[PATCH] Permit nommu MAP_SHARED of memory backed files

The attached patch applies some further fixes and extensions to the nommu mmap
implementation:

 (1) /proc/maps distinguishes shareable private mappings and real shared
     mappings by marking the former with 's' and the latter with 'S'.

 (2) Rearrange and optimise the checking portion of do_mmap_pgoff() to make it
     easier to follow.

 (3) Only set VM_SHARED on MAP_SHARED mappings. Its presence indicates that the
     backing memory is supplied by the underlying file or chardev.

     VM_MAYSHARE indicates that a VMA may be shared if it's a private VMA. The
     memory for a private VMA is allocated by do_mmap_pgoff() from a kmalloc
     slab and then the file contents are read into it before returning.

 (4) Permit MAP_SHARED + PROT_WRITE on memory-backed files[*] and chardevs to
     indicate a contiguous area of memory when its get_unmapped_area() is
     called if the backing fs/chardev is willing.

     [*] file->f_mapping->backing_dev_info->memory_backed == 1

 (5) Require chardevs and files that support to provide a get_unmapped_area()
     file operation.

 (6) Made sure a private mapping of /dev/zero is possible. Shared mappings of
     /dev/zero are not currently supported because this'd need greater
     interaction of mmap with the chardev driver than is currently supported.

 (7) Add in some extra checks from mm/mmap.c: security, file having write
     access for a writable shared mapping, file not being in append mode.

 (8) Only account the mapping memory if it's allocated here; memory belonging
     to a shared chardev or file is not accounted.

With this patch it should be possible to map contiguous flash files directly
out of ROM simply by providing get_unmapped_area() for a read-only/shared
mapping.

I think that it might be worth splitting do_mmap_pgoff() up into smaller
subfunctions: one to handle the checking, one to handle shared mappings and
one to handle private mappings.

Signed-Off-By: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/fs/proc/nommu.c b/fs/proc/nommu.c
index 3423101bb0807f..f3bf016d5ee309 100644
--- a/fs/proc/nommu.c
+++ b/fs/proc/nommu.c
@@ -62,7 +62,7 @@ static int nommu_vma_list_show(struct seq_file *m, void *v)
 		   flags & VM_READ ? 'r' : '-',
 		   flags & VM_WRITE ? 'w' : '-',
 		   flags & VM_EXEC ? 'x' : '-',
-		   flags & VM_MAYSHARE ? 's' : 'p',
+		   flags & VM_MAYSHARE ? flags & VM_SHARED ? 'S' : 's' : 'p',
 		   vma->vm_pgoff << PAGE_SHIFT,
 		   MAJOR(dev), MINOR(dev), ino, &len);