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Subject[PATCH] drivers/char/mem.c, kernel 2.4.20: fix mmap /dev/kmem and vmalloc'd addresses
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mmap'ing kernel virtual addresses above high_memory using /dev/kmem returns different results than reads of /dev/kmem.  The problem occurs because the call to remap_page_range in mmap_mem assumes a physically-contiguous region of memory that is allocated below high_memory; this isn't the case for (e.g.) vmalloc'd memory.

The attached patch creates a unique mmap routine for /dev/kmem that uses the "nopage" vm_operations_struct function to construct a correct mapping to kernel memory when a page is accessed via the user virtual address. A simple test program verifies that the patch corrects the different values returned when reading /dev/kmem as compared to mmap'ing /dev/kmem.

Joseph "Tony" Dziedzic
Storigen Systems, Inc.

--- linux-2.4.20/drivers/char/mem.c Fri Aug 2 20:39:43 2002
+++ linux-2.4.20-patch/drivers/char/mem.c Thu Jan 16 11:26:36 2003
@@ -520,7 +520,91 @@
return capable(CAP_SYS_RAWIO) ? 0 : -EPERM;
}

-#define mmap_kmem mmap_mem
+struct page *kmem_vm_nopage(struct vm_area_struct *vma, unsigned long address, int write)
+{
+ unsigned long offset = vma->vm_pgoff << PAGE_SHIFT;
+ unsigned long kaddr;
+ pgd_t *pgd;
+ pmd_t *pmd;
+ pte_t *ptep, pte;
+ struct page *page = NULL;
+
+ /* address is user VA; convert to kernel VA of desired page */
+ kaddr = (address - vma->vm_start) + offset;
+ kaddr = VMALLOC_VMADDR(kaddr);
+
+ spin_lock(&init_mm.page_table_lock);
+
+ /* Lookup page structure for kernel VA */
+ pgd = pgd_offset(&init_mm, kaddr);
+ if (pgd_none(*pgd) || pgd_bad(*pgd))
+ goto out;
+ pmd = pmd_offset(pgd, kaddr);
+ if (pmd_none(*pmd) || pmd_bad(*pmd))
+ goto out;
+ ptep = pte_offset(pmd, kaddr);
+ if (!ptep)
+ goto out;
+ pte = *ptep;
+ if (!pte_present(pte))
+ goto out;
+ if (write && !pte_write(pte))
+ goto out;
+ page = pte_page(pte);
+ if (!VALID_PAGE(page)) {
+ page = NULL;
+ goto out;
+ }
+
+ /* Increment reference count on page */
+ get_page(page);
+
+out:
+ spin_unlock(&init_mm.page_table_lock);
+
+ return page;
+}
+
+struct vm_operations_struct kmem_vm_ops = {
+ nopage: kmem_vm_nopage,
+};
+
+static int mmap_kmem(struct file * file, struct vm_area_struct * vma)
+{
+ unsigned long offset = vma->vm_pgoff << PAGE_SHIFT;
+ unsigned long size = vma->vm_end - vma->vm_start;
+
+ /*
+ * If the user is not attempting to mmap a high memory address then
+ * the standard mmap_mem mechanism will work. High memory addresses
+ * need special handling, as remap_page_range expects a physically-
+ * contiguous range of kernel addresses (such as obtained in kmalloc).
+ */
+ if ((offset + size) < (unsigned long) high_memory)
+ return mmap_mem(file, vma);
+
+ /*
+ * Accessing memory above the top the kernel knows about or
+ * through a file pointer that was marked O_SYNC will be
+ * done non-cached.
+ */
+ if (noncached_address(offset) || (file->f_flags & O_SYNC))
+ vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
+
+ /* Don't do anything here; "nopage" will fill the holes */
+ vma->vm_ops = &kmem_vm_ops;
+
+ /* Don't try to swap out physical pages.. */
+ vma->vm_flags |= VM_RESERVED;
+
+ /*
+ * Don't dump addresses that are not real memory to a core file.
+ */
+ vma->vm_flags |= VM_IO;
+
+ return 0;
+}
+
#define zero_lseek null_lseek
#define full_lseek null_lseek
#define write_zero write_null
-
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