[PATCH] mm: add a note about partially hardcoded VM_* flags

Hugh made me note this line for permission checking in mprotect(): if ((newflags & ~(newflags >> 4)) & 0xf) { after figuring out what's that about, I decided it's nasty enough. Btw Hugh itself didn't like the 0xf. We can safely change it to VM_READ|VM_WRITE|VM_EXEC because we never change VM_SHARED, so no need to check that. Signed-off-by: Paolo 'Blaisorblade' Giarrusso <[email protected]> Acked-by: Hugh Dickins <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
kyagi · Sep 21, 2005 · 7e2cff4 · 7e2cff4
1 parent 7e871b6
commit 7e2cff4
Show file tree

Hide file tree

Showing 2 changed files with 3 additions and 1 deletion.
diff --git a/include/linux/mm.h b/include/linux/mm.h
@@ -136,6 +136,7 @@ extern unsigned int kobjsize(const void *objp);
 #define VM_EXEC		0x00000004
 #define VM_SHARED	0x00000008
 
+/* mprotect() hardcodes VM_MAYREAD >> 4 == VM_READ, and so for r/w/x bits. */
 #define VM_MAYREAD	0x00000010	/* limits for mprotect() etc */
 #define VM_MAYWRITE	0x00000020
 #define VM_MAYEXEC	0x00000040

diff --git a/mm/mprotect.c b/mm/mprotect.c
@@ -248,7 +248,8 @@ sys_mprotect(unsigned long start, size_t len, unsigned long prot)
 
 		newflags = vm_flags | (vma->vm_flags & ~(VM_READ | VM_WRITE | VM_EXEC));
 
-		if ((newflags & ~(newflags >> 4)) & 0xf) {
+		/* newflags >> 4 shift VM_MAY% in place of VM_% */
+		if ((newflags & ~(newflags >> 4)) & (VM_READ | VM_WRITE | VM_EXEC)) {
 			error = -EACCES;
 			goto out;
 		}