[libc-commits] [libc] Code Coverage Prototype - Linux Extraction Hook (PR #210062)
Tapiwa Gonga via libc-commits
libc-commits at lists.llvm.org
Fri Jul 17 06:46:50 PDT 2026
https://github.com/tapiwagonga updated https://github.com/llvm/llvm-project/pull/210062
>From f4f50f1df5d1b8e8faf74583cacfc13bd3e3bcf1 Mon Sep 17 00:00:00 2001
From: Tapiwa Gonga <tapiwagonga at google.com>
Date: Thu, 16 Jul 2026 13:26:20 +0000
Subject: [PATCH 1/2] [libc][WIP] Implement freestanding coverage extraction
prototype using raw syscalls
---
libc/test/UnitTest/LibcTestMain.cpp | 91 ++++++++++++++++++++++++++++-
1 file changed, 88 insertions(+), 3 deletions(-)
diff --git a/libc/test/UnitTest/LibcTestMain.cpp b/libc/test/UnitTest/LibcTestMain.cpp
index c348d5ef1aa1b..00f747d1f6f1e 100644
--- a/libc/test/UnitTest/LibcTestMain.cpp
+++ b/libc/test/UnitTest/LibcTestMain.cpp
@@ -43,8 +43,90 @@ TestOptions parseOptions(int argc, char **argv) {
} // anonymous namespace
-// The C++ standard forbids declaring the main function with a linkage specifier
-// outisde of 'freestanding' mode, only define the linkage for hermetic tests.
+#if defined(__linux__)
+#include "src/__support/OSUtil/syscall.h"
+#include <errno.h>
+#include <fcntl.h>
+#include <sys/mman.h>
+#include <sys/syscall.h>
+#include <time.h>
+
+extern "C" {
+__attribute__((weak)) uint64_t __llvm_profile_get_size_for_buffer(void);
+__attribute__((weak)) int __llvm_profile_write_buffer(char *Buffer);
+__attribute__((weak)) void __llvm_profile_set_filename(const char *FilenamePat);
+
+// Override compiler-rt's weak filename symbol. This redirects the default
+// filename to /dev/null to silence the default dumper by default.
+char __llvm_profile_filename[] = "/dev/null";
+}
+
+namespace {
+void write_raw_profile() {
+ if (!__llvm_profile_get_size_for_buffer || !__llvm_profile_write_buffer)
+ return;
+
+ uint64_t required_size = __llvm_profile_get_size_for_buffer();
+ if (required_size == 0)
+ return;
+
+ long mmap_ret = LIBC_NAMESPACE::syscall_impl<long>(
+ SYS_mmap, nullptr, required_size, PROT_READ | PROT_WRITE,
+ MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+ if (mmap_ret < 0 && mmap_ret > -4096)
+ return;
+ char *profile_buffer = reinterpret_cast<char *>(mmap_ret);
+
+ if (__llvm_profile_write_buffer(profile_buffer) != 0) {
+ LIBC_NAMESPACE::syscall_impl<long>(SYS_munmap, profile_buffer, required_size);
+ return;
+ }
+
+ // Create a minimal filename: libc_cov_<pid>.profraw
+ long pid = LIBC_NAMESPACE::syscall_impl<long>(SYS_getpid);
+ if (pid <= 0) pid = 1;
+
+ char filename[64] = "libc_cov_";
+ int idx = 9;
+
+ long temp = pid;
+ char digits[32];
+ int d_len = 0;
+ while (temp > 0) { digits[d_len++] = static_cast<char>('0' + (temp % 10)); temp /= 10; }
+ if (d_len == 0) digits[d_len++] = '0';
+ for (int i = d_len - 1; i >= 0; --i) filename[idx++] = digits[i];
+
+ const char *suffix = ".profraw";
+ for (int i = 0; suffix[i] != '\0'; ++i) filename[idx++] = suffix[i];
+ filename[idx] = '\0';
+
+ long fd = LIBC_NAMESPACE::syscall_impl<long>(
+ SYS_openat, AT_FDCWD, filename, O_WRONLY | O_CREAT | O_TRUNC, 0644);
+
+ if (fd >= 0) {
+ uint64_t bytes_written = 0;
+ while (bytes_written < required_size) {
+ long ret = LIBC_NAMESPACE::syscall_impl<long>(
+ SYS_write, fd, profile_buffer + bytes_written, required_size - bytes_written);
+ if (ret <= 0 && ret != -EINTR) break;
+ if (ret > 0) bytes_written += ret;
+ }
+ LIBC_NAMESPACE::syscall_impl<long>(SYS_close, fd);
+ }
+
+ LIBC_NAMESPACE::syscall_impl<long>(SYS_munmap, profile_buffer, required_size);
+
+ // Clear the filename pattern to prevent compiler-rt from writing at exit.
+ if (__llvm_profile_set_filename)
+ __llvm_profile_set_filename("/dev/null");
+}
+} // anonymous namespace
+#else
+namespace {
+void write_raw_profile() {}
+} // anonymous namespace
+#endif
+
#if __STDC_HOSTED__
#define TEST_MAIN int main
#else
@@ -56,5 +138,8 @@ TEST_MAIN(int argc, char **argv, char **envp) {
LIBC_NAMESPACE::testing::argv = argv;
LIBC_NAMESPACE::testing::envp = envp;
- return LIBC_NAMESPACE::testing::Test::runTests(parseOptions(argc, argv));
+ int result =
+ LIBC_NAMESPACE::testing::Test::runTests(parseOptions(argc, argv));
+ write_raw_profile();
+ return result;
}
>From 6dccbeeddbed1442917885a1a91398c4e5e521b3 Mon Sep 17 00:00:00 2001
From: Tapiwa Gonga <tapiwagonga at google.com>
Date: Fri, 17 Jul 2026 13:45:22 +0000
Subject: [PATCH 2/2] [libc] Integrate robust error handling and linux_syscall
wrappers for coverage
---
libc/test/UnitTest/CMakeLists.txt | 11 +++
libc/test/UnitTest/LibcTestMain.cpp | 123 ++++++++++++++++++----------
2 files changed, 93 insertions(+), 41 deletions(-)
diff --git a/libc/test/UnitTest/CMakeLists.txt b/libc/test/UnitTest/CMakeLists.txt
index 4a47597ae3a45..b8142f48ef08a 100644
--- a/libc/test/UnitTest/CMakeLists.txt
+++ b/libc/test/UnitTest/CMakeLists.txt
@@ -69,6 +69,16 @@ else()
set(test_logger_osutil "libc.src.__support.OSUtil.osutil")
endif()
+if(LIBC_TARGET_OS STREQUAL "linux")
+ set(libctest_syscall_wrappers
+ libc.src.__support.OSUtil.linux.syscall_wrappers.close
+ libc.src.__support.OSUtil.linux.syscall_wrappers.mmap
+ libc.src.__support.OSUtil.linux.syscall_wrappers.munmap
+ libc.src.__support.OSUtil.linux.syscall_wrappers.open
+ libc.src.__support.OSUtil.linux.syscall_wrappers.write
+ )
+endif()
+
add_unittest_framework_library(
LibcTest
SRCS
@@ -94,6 +104,7 @@ add_unittest_framework_library(
libc.src.__support.macros.properties.types
libc.src.__support.uint128
${test_logger_osutil}
+ ${libctest_syscall_wrappers}
)
set(libc_death_test_srcs LibcDeathTestExecutors.cpp)
diff --git a/libc/test/UnitTest/LibcTestMain.cpp b/libc/test/UnitTest/LibcTestMain.cpp
index 00f747d1f6f1e..8ddb1349d17d5 100644
--- a/libc/test/UnitTest/LibcTestMain.cpp
+++ b/libc/test/UnitTest/LibcTestMain.cpp
@@ -44,17 +44,24 @@ TestOptions parseOptions(int argc, char **argv) {
} // anonymous namespace
#if defined(__linux__)
+#include "hdr/errno_macros.h"
+#include "hdr/fcntl_macros.h"
+#include "hdr/sys_mman_macros.h"
+#include "src/__support/OSUtil/linux/syscall_wrappers/close.h"
+#include "src/__support/OSUtil/linux/syscall_wrappers/mmap.h"
+#include "src/__support/OSUtil/linux/syscall_wrappers/munmap.h"
+#include "src/__support/OSUtil/linux/syscall_wrappers/open.h"
+#include "src/__support/OSUtil/linux/syscall_wrappers/write.h"
#include "src/__support/OSUtil/syscall.h"
-#include <errno.h>
-#include <fcntl.h>
-#include <sys/mman.h>
+#include "src/__support/integer_to_string.h"
+#include "src/string/memory_utils/inline_memcpy.h"
#include <sys/syscall.h>
-#include <time.h>
extern "C" {
__attribute__((weak)) uint64_t __llvm_profile_get_size_for_buffer(void);
-__attribute__((weak)) int __llvm_profile_write_buffer(char *Buffer);
-__attribute__((weak)) void __llvm_profile_set_filename(const char *FilenamePat);
+__attribute__((weak)) int __llvm_profile_write_buffer(char *buffer);
+__attribute__((weak)) void
+__llvm_profile_set_filename(const char *filename_pat);
// Override compiler-rt's weak filename symbol. This redirects the default
// filename to /dev/null to silence the default dumper by default.
@@ -62,6 +69,26 @@ char __llvm_profile_filename[] = "/dev/null";
}
namespace {
+struct FixedSizeBuffer {
+ char data[64];
+ size_t idx = 0;
+
+ FixedSizeBuffer() { data[0] = '\0'; }
+
+ bool append(string_view str) {
+ if (idx + str.size() >= sizeof(data))
+ return false;
+ LIBC_NAMESPACE::inline_memcpy(data + idx, str.data(), str.size());
+ idx += str.size();
+ data[idx] = '\0';
+ return true;
+ }
+};
+
+LIBC_INLINE void report_error(string_view msg) {
+ LIBC_NAMESPACE::syscall_impl<long>(SYS_write, 2, msg.data(), msg.size());
+}
+
void write_raw_profile() {
if (!__llvm_profile_get_size_for_buffer || !__llvm_profile_write_buffer)
return;
@@ -70,51 +97,65 @@ void write_raw_profile() {
if (required_size == 0)
return;
- long mmap_ret = LIBC_NAMESPACE::syscall_impl<long>(
- SYS_mmap, nullptr, required_size, PROT_READ | PROT_WRITE,
+ auto mmap_or_error = LIBC_NAMESPACE::linux_syscalls::mmap(
+ nullptr, required_size, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
- if (mmap_ret < 0 && mmap_ret > -4096)
- return;
- char *profile_buffer = reinterpret_cast<char *>(mmap_ret);
+ if (!mmap_or_error)
+ return report_error("error: libc coverage failed to mmap buffer\n");
+ char *profile_buffer = reinterpret_cast<char *>(mmap_or_error.value());
if (__llvm_profile_write_buffer(profile_buffer) != 0) {
- LIBC_NAMESPACE::syscall_impl<long>(SYS_munmap, profile_buffer, required_size);
- return;
+ LIBC_NAMESPACE::linux_syscalls::munmap(profile_buffer, required_size);
+ return report_error(
+ "error: libc coverage failed to write profile buffer\n");
}
// Create a minimal filename: libc_cov_<pid>.profraw
long pid = LIBC_NAMESPACE::syscall_impl<long>(SYS_getpid);
- if (pid <= 0) pid = 1;
-
- char filename[64] = "libc_cov_";
- int idx = 9;
-
- long temp = pid;
- char digits[32];
- int d_len = 0;
- while (temp > 0) { digits[d_len++] = static_cast<char>('0' + (temp % 10)); temp /= 10; }
- if (d_len == 0) digits[d_len++] = '0';
- for (int i = d_len - 1; i >= 0; --i) filename[idx++] = digits[i];
-
- const char *suffix = ".profraw";
- for (int i = 0; suffix[i] != '\0'; ++i) filename[idx++] = suffix[i];
- filename[idx] = '\0';
-
- long fd = LIBC_NAMESPACE::syscall_impl<long>(
- SYS_openat, AT_FDCWD, filename, O_WRONLY | O_CREAT | O_TRUNC, 0644);
-
- if (fd >= 0) {
- uint64_t bytes_written = 0;
- while (bytes_written < required_size) {
- long ret = LIBC_NAMESPACE::syscall_impl<long>(
- SYS_write, fd, profile_buffer + bytes_written, required_size - bytes_written);
- if (ret <= 0 && ret != -EINTR) break;
- if (ret > 0) bytes_written += ret;
+ if (pid <= 0)
+ pid = 1;
+
+ FixedSizeBuffer filename;
+ LIBC_NAMESPACE::IntegerToString<long> pid_str(pid);
+ if (!filename.append("libc_cov_") || !filename.append(pid_str.view()) ||
+ !filename.append(".profraw")) {
+ LIBC_NAMESPACE::linux_syscalls::munmap(profile_buffer, required_size);
+ return report_error("error: libc coverage filename buffer overflow\n");
+ }
+
+ auto fd_or_error = LIBC_NAMESPACE::linux_syscalls::open(
+ filename.data, O_WRONLY | O_CREAT | O_TRUNC, 0644);
+ if (!fd_or_error) {
+ LIBC_NAMESPACE::linux_syscalls::munmap(profile_buffer, required_size);
+ return report_error("error: libc coverage failed to open output file\n");
+ }
+ int fd = fd_or_error.value();
+
+ uint64_t bytes_written = 0;
+ bool write_error_occurred = false;
+ while (bytes_written < required_size) {
+ auto write_or_error = LIBC_NAMESPACE::linux_syscalls::write(
+ fd, profile_buffer + bytes_written, required_size - bytes_written);
+ if (!write_or_error) {
+ if (write_or_error.error() == EINTR)
+ continue;
+ write_error_occurred = true;
+ break;
}
- LIBC_NAMESPACE::syscall_impl<long>(SYS_close, fd);
+ ssize_t ret = write_or_error.value();
+ if (ret == 0) {
+ write_error_occurred = true;
+ break;
+ }
+ bytes_written += ret;
}
- LIBC_NAMESPACE::syscall_impl<long>(SYS_munmap, profile_buffer, required_size);
+ LIBC_NAMESPACE::linux_syscalls::close(fd);
+ LIBC_NAMESPACE::linux_syscalls::munmap(profile_buffer, required_size);
+
+ if (write_error_occurred || bytes_written < required_size)
+ return report_error(
+ "error: libc coverage failed to write all data to file\n");
// Clear the filename pattern to prevent compiler-rt from writing at exit.
if (__llvm_profile_set_filename)
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