sslkeylogfile.c

/* For RTLD_NEXT */
#define _GNU_SOURCE

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <dlfcn.h>
#include <errno.h>
#include <openssl/ssl.h>

/* Header of key log file. */
#define HEADER "# SSL key log file generated by sslkeylogfile.c\n"

/* Environment variable indicates filename of SSLKEYLOGFILE. */
#define FILENAME_ENVVAR "LIBSSL_SSLKEYLOGFILE"

static int fd_sslkeylogfile = -1;

static void open_sslkeylogfile(void);
static void write_sslkeylogfile(const SSL *ssl, const char *line);

SSL *SSL_new(SSL_CTX *ctx)
{
	SSL *(*SSL_new_orig)() = dlsym(RTLD_NEXT, __func__);

	if (SSL_new_orig == NULL) {
		fprintf(stderr, "Could not get symbol \"%s\": %s\n", __func__, dlerror());
		abort();
	}

	SSL_CTX_set_keylog_callback(ctx, write_sslkeylogfile);

	return SSL_new_orig(ctx);
}

static void open_sslkeylogfile(void)
{
	// Already opened, no need to do anything.
	if (fd_sslkeylogfile != -1)
		return;

	const char *filename = getenv(FILENAME_ENVVAR);

	// FILENAME_ENVVAR is not defined ...
	if (filename == NULL) {
		fprintf(stderr, "Environment variable \""FILENAME_ENVVAR"\" is not defined.");
		abort();
	}

	// Open log file in write-only and append mode,
	// create mode 0644 if not exist.
	fd_sslkeylogfile = open(
		filename,
		O_WRONLY | O_APPEND | O_CREAT,
		S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH
	);

	// If we could not do so ...
	if (fd_sslkeylogfile == -1) {
		fprintf(stderr, "Failed open SSLKEYLOGFILE (%s): %s\n", filename, strerror(errno));
		abort();
	}

	// Move pointer to the tail of the file.
	// Dump the header to file if the file is newly created (empty, or 0 is the last byte)
	if (lseek(fd_sslkeylogfile, 0, SEEK_END) == 0)
		write(fd_sslkeylogfile, HEADER, sizeof(HEADER) - 1);
}

static void write_sslkeylogfile(const SSL *ssl, const char *line)
{
	open_sslkeylogfile();

	if (fd_sslkeylogfile != -1) {
		write(fd_sslkeylogfile, line, strlen(line));
		write(fd_sslkeylogfile, "\n", 1); // New line
	}
}