It only assumes that mmap is available if the mmap() call returns success.
That's not _too_ unreasonable an assumption.
mmap.c has an optimisation - if it's a zero-length map that's requested,
return success immediately.
Perhaps if we checked that the filedescriptor has mmap() capability _first_,
rather than later, the desired effect would be achieved...
Index: mmap.c
===================================================================
RCS file: /cvs/linux/mm/mmap.c,v
retrieving revision 1.2
diff -u -w -r1.2 mmap.c
--- mmap.c 1999/04/20 11:03:16 1.2
+++ mmap.c 1999/05/04 14:57:43
@@ -176,6 +176,9 @@
struct vm_area_struct * vma;
int error;
+ if (file && (!file->f_op || !file->f_op->mmap))
+ return -ENODEV;
+
if ((len = PAGE_ALIGN(len)) == 0)
return addr;
@@ -244,8 +247,6 @@
* specific mapper. the address has already been validated, but
* not unmapped, but the maps are removed from the list.
*/
- if (file && (!file->f_op || !file->f_op->mmap))
- return -ENODEV;
vma = kmem_cache_alloc(vm_area_cachep, SLAB_KERNEL);
if (!vma)
---- ---- ----
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