src.unistd.getpid
     libc.src.unistd.getppid
diff --git a/libc/config/windows/entrypoints.txt b/libc/config/windows/entrypoints.txt
@@ -101,6 +101,9 @@ set(TARGET_LIBC_ENTRYPOINTS
     # time.h entrypoints
     libc.src.time.time
     libc.src.time.clock_getres
+
+    # unistd.h entrypoints
+    libc.src.unistd.getentropy
 )
 
 set(TARGET_LIBM_ENTRYPOINTS
diff --git a/libc/config/windows/headers.txt b/libc/config/windows/headers.txt
@@ -6,4 +6,5 @@ set(TARGET_PUBLIC_HEADERS
     libc.include.errno
     libc.include.fenv
     libc.include.math
+    libc.include.unistd
 )
diff --git a/libc/include/sys/random.yaml b/libc/include/sys/random.yaml
@@ -15,3 +15,10 @@ functions:
       - type: void *
       - type: size_t
       - type: unsigned int
+  - name: getentropy
+    standards:
+      - GNUExtensions
+    return_type: int
+    arguments:
+      - type: void *
+      - type: size_t
diff --git a/libc/include/unistd.yaml b/libc/include/unistd.yaml
@@ -128,6 +128,13 @@ functions:
     arguments:
       - type: char *
       - type: size_t
+  - name: getentropy
+    standards:
+      - GNUExtensions
+    return_type: int
+    arguments:
+      - type: void *
+      - type: size_t
   - name: geteuid
     standards:
       - POSIX
diff --git a/libc/src/CMakeLists.txt b/libc/src/CMakeLists.txt
@@ -15,6 +15,7 @@ add_subdirectory(string)
 add_subdirectory(strings)
 add_subdirectory(wchar)
 add_subdirectory(time)
+add_subdirectory(unistd)
 
 if(${LIBC_TARGET_OS} STREQUAL "linux")
   add_subdirectory(dirent)
@@ -23,7 +24,6 @@ if(${LIBC_TARGET_OS} STREQUAL "linux")
   add_subdirectory(sched)
   add_subdirectory(sys)
   add_subdirectory(termios)
-  add_subdirectory(unistd)
 endif()
 
 if(NOT LLVM_LIBC_FULL_BUILD)
diff --git a/libc/src/unistd/CMakeLists.txt b/libc/src/unistd/CMakeLists.txt
@@ -350,3 +350,10 @@ add_entrypoint_object(
   DEPENDS
     libc.src.__support.threads.identifier
 )
+
+add_entrypoint_object(
+  getentropy
+  ALIAS
+  DEPENDS
+    .${LIBC_TARGET_OS}.getentropy
+)
diff --git a/libc/src/unistd/getentropy.h b/libc/src/unistd/getentropy.h
@@ -0,0 +1,19 @@
+//===-- Implementation header for getentropy ------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "hdr/types/size_t.h"
+#include "src/__support/common.h"
+
+#ifndef LLVM_LIBC_SRC_UNISTD_GETENTROPY_H
+#define LLVM_LIBC_SRC_UNISTD_GETENTROPY_H
+
+namespace LIBC_NAMESPACE_DECL {
+int getentropy(void *buffer, size_t length);
+}
+
+#endif // LLVM_LIBC_SRC_UNISTD_GETENTROPY_H
diff --git a/libc/src/unistd/linux/CMakeLists.txt b/libc/src/unistd/linux/CMakeLists.txt
@@ -570,3 +570,18 @@ add_entrypoint_object(
     libc.src.__support.OSUtil.osutil
     libc.src.errno.errno
 )
+
+add_entrypoint_object(
+  getentropy
+  SRCS
+    getentropy.cpp
+  HDRS
+    ../getentropy.h
+  DEPENDS
+    libc.hdr.types.size_t
+    libc.hdr.types.ssize_t
+    libc.hdr.errno_macros
+    libc.include.sys_syscall
+    libc.src.__support.OSUtil.osutil
+    libc.src.errno.errno
+)
diff --git a/libc/src/unistd/linux/getentropy.cpp b/libc/src/unistd/linux/getentropy.cpp
@@ -0,0 +1,51 @@
+//===-- Linux implementation of getentropy --------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/unistd/getentropy.h"
+#include "hdr/errno_macros.h"
+#include "src/__support/OSUtil/syscall.h"
+#include "src/__support/common.h"
+#include "src/errno/libc_errno.h"
+
+#include <sys/syscall.h> // For syscall numbers.
+
+namespace LIBC_NAMESPACE_DECL {
+LLVM_LIBC_FUNCTION(int, getentropy, (void *buffer, size_t length)) {
+  // check the length limit
+  if (length > 256) {
+    libc_errno = EIO;
+    return -1;
+  }
+
+  char *cursor = static_cast<char *>(buffer);
+  while (length != 0) {
+    // 0 flag means urandom and blocking, which meets the assumption of
+    // getentropy
+    auto ret = syscall_impl<long>(SYS_getrandom, cursor, length, 0);
+
+    // on success, advance the buffer pointer
+    if (ret >= 0) {
+      length -= static_cast<size_t>(ret);
+      cursor += ret;
+      continue;
+    }
+
+    auto error = -static_cast<int>(ret);
+
+    // on EINTR, try again
+    if (error == EINTR)
+      continue;
+
+    // on ENOSYS, forward errno and exit;
+    // otherwise, set EIO and exit
+    libc_errno = (error == ENOSYS) ? ENOSYS : EIO;
+    return -1;
+  }
+  return 0;
+}
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/unistd/windows/CMakeLists.txt b/libc/src/unistd/windows/CMakeLists.txt
@@ -0,0 +1,11 @@
+add_entrypoint_object(
+  getentropy
+  SRCS
+    getentropy.cpp
+  HDRS
+    ../getentropy.h
+  DEPENDS
+    libc.hdr.types.size_t
+    libc.hdr.errno_macros
+    libc.src.errno.errno
+)
diff --git a/libc/src/unistd/windows/getentropy.cpp b/libc/src/unistd/windows/getentropy.cpp
@@ -0,0 +1,42 @@
+//===-- Windows implementation of getentropy ------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/unistd/getentropy.h"
+#include "hdr/errno_macros.h"
+#include "src/__support/common.h"
+#include "src/errno/libc_errno.h"
+
+#define WIN32_LEAN_AND_MEAN
+#include <Windows.h>
+#include <bcrypt.h>
+#include <ntstatus.h>
+#pragma comment(lib, "bcrypt.lib")
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(int, getentropy, (void *buffer, size_t length)) {
+  __try {
+    // check the length limit
+    if (length > 256)
+      __leave;
+
+    NTSTATUS result = ::BCryptGenRandom(nullptr, static_cast<PUCHAR>(buffer),
+                                        static_cast<ULONG>(length),
+                                        BCRYPT_USE_SYSTEM_PREFERRED_RNG);
+
+    if (result == STATUS_SUCCESS)
+      return 0;
+
+  } __except (EXCEPTION_EXECUTE_HANDLER) {
+    // no need to handle exceptions specially
+  }
+
+  libc_errno = EIO;
+  return -1;
+}
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/test/src/CMakeLists.txt b/libc/test/src/CMakeLists.txt
@@ -61,6 +61,7 @@ add_subdirectory(string)
 add_subdirectory(strings)
 add_subdirectory(wchar)
 add_subdirectory(time)
+add_subdirectory(unistd)
 
 # Depends on utilities in stdlib
 add_subdirectory(inttypes)
@@ -70,7 +71,6 @@ if(${LIBC_TARGET_OS} STREQUAL "linux")
   add_subdirectory(sched)
   add_subdirectory(sys)
   add_subdirectory(termios)
-  add_subdirectory(unistd)
 endif()
 
 if(NOT LLVM_LIBC_FULL_BUILD)
diff --git a/libc/test/src/unistd/CMakeLists.txt b/libc/test/src/unistd/CMakeLists.txt
@@ -488,6 +488,18 @@ add_libc_test(
     libc.src.stdio.fflush
 )
 
+add_libc_test(
+  getentropy_test
+  SUITE
+    libc_unistd_unittests
+  SRCS
+    getentropy_test.cpp
+  DEPENDS
+    libc.src.unistd.getentropy
+    libc.src.errno.errno
+    libc.test.UnitTest.ErrnoSetterMatcher
+)
+
 if(LLVM_LIBC_FULL_BUILD)
   add_libc_test(
     _exit_test
diff --git a/libc/test/src/unistd/getentropy_test.cpp b/libc/test/src/unistd/getentropy_test.cpp
@@ -0,0 +1,28 @@
+//===-- Unittests for getentropy ------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "hdr/errno_macros.h"
+#include "src/unistd/getentropy.h"
+#include "test/UnitTest/ErrnoSetterMatcher.h"
+#include "test/UnitTest/Test.h"
+
+using namespace LIBC_NAMESPACE::testing::ErrnoSetterMatcher;
+
+TEST(LlvmLibcUnistdGetEntropyTest, LengthTooLong) {
+  char buf[1024];
+  ASSERT_THAT(LIBC_NAMESPACE::getentropy(buf, 257), Fails(EIO));
+}
+
+TEST(LlvmLibcUnistdGetEntropyTest, SmokeTest) {
+  char buf[256];
+  ASSERT_THAT(LIBC_NAMESPACE::getentropy(buf, 256), Succeeds());
+}
+
+TEST(LlvmLibcUnistdGetEntropyTest, OtherError) {
+  ASSERT_THAT(LIBC_NAMESPACE::getentropy(nullptr, 1), Fails(EIO));
+}

Original file line number	Diff line number	Diff line change
`@@ -101,6 +101,9 @@ set(TARGET_LIBC_ENTRYPOINTS`
`101`	`101`	`# time.h entrypoints`
`102`	`102`	`libc.src.time.time`
`103`	`103`	`libc.src.time.clock_getres`
	`104`	`+`
	`105`	`+ # unistd.h entrypoints`
	`106`	`+ libc.src.unistd.getentropy`
`104`	`107`	`)`
`105`	`108`
`106`	`109`	`set(TARGET_LIBM_ENTRYPOINTS`
Original file line number	Diff line number	Diff line change
`@@ -6,4 +6,5 @@ set(TARGET_PUBLIC_HEADERS`
`6`	`6`	`libc.include.errno`
`7`	`7`	`libc.include.fenv`
`8`	`8`	`libc.include.math`
	`9`	`+ libc.include.unistd`
`9`	`10`	`)`