arch/x86/vdso/vma.c

/*
 * Set up the VMAs to tell the VM about the vDSO.
 * Copyright 2007 Andi Kleen, SUSE Labs.
 * Subject to the GPL, v.2
 */
#include <linux/mm.h>
#include <linux/err.h>
#include <linux/sched.h>
#include <linux/init.h>
#include <linux/random.h>
#include <linux/elf.h>
#include <asm/vsyscall.h>
#include <asm/vgtod.h>
#include <asm/proto.h>
#include <asm/vdso.h>

#include "vextern.h"		/* Just for VMAGIC.  */
#undef VEXTERN

unsigned int __read_mostly vdso_enabled = 1;

extern char vdso_start[], vdso_end[];
extern unsigned short vdso_sync_cpuid;

static struct page **vdso_pages;
static unsigned vdso_size;

static inline void *var_ref(void *p, char *name)
{
	if (*(void **)p != (void *)VMAGIC) {
		printk("VDSO: variable %s broken\n", name);
		vdso_enabled = 0;
	}
	return p;
}

static int __init init_vdso_vars(void)
{
	int npages = (vdso_end - vdso_start + PAGE_SIZE - 1) / PAGE_SIZE;
	int i;
	char *vbase;

	vdso_size = npages << PAGE_SHIFT;
	vdso_pages = kmalloc(sizeof(struct page *) * npages, GFP_KERNEL);
	if (!vdso_pages)
		goto oom;
	for (i = 0; i < npages; i++) {
		struct page *p;
		p = alloc_page(GFP_KERNEL);
		if (!p)
			goto oom;
		vdso_pages[i] = p;
		copy_page(page_address(p), vdso_start + i*PAGE_SIZE);
	}

	vbase = vmap(vdso_pages, npages, 0, PAGE_KERNEL);
	if (!vbase)
		goto oom;

	if (memcmp(vbase, "\177ELF", 4)) {
		printk("VDSO: I'm broken; not ELF\n");
		vdso_enabled = 0;
	}

#define VEXTERN(x) \
	*(typeof(__ ## x) **) var_ref(VDSO64_SYMBOL(vbase, x), #x) = &__ ## x;
#include "vextern.h"
#undef VEXTERN
	return 0;

 oom:
	printk("Cannot allocate vdso\n");
	vdso_enabled = 0;
	return -ENOMEM;
}
__initcall(init_vdso_vars);

struct linux_binprm;

/*
 * Put the vdso above the (randomized) stack with another randomized
 * offset.  This way there is no hole in the middle of address space.
 * To save memory make sure it is still in the same PTE as the stack
 * top.  This doesn't give that many random bits.
 *
 * Note that this algorithm is imperfect: the distribution of the vdso
 * start address within a PMD is biased toward the end.
 */
static unsigned long vdso_addr(unsigned long start, unsigned len)
{
	unsigned long addr, end;
	unsigned offset;

	/*
	 * Round up the start address.  It can start out unaligned as a result
	 * of stack start randomization.
	 */
	start = PAGE_ALIGN(start);

	/* Round the lowest possible end address up to a PMD boundary. */
	end = (start + len + PMD_SIZE - 1) & PMD_MASK;
	if (end >= TASK_SIZE_MAX)
		end = TASK_SIZE_MAX;
	end -= len;

	if (end > start) {
		offset = get_random_int() % (((end - start) >> PAGE_SHIFT) + 1);
		addr = start + (offset << PAGE_SHIFT);
	} else {
		addr = start;
	}

	return addr;
}

/* Setup a VMA at program startup for the vsyscall page.
   Not called for compat tasks */
int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
{
	struct mm_struct *mm = current->mm;
	unsigned long addr;
	int ret;

	if (!vdso_enabled)
		return 0;

	down_write(&mm->mmap_sem);
	addr = vdso_addr(mm->start_stack, vdso_size);
	addr = get_unmapped_area(NULL, addr, vdso_size, 0, 0);
	if (IS_ERR_VALUE(addr)) {
		ret = addr;
		goto up_fail;
	}

	current->mm->context.vdso = (void *)addr;

	ret = install_special_mapping(mm, addr, vdso_size,
				      VM_READ|VM_EXEC|
				      VM_MAYREAD|VM_MAYWRITE|VM_MAYEXEC|
				      VM_ALWAYSDUMP,
				      vdso_pages);
	if (ret) {
		current->mm->context.vdso = NULL;
		goto up_fail;
	}

up_fail:
	up_write(&mm->mmap_sem);
	return ret;
}

static __init int vdso_setup(char *s)
{
	vdso_enabled = simple_strtoul(s, NULL, 0);
	return 0;
}
__setup("vdso=", vdso_setup);