Re: [PATCH v5 3/5] mm: introduce mmap3 for safely defining new mmap flags

On Wed, Aug 16, 2017 at 9:47 AM, Kirill A. Shutemov
[off-list ref] wrote:

On Wed, Aug 16, 2017 at 09:35:11AM -0700, Dan Williams wrote:

quoted

On Wed, Aug 16, 2017 at 4:15 AM, Kirill A. Shutemov
[off-list ref] wrote:

quoted

On Wed, Aug 16, 2017 at 12:44:22AM -0700, Dan Williams wrote:

quoted

diff --git a/include/linux/mman.h b/include/linux/mman.h
index c8367041fafd..0e1de42c836f 100644
--- a/include/linux/mman.h
+++ b/include/linux/mman.h

@@ -7,6 +7,40 @@
 #include <linux/atomic.h>
 #include <uapi/linux/mman.h>

+#ifndef MAP_32BIT
+#define MAP_32BIT 0
+#endif
+#ifndef MAP_HUGE_2MB
+#define MAP_HUGE_2MB 0
+#endif
+#ifndef MAP_HUGE_1GB
+#define MAP_HUGE_1GB 0
+#endif
+
+/*
+ * The historical set of flags that all mmap implementations implicitly
+ * support when file_operations.mmap_supported_mask is zero.
+ */
+#define LEGACY_MAP_SUPPORTED_MASK (MAP_SHARED \
+             | MAP_PRIVATE \
+             | MAP_FIXED \
+             | MAP_ANONYMOUS \
+             | MAP_UNINITIALIZED \
+             | MAP_GROWSDOWN \
+             | MAP_DENYWRITE \
+             | MAP_EXECUTABLE \
+             | MAP_LOCKED \
+             | MAP_NORESERVE \
+             | MAP_POPULATE \
+             | MAP_NONBLOCK \
+             | MAP_STACK \
+             | MAP_HUGETLB \
+             | MAP_32BIT \
+             | MAP_HUGE_2MB \
+             | MAP_HUGE_1GB)
+
+#define      MAP_SUPPORTED_MASK (LEGACY_MAP_SUPPORTED_MASK)
+
 extern int sysctl_overcommit_memory;
 extern int sysctl_overcommit_ratio;
 extern unsigned long sysctl_overcommit_kbytes;

Since we looking into mmap(2) ABI, maybe we should consider re-defining
MAP_DENYWRITE and MAP_EXECUTABLE as 0 in hope that we would be able to
re-use these bits in the future? These flags are ignored now anyway.

Yes, we can make these -EOPNOTSUPP in the new syscall.

You cannot detect them, if we would redefine them as 0. :)

Yes, we can, there will now be missing bits in
LEGACY_MAP_SUPPORTED_MASK that will fail those bit values until we
re-define them. Everything else is a an exercise for libc about what
it wants to do when it sees those values.