open_log.c

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <errno.h>

static FILE *log_file = NULL;

static void init_log(void)
{
	const char *log_path;

	if (log_file)
		return;

	log_path = getenv("GWLOG_PATH");
	if (!log_path)
		log_path = "/dev/null";

	log_file = fopen(log_path, "a");
	assert(log_file);
}

int open(const char *file, int flags, int mode)
{
	int fd;

	__asm__ volatile (
		"syscall"
		: "=a" (fd)	/* %rax */
		: "a" (2),	/* %rax */
		  "D" (file),	/* %rdi */
		  "S" (flags),	/* %rsi */
		  "d" (mode)	/* %rdx */
		: "memory", "rcx", "r11", "cc"
	);

	init_log();
	fprintf(log_file, "open(\"%s\", %d, %d) = %d\n", file, flags, mode, fd);
	fflush(log_file);

	if (fd < 0) {
		errno = -fd;
		fd = -1;
	}

	return fd;
}

int openat(int fd, const char *file, int flags, int mode)
{
	register int __mode __asm__("%r10") = mode;
	int fd2;

	__asm__ volatile (
		"syscall"
		: "=a" (fd2)	/* %rax */
		: "a" (257),	/* %rax */
		  "D" (fd),	/* %rdi */
		  "S" (file),	/* %rsi */
		  "d" (flags),	/* %rdx */
		  "r" (__mode)	/* %r10 */
		: "memory", "rcx", "r11", "cc"
	);

	init_log();
	fprintf(log_file, "openat(%d, %s, %d, %d) = %d\n", fd, file, flags, mode, fd2);
	fflush(log_file);

	if (fd2 < 0) {
		errno = -fd2;
		fd2 = -1;
	}

	return fd2;
}