[libc] Set default visibility to 'hidden' and make entrypoints default

jhuber6 · jhuber6 · commit 1819de0f0f4c · 2024-07-03T14:32:21.000-05:00
Summary: See for visibility: https://llvm.org/docs/LangRef.html#visibility-styles Currently we build everything with default visibility, meaning that all internal symbols are preemptable and visible to any `.so` they're linked into. What we want is hidden visibility for all the internal parts, and default visibility for the API functions / exposed globals. This patch is based on #97109.
diff --git a/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake b/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
@@ -41,6 +41,7 @@ function(_get_common_compile_options output_var flags)
 
   if(LLVM_COMPILER_IS_GCC_COMPATIBLE)
     list(APPEND compile_options "-fpie")
+    list(APPEND compile_options "-fvisibility=hidden")
 
     if(LLVM_LIBC_FULL_BUILD)
       list(APPEND compile_options "-DLIBC_FULL_BUILD")
@@ -78,6 +79,9 @@ function(_get_common_compile_options output_var flags)
     if (LIBC_CONF_ENABLE_STACK_PROTECTOR)
       list(APPEND compile_options "-fstack-protector-strong")
     endif()
+    if(LIBC_CONF_VISIBILITY)
+      list(APPEND compile_options "-DLIBC_VISIBILITY=${LIBC_CONF_VISIBILITY}")
+    endif()
     list(APPEND compile_options "-Wall")
     list(APPEND compile_options "-Wextra")
     # -DLIBC_WNO_ERROR=ON if you can't build cleanly with -Werror.
@@ -102,7 +106,6 @@ function(_get_common_compile_options output_var flags)
   endif()
   if (LIBC_TARGET_OS_IS_GPU)
     list(APPEND compile_options "-nogpulib")
-    list(APPEND compile_options "-fvisibility=hidden")
     list(APPEND compile_options "-fconvergent-functions")
     list(APPEND compile_options "-flto")
     list(APPEND compile_options "-Wno-multi-gpu")
diff --git a/libc/config/config.json b/libc/config/config.json
@@ -32,6 +32,10 @@
     }
   },
   "codegen": {
+    "LIBC_CONF_VISIBILITY": {
+      "value": "default",
+      "doc": "Visibility to use on all exported C library symbols."
+    },
     "LIBC_CONF_KEEP_FRAME_POINTER": {
       "value": true,
       "doc": "Keep frame pointer in functions for better debugging experience."
diff --git a/libc/config/gpu/config.json b/libc/config/gpu/config.json
@@ -12,5 +12,10 @@
     "LIBC_CONF_PRINTF_FLOAT_TO_STR_USE_MEGA_LONG_DOUBLE_TABLE": {
       "value": false
     }
+  },
+  "codegen": {
+    "LIBC_CONF_VISIBILITY": {
+      "value": "hidden"
+    }
   }
 }
diff --git a/libc/docs/configure.rst b/libc/docs/configure.rst
@@ -28,6 +28,7 @@ to learn about the defaults for your platform and target.
 * **"codegen" options**
     - ``LIBC_CONF_ENABLE_STRONG_STACK_PROTECTOR``: Enable -fstack-protector-strong to defend against stack smashing attack.
     - ``LIBC_CONF_KEEP_FRAME_POINTER``: Keep frame pointer in functions for better debugging experience.
+    - ``LIBC_CONF_VISIBILITY``: Visibility to use on all exported C library symbols.
 * **"malloc" options**
     - ``LIBC_CONF_FREELIST_MALLOC_BUFFER_SIZE``: Default size for the constinit freelist buffer used for the freelist malloc implementation (default 1o 1GB).
 * **"printf" options**
diff --git a/libc/docs/dev/entrypoints.rst b/libc/docs/dev/entrypoints.rst
@@ -5,4 +5,5 @@ Entrypoints in LLVM libc
 
 A public function or a global variable provided by LLVM-libc is called an
 entrypoint. The notion of entrypoints is ingrained in LLVM-libc's
-source layout, build system and source code.
+source layout, build system and source code. Entrypoints will be given the
+visibility defined in the ``LIBC_CONF_VISIBILITY`` configuration.
diff --git a/libc/src/__support/File/linux/stderr.cpp b/libc/src/__support/File/linux/stderr.cpp
@@ -6,7 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "file.h"
+#include "src/__support/File/linux/file.h"
+#include "src/__support/common.h"
+
 #include <stdio.h>
 
 namespace LIBC_NAMESPACE {
@@ -18,6 +20,5 @@ File *stderr = &StdErr;
 
 } // namespace LIBC_NAMESPACE
 
-extern "C" {
-FILE *stderr = reinterpret_cast<FILE *>(&LIBC_NAMESPACE::StdErr);
-}
+LLVM_LIBC_GLOBAL(FILE *,
+                 stderr) = reinterpret_cast<FILE *>(&LIBC_NAMESPACE::StdErr);
diff --git a/libc/src/__support/File/linux/stdin.cpp b/libc/src/__support/File/linux/stdin.cpp
@@ -6,7 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "file.h"
+#include "src/__support/File/linux/file.h"
+#include "src/__support/common.h"
+
 #include <stdio.h>
 
 namespace LIBC_NAMESPACE {
@@ -19,6 +21,5 @@ File *stdin = &StdIn;
 
 } // namespace LIBC_NAMESPACE
 
-extern "C" {
-FILE *stdin = reinterpret_cast<FILE *>(&LIBC_NAMESPACE::StdIn);
-} // extern "C"
+LLVM_LIBC_GLOBAL(FILE *,
+                 stdin) = reinterpret_cast<FILE *>(&LIBC_NAMESPACE::StdIn);
diff --git a/libc/src/__support/File/linux/stdout.cpp b/libc/src/__support/File/linux/stdout.cpp
@@ -6,7 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "file.h"
+#include "src/__support/File/linux/file.h"
+#include "src/__support/common.h"
+
 #include <stdio.h>
 
 namespace LIBC_NAMESPACE {
@@ -19,6 +21,5 @@ File *stdout = &StdOut;
 
 } // namespace LIBC_NAMESPACE
 
-extern "C" {
-FILE *stdout = reinterpret_cast<FILE *>(&LIBC_NAMESPACE::StdOut);
-} // extern "C"
+LLVM_LIBC_GLOBAL(FILE *,
+                 stdout) = reinterpret_cast<FILE *>(&LIBC_NAMESPACE::StdOut);
diff --git a/libc/src/__support/common.h b/libc/src/__support/common.h
@@ -16,14 +16,26 @@
 #include "src/__support/macros/attributes.h"
 #include "src/__support/macros/properties/architectures.h"
 
+#define __LIBC_MACRO_TO_STRING(str) #str
+#define LIBC_MACRO_TO_STRING(str) __LIBC_MACRO_TO_STRING(str)
+
 #ifndef LLVM_LIBC_FUNCTION_ATTR
 #define LLVM_LIBC_FUNCTION_ATTR
 #endif
 
+#ifndef LIBC_VISIBILITY
+#define LIBC_VISIBILITY default
+#endif
+
+#ifndef LLVM_LIBC_VISIBILITY
+#define LLVM_LIBC_VISIBILITY                                                   \
+  [[gnu::visibility(LIBC_MACRO_TO_STRING(LIBC_VISIBILITY))]]
+#endif
+
 // MacOS needs to be excluded because it does not support aliasing.
 #if defined(LIBC_COPT_PUBLIC_PACKAGING) && (!defined(__APPLE__))
 #define LLVM_LIBC_FUNCTION_IMPL(type, name, arglist)                           \
-  LLVM_LIBC_FUNCTION_ATTR decltype(LIBC_NAMESPACE::name)                       \
+  LLVM_LIBC_FUNCTION_ATTR LLVM_LIBC_VISIBILITY decltype(LIBC_NAMESPACE::name)  \
       __##name##_impl__ __asm__(#name);                                        \
   decltype(LIBC_NAMESPACE::name) name [[gnu::alias(#name)]];                   \
   type __##name##_impl__ arglist
@@ -35,6 +47,10 @@
 #define LLVM_LIBC_FUNCTION(type, name, arglist)                                \
   LLVM_LIBC_FUNCTION_IMPL(type, name, arglist)
 
+// Defines a global variable meant to be exported by the C library.
+#define LLVM_LIBC_GLOBAL(type, name)                                           \
+  LLVM_LIBC_VISIBILITY type name __asm__(#name)
+
 namespace LIBC_NAMESPACE {
 namespace internal {
 LIBC_INLINE constexpr bool same_string(char const *lhs, char const *rhs) {
@@ -46,9 +62,6 @@ LIBC_INLINE constexpr bool same_string(char const *lhs, char const *rhs) {
 } // namespace internal
 } // namespace LIBC_NAMESPACE
 
-#define __LIBC_MACRO_TO_STRING(str) #str
-#define LIBC_MACRO_TO_STRING(str) __LIBC_MACRO_TO_STRING(str)
-
 // LLVM_LIBC_IS_DEFINED checks whether a particular macro is defined.
 // Usage: constexpr bool kUseAvx = LLVM_LIBC_IS_DEFINED(__AVX__);
 //
diff --git a/libc/src/errno/libc_errno.cpp b/libc/src/errno/libc_errno.cpp
@@ -8,13 +8,12 @@
 
 #include "libc_errno.h"
 #include "src/__support/CPP/atomic.h"
+#include "src/__support/common.h"
 
 #ifdef LIBC_TARGET_ARCH_IS_GPU
 // LIBC_THREAD_LOCAL on GPU currently does nothing. So essentially this is just
 // a global errno for gpu to use for now.
-extern "C" {
-LIBC_THREAD_LOCAL LIBC_NAMESPACE::cpp::Atomic<int> __llvmlibc_errno;
-}
+LLVM_LIBC_GLOBAL(LIBC_NAMESPACE::cpp::Atomic<int>, __llvmlibc_errno);
 
 void LIBC_NAMESPACE::Errno::operator=(int a) {
   __llvmlibc_errno.store(a, cpp::MemoryOrder::RELAXED);
@@ -33,9 +32,7 @@ LIBC_NAMESPACE::Errno::operator int() { return __llvmlibc_internal_errno; }
 #elif defined(LIBC_FULL_BUILD)
 // This mode is for public libc archive, hermetic, and integration tests.
 // In full build mode, we provide the errno storage ourselves.
-extern "C" {
-LIBC_THREAD_LOCAL int __llvmlibc_errno;
-}
+LLVM_LIBC_GLOBAL(LIBC_THREAD_LOCAL int, __llvmlibc_errno);
 
 void LIBC_NAMESPACE::Errno::operator=(int a) { __llvmlibc_errno = a; }
 LIBC_NAMESPACE::Errno::operator int() { return __llvmlibc_errno; }
diff --git a/libc/src/stdio/generic/stderr.cpp b/libc/src/stdio/generic/stderr.cpp
@@ -7,7 +7,8 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/__support/File/file.h"
+#include "src/__support/common.h"
 
 #include <stdio.h>
 
-extern "C" FILE *stderr;
+LLVM_LIBC_GLOBAL(FILE *, stderr);
diff --git a/libc/src/stdio/generic/stdin.cpp b/libc/src/stdio/generic/stdin.cpp
@@ -7,7 +7,8 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/__support/File/file.h"
+#include "src/__support/common.h"
 
 #include <stdio.h>
 
-extern "C" FILE *stdin;
+LLVM_LIBC_GLOBAL(FILE *, stdin);
diff --git a/libc/src/stdio/generic/stdout.cpp b/libc/src/stdio/generic/stdout.cpp
@@ -7,7 +7,8 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/__support/File/file.h"
+#include "src/__support/common.h"
 
 #include <stdio.h>
 
-extern "C" FILE *stdout;
+LLVM_LIBC_GLOBAL(FILE *, stdout);
diff --git a/libc/src/stdio/gpu/stderr.cpp b/libc/src/stdio/gpu/stderr.cpp
@@ -6,11 +6,14 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "src/__support/common.h"
+
 #include <stdio.h>
 
 namespace LIBC_NAMESPACE {
 static struct {
 } stub;
 FILE *stderr = reinterpret_cast<FILE *>(&stub);
 } // namespace LIBC_NAMESPACE
-extern "C" FILE *stderr = reinterpret_cast<FILE *>(&LIBC_NAMESPACE::stub);
+LLVM_LIBC_GLOBAL(FILE *,
+                 stderr) = reinterpret_cast<FILE *>(&LIBC_NAMESPACE::stub);
diff --git a/libc/src/stdio/gpu/stdin.cpp b/libc/src/stdio/gpu/stdin.cpp
@@ -6,11 +6,14 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "src/__support/common.h"
+
 #include <stdio.h>
 
 namespace LIBC_NAMESPACE {
 static struct {
 } stub;
 FILE *stdin = reinterpret_cast<FILE *>(&stub);
 } // namespace LIBC_NAMESPACE
-extern "C" FILE *stdin = reinterpret_cast<FILE *>(&LIBC_NAMESPACE::stub);
+LLVM_LIBC_GLOBAL(FILE *,
+                 stdin) = reinterpret_cast<FILE *>(&LIBC_NAMESPACE::stub);
diff --git a/libc/src/stdio/gpu/stdout.cpp b/libc/src/stdio/gpu/stdout.cpp
@@ -6,11 +6,14 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "src/__support/common.h"
+
 #include <stdio.h>
 
 namespace LIBC_NAMESPACE {
 static struct {
 } stub;
 FILE *stdout = reinterpret_cast<FILE *>(&stub);
 } // namespace LIBC_NAMESPACE
-extern "C" FILE *stdout = reinterpret_cast<FILE *>(&LIBC_NAMESPACE::stub);
+LLVM_LIBC_GLOBAL(FILE *,
+                 stdout) = reinterpret_cast<FILE *>(&LIBC_NAMESPACE::stub);
diff --git a/libc/src/stdlib/atexit.cpp b/libc/src/stdlib/atexit.cpp
@@ -13,19 +13,16 @@
 
 namespace LIBC_NAMESPACE {
 
-extern "C" {
-
-int __cxa_atexit(AtExitCallback *callback, void *payload, void *) {
+LLVM_LIBC_FUNCTION(int, __cxa_atexit,
+                   (AtExitCallback * callback, void *payload, void *)) {
   return add_atexit_unit(atexit_callbacks, {callback, payload});
 }
 
-void __cxa_finalize(void *dso) {
+LLVM_LIBC_FUNCTION(void, __cxa_finalize, (void *dso)) {
   if (!dso)
     call_exit_callbacks(atexit_callbacks);
 }
 
-} // extern "C"
-
 LLVM_LIBC_FUNCTION(int, atexit, (__atexithandler_t callback)) {
   return add_atexit_unit(
       atexit_callbacks,
diff --git a/libc/src/stdlib/atexit.h b/libc/src/stdlib/atexit.h
@@ -10,8 +10,14 @@
 #define LLVM_LIBC_SRC_STDLIB_ATEXIT_H
 
 #include "hdr/types/atexithandler_t.h"
+#include "src/stdlib/exit_handler.h"
+
 namespace LIBC_NAMESPACE {
 
+int __cxa_atexit(AtExitCallback *callback, void *payload, void *);
+
+void __cxa_finalize(void *dso);
+
 int atexit(__atexithandler_t);
 
 } // namespace LIBC_NAMESPACE
diff --git a/libc/src/stdlib/exit.cpp b/libc/src/stdlib/exit.cpp
@@ -9,8 +9,7 @@
 #include "src/stdlib/exit.h"
 #include "src/__support/OSUtil/exit.h"
 #include "src/__support/common.h"
-
-extern "C" void __cxa_finalize(void *);
+#include "src/stdlib/atexit.h"
 
 namespace LIBC_NAMESPACE {
 
diff --git a/libc/src/unistd/environ.cpp b/libc/src/unistd/environ.cpp
@@ -6,11 +6,12 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "src/unistd/environ.h"
+#include "src/__support/common.h"
+
 namespace LIBC_NAMESPACE {
 
 // This is initialized to the correct value by the statup code.
-extern "C" {
-char **environ = nullptr;
-}
+LLVM_LIBC_GLOBAL(char **, environ) = nullptr;
 
 } // namespace LIBC_NAMESPACE
diff --git a/libc/src/unistd/getopt.cpp b/libc/src/unistd/getopt.cpp
@@ -175,12 +175,10 @@ int getopt_r(int argc, char *const argv[], const char *optstring,
 
 namespace impl {
 
-extern "C" {
-char *optarg = nullptr;
-int optind = 1;
-int optopt = 0;
-int opterr = 0;
-}
+LLVM_LIBC_GLOBAL(char, *optarg) = nullptr;
+LLVM_LIBC_GLOBAL(int, optind) = 1;
+LLVM_LIBC_GLOBAL(int, optopt) = 0;
+LLVM_LIBC_GLOBAL(int, opterr) = 0;
 
 static unsigned optpos;
 
diff --git a/libc/test/IntegrationTest/test.cpp b/libc/test/IntegrationTest/test.cpp
@@ -28,6 +28,8 @@ void *memcpy(void *__restrict, const void *__restrict, size_t);
 void *memmove(void *dst, const void *src, size_t count);
 void *memset(void *ptr, int value, size_t count);
 int atexit(void (*func)(void));
+int __cxa_atexit(void (*func)(void *), void *payload, void *dso);
+void __cxa_finalize(void *dso);
 
 } // namespace LIBC_NAMESPACE
 
@@ -53,6 +55,12 @@ void *memset(void *ptr, int value, size_t count) {
 // This is needed if the test was compiled with '-fno-use-cxa-atexit'.
 int atexit(void (*func)(void)) { return LIBC_NAMESPACE::atexit(func); }
 
+int __cxa_atexit(void (*callback)(void *), void *payload, void *dso) {
+  return LIBC_NAMESPACE::__cxa_atexit(callback, payload, dso);
+}
+
+void __cxa_finalize(void *dso) { LIBC_NAMESPACE::__cxa_finalize(dso); }
+
 } // extern "C"
 
 // Integration tests cannot use the SCUDO standalone allocator as SCUDO pulls
diff --git a/libc/test/UnitTest/HermeticTestUtils.cpp b/libc/test/UnitTest/HermeticTestUtils.cpp

Original file line number	Diff line number	Diff line change
`@@ -12,5 +12,10 @@`
`12`	`12`	`"LIBC_CONF_PRINTF_FLOAT_TO_STR_USE_MEGA_LONG_DOUBLE_TABLE": {`
`13`	`13`	`"value": false`
`14`	`14`	`}`
	`15`	`+ },`
	`16`	`+ "codegen": {`
	`17`	`+ "LIBC_CONF_VISIBILITY": {`
	`18`	`+ "value": "hidden"`
	`19`	`+ }`
`15`	`20`	`}`
`16`	`21`	`}`