dump-vvar.c

#define _GNU_SOURCE
#include <stdio.h>
#include <string.h>
#include <sys/mman.h>
#include <unistd.h>
#include <setjmp.h>
#include <signal.h>
#include <err.h>

static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
                       int flags)
{
	struct sigaction sa;
	memset(&sa, 0, sizeof(sa));
	sa.sa_sigaction = handler;
	sa.sa_flags = SA_SIGINFO | flags;
	sigemptyset(&sa.sa_mask);
	if (sigaction(sig, &sa, 0))
		err(1, "sigaction");
}

static jmp_buf fail_jmp;

static void handler(int sig, siginfo_t *si, void *ctx_void)
{
	siglongjmp(fail_jmp, 1);
}

static void dump_page(int n, const void *base)
{
	unsigned char data[4096];

	sethandler(SIGBUS, handler, 0);

	if (sigsetjmp(fail_jmp, 1)) {
		fprintf(stderr, "Cannot read vvar page %d\n", n);
		memset(data, 0xff, sizeof(data));
	} else {
		memcpy(data, base, sizeof(data));
	}

	write(1, data, 4096);
}

int main()
{
	FILE *maps;
	void *vvar_begin, *vvar_end;
	int found_vvar = 0;
	int npages;

	maps = fopen("/proc/self/maps", "r");
	char buf[1024];
	while (fgets(buf, 1024, maps)) {
		if (strstr(buf, "[vvar]")) {
			found_vvar = 1;
			break;
		}
	}
	fclose(maps);

	if (!found_vvar) {
		fprintf(stderr, "Could not find vvar mapping\n");
		return 1;
	}

	sscanf(buf, "%p-%p", &vvar_begin, &vvar_end);
	npages = ((char *)vvar_end - (char *)vvar_begin) / 4096;;

	fprintf(stderr, "vvar mapping is %d pages (0x%lx - 0x%lx)\n",
		npages, (unsigned long)vvar_begin, (unsigned long)vvar_end);

	for (int i = 0; i < npages; i++)
		dump_page(i, (char *)vvar_begin + i * 4096);

	mprotect(vvar_begin, vvar_end - vvar_begin, PROT_READ | PROT_WRITE);

	return 0;
}