Commit 1c972597 authored by Dan Williams's avatar Dan Williams
Browse files

mm: introduce MAP_SHARED_VALIDATE, a mechanism to safely define new mmap flags 



The mmap(2) syscall suffers from the ABI anti-pattern of not validating
unknown flags. However, proposals like MAP_SYNC need a mechanism to
define new behavior that is known to fail on older kernels without the
support. Define a new MAP_SHARED_VALIDATE flag pattern that is
guaranteed to fail on all legacy mmap implementations.

It is worth noting that the original proposal was for a standalone
MAP_VALIDATE flag. However, when that  could not be supported by all
archs Linus observed:

    I see why you *think* you want a bitmap. You think you want
    a bitmap because you want to make MAP_VALIDATE be part of MAP_SYNC
    etc, so that people can do

    ret = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED
		    | MAP_SYNC, fd, 0);

    and "know" that MAP_SYNC actually takes.

    And I'm saying that whole wish is bogus. You're fundamentally
    depending on special semantics, just make it explicit. It's already
    not portable, so don't try to make it so.

    Rename that MAP_VALIDATE as MAP_SHARED_VALIDATE, make it have a value
    of 0x3, and make people do

    ret = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED_VALIDATE
		    | MAP_SYNC, fd, 0);

    and then the kernel side is easier too (none of that random garbage
    playing games with looking at the "MAP_VALIDATE bit", but just another
    case statement in that map type thing.

    Boom. Done.

Similar to ->fallocate() we also want the ability to validate the
support for new flags on a per ->mmap() 'struct file_operations'
instance basis.  Towards that end arrange for flags to be generically
validated against a mmap_supported_flags exported by 'struct
file_operations'. By default all existing flags are implicitly
supported, but new flags require MAP_SHARED_VALIDATE and
per-instance-opt-in.

Cc: Jan Kara <jack@suse.cz>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Suggested-by: default avatarChristoph Hellwig <hch@lst.de>
Suggested-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: default avatarRoss Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: default avatarDan Williams <dan.j.williams@intel.com>
Signed-off-by: default avatarJan Kara <jack@suse.cz>
Signed-off-by: default avatarDan Williams <dan.j.williams@intel.com>
parent 592e2545

arch/alpha/include/uapi/asm/mman.h

+1 −0
Original line number Diff line number Diff line
@@ -11,6 +11,7 @@ 


#define MAP_SHARED	0x01		/* Share changes */

#define MAP_PRIVATE	0x02		/* Changes are private */

#define MAP_SHARED_VALIDATE 0x03	/* share + validate extension flags */

#define MAP_TYPE	0x0f		/* Mask for type of mapping (OSF/1 is _wrong_) */

#define MAP_FIXED	0x100		/* Interpret addr exactly */

#define MAP_ANONYMOUS	0x10		/* don't use a file */

arch/mips/include/uapi/asm/mman.h

+1 −0
Original line number Diff line number Diff line
@@ -28,6 +28,7 @@ 
 */

#define MAP_SHARED	0x001		/* Share changes */

#define MAP_PRIVATE	0x002		/* Changes are private */

#define MAP_SHARED_VALIDATE 0x003	/* share + validate extension flags */

#define MAP_TYPE	0x00f		/* Mask for type of mapping */

#define MAP_FIXED	0x010		/* Interpret addr exactly */

arch/parisc/include/uapi/asm/mman.h

+1 −0
Original line number Diff line number Diff line
@@ -11,6 +11,7 @@ 


#define MAP_SHARED	0x01		/* Share changes */

#define MAP_PRIVATE	0x02		/* Changes are private */

#define MAP_SHARED_VALIDATE 0x03	/* share + validate extension flags */

#define MAP_TYPE	0x03		/* Mask for type of mapping */

#define MAP_FIXED	0x04		/* Interpret addr exactly */

#define MAP_ANONYMOUS	0x10		/* don't use a file */

arch/xtensa/include/uapi/asm/mman.h

+1 −0
Original line number Diff line number Diff line
@@ -35,6 +35,7 @@ 
 */

#define MAP_SHARED	0x001		/* Share changes */

#define MAP_PRIVATE	0x002		/* Changes are private */

#define MAP_SHARED_VALIDATE 0x003	/* share + validate extension flags */

#define MAP_TYPE	0x00f		/* Mask for type of mapping */

#define MAP_FIXED	0x010		/* Interpret addr exactly */

include/linux/fs.h

+1 −0
Original line number Diff line number Diff line
@@ -1701,6 +1701,7 @@ struct file_operations { 
	long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);

	long (*compat_ioctl) (struct file *, unsigned int, unsigned long);

	int (*mmap) (struct file *, struct vm_area_struct *);

	unsigned long mmap_supported_flags;

	int (*open) (struct inode *, struct file *);

	int (*flush) (struct file *, fl_owner_t id);

	int (*release) (struct inode *, struct file *);

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