Implement basic `read` and `write` interposition

The goal is to implement a shared library to be loaded using `LD_PRELOAD` that tracks all file changes performed by a process.
This information is required to enable general purpose change logging inside a shell session. i.e. I want to be able to display diffs and before- / after-hashes of all changes performed by every single command without requiring every changed file to be known prior to execution.

1 files changed, 76 insertions, 0 deletions

diff --git a/change_log.cc b/change_log.cc
new file mode 100644
index 0000000..d4c3dc4
--- /dev/null
+++ b/change_log.cc
@@ -0,0 +1,76 @@
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
+#include <unistd.h>
+#include <dlfcn.h>
+#include <sys/stat.h>
+
+#include <cstring>
+#include <iostream>
+#include <functional>
+
+template <class Result, typename... Arguments>
+std::function<Result(Arguments...)> get_real_function(
+	const std::string& symbol_name) {
+
+	Result (*real_function)(Arguments...) = nullptr;
+	const void* symbol_address = dlsym(RTLD_NEXT, symbol_name.c_str());
+
+	std::memcpy(&real_function, &symbol_address, sizeof(symbol_address));
+
+	return std::function<Result(Arguments...)>(real_function);
+}
+
+std::string get_file_name(int fd) {
+	char proc_link[20];
+	char file_name[256];
+
+	snprintf(proc_link, sizeof(proc_link), "/proc/self/fd/%d", fd);
+	const ssize_t name_size = readlink(proc_link, file_name, sizeof(file_name));
+
+	if ( name_size > 0 ) {
+		file_name[name_size] = '\0';
+
+		return std::string(file_name);
+	} else {
+		return std::string();
+	}
+}
+
+bool is_regular_file(int fd) {
+	struct stat fd_stat;
+	fstat(fd, &fd_stat);
+
+	return S_ISREG(fd_stat.st_mode);
+}
+
+ssize_t read(int fd, void *buffer, size_t count) {
+	static auto real_read = get_real_function<ssize_t, int, void*, size_t>("read");
+
+	const ssize_t result = real_read(fd, buffer, count);
+
+	if ( is_regular_file(fd) ) {
+		std::cerr << "read size "
+		          << count
+		          << " of "
+		          << get_file_name(fd)
+		          << std::endl;
+	}
+
+	return result;
+}
+
+ssize_t write(int fd, const void* buffer, size_t count) {
+	static auto real_write = get_real_function<ssize_t, int, const void*, size_t>("write");
+
+	if ( is_regular_file(fd) ) {
+		std::cerr << "write size "
+		          << count
+		          << " to "
+		          << get_file_name(fd)
+		          << std::endl;
+	}
+
+	return real_write(fd, buffer, count);
+}