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nommu: fix race between ramfs truncation and shared mmap 

Fix the race between the truncation of a ramfs file and an attempt to make
a shared mmap of region of that file.

The problem is that do_mmap_pgoff() calls f_op->get_unmapped_area() to
verify that the file region is made of contiguous pages and to find its
base address - but there isn't any locking to guarantee this region until
vma_prio_tree_insert() is called by add_vma_to_mm().

Note that moving the functionality into f_op->mmap() doesn't help as that
is also called before vma_prio_tree_insert().

Instead make ramfs_nommu_check_mappings() grab nommu_region_sem whilst it
does its checks.  This means that this function will wait whilst mmaps
take place.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Greg Ungerer <gerg@snapgear.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
David Howells 2010-01-15 17:01:36 -08:00 committed by Linus Torvalds
parent efc1a3b169
commit 81759b5b22
1 changed files with 6 additions and 1 deletions
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7
fs/ramfs/file-nommu.c
View File

@ -131,6 +131,8 @@ static int ramfs_nommu_check_mappings(struct inode *inode,
struct vm_area_struct *vma;
struct prio_tree_iter iter;
down_write(&nommu_region_sem);
/* search for VMAs that fall within the dead zone */
vma_prio_tree_foreach(vma, &iter, &inode->i_mapping->i_mmap,
newsize >> PAGE_SHIFT,
@ -138,10 +140,13 @@ static int ramfs_nommu_check_mappings(struct inode *inode,
) {
/* found one - only interested if it's shared out of the page
* cache */
if (vma->vm_flags & VM_SHARED)
if (vma->vm_flags & VM_SHARED) {
up_write(&nommu_region_sem);
return -ETXTBSY; /* not quite true, but near enough */
}
}
up_write(&nommu_region_sem);
return 0;
}