Commit b6b7a8fa authored by David Hildenbrand's avatar David Hildenbrand Committed by Andrew Morton
Browse files

mm/nommu: don't use VM_MAYSHARE for MAP_PRIVATE mappings 

Let's stop using VM_MAYSHARE for MAP_PRIVATE mappings and use
VM_MAYOVERLAY instead.  Rewrite determine_vm_flags() to make the whole
logic easier to digest, and to cleanly separate MAP_PRIVATE vs. 
MAP_SHARED.

No functional change intended.

Link: https://lkml.kernel.org/r/20230102160856.500584-3-david@redhat.com


Signed-off-by: default avatarDavid Hildenbrand <david@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Nicolas Pitre <nico@fluxnic.net>
Cc: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent fc4f4be9

include/linux/mm.h

+6 −1
Original line number Diff line number Diff line
@@ -276,7 +276,12 @@ extern unsigned int kobjsize(const void *objp); 
#define VM_MAYSHARE	0x00000080



#define VM_GROWSDOWN	0x00000100	/* general info on the segment */

#ifdef CONFIG_MMU

#define VM_UFFD_MISSING	0x00000200	/* missing pages tracking */

#else /* CONFIG_MMU */

#define VM_MAYOVERLAY	0x00000200	/* nommu: R/O MAP_PRIVATE mapping that might overlay a file mapping */

#define VM_UFFD_MISSING	0

#endif /* CONFIG_MMU */

#define VM_PFNMAP	0x00000400	/* Page-ranges managed without "struct page", just pure PFN */

#define VM_UFFD_WP	0x00001000	/* wrprotect pages tracking */
@@ -1358,7 +1363,7 @@ static inline bool is_nommu_shared_mapping(vm_flags_t flags) 
	 * ptrace does not apply. Note that there is no mprotect() to upgrade

	 * write permissions later.

	 */

	return flags & VM_MAYSHARE;

	return flags & (VM_MAYSHARE | VM_MAYOVERLAY);

}

#endif

mm/nommu.c

+30 −21
Original line number Diff line number Diff line
@@ -892,28 +892,35 @@ static unsigned long determine_vm_flags(struct file *file, 
	unsigned long vm_flags;



	vm_flags = calc_vm_prot_bits(prot, 0) | calc_vm_flag_bits(flags);

	/* vm_flags |= mm->def_flags; */



	if (!(capabilities & NOMMU_MAP_DIRECT)) {

		/* attempt to share read-only copies of mapped file chunks */

	if (!file) {

		/*

		 * MAP_ANONYMOUS. MAP_SHARED is mapped to MAP_PRIVATE, because

		 * there is no fork().

		 */

		vm_flags |= VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC;

	} else if (flags & MAP_PRIVATE) {

		/* MAP_PRIVATE file mapping */

		if (capabilities & NOMMU_MAP_DIRECT)

			vm_flags |= (capabilities & NOMMU_VMFLAGS);

		else

			vm_flags |= VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC;

		if (file && !(prot & PROT_WRITE))

			vm_flags |= VM_MAYSHARE;

	} else {

		/* overlay a shareable mapping on the backing device or inode

		 * if possible - used for chardevs, ramfs/tmpfs/shmfs and

		 * romfs/cramfs */

		vm_flags |= VM_MAYSHARE | (capabilities & NOMMU_VMFLAGS);

		if (flags & MAP_SHARED)

			vm_flags |= VM_SHARED;

	}



	/* refuse to let anyone share private mappings with this process if

	 * it's being traced - otherwise breakpoints set in it may interfere

	 * with another untraced process

		if (!(prot & PROT_WRITE) && !current->ptrace)

			/*

			 * R/O private file mapping which cannot be used to

			 * modify memory, especially also not via active ptrace

			 * (e.g., set breakpoints) or later by upgrading

			 * permissions (no mprotect()). We can try overlaying

			 * the file mapping, which will work e.g., on chardevs,

			 * ramfs/tmpfs/shmfs and romfs/cramf.

			 */

	if ((flags & MAP_PRIVATE) && current->ptrace)

		vm_flags &= ~VM_MAYSHARE;

			vm_flags |= VM_MAYOVERLAY;

	} else {

		/* MAP_SHARED file mapping: NOMMU_MAP_DIRECT is set. */

		vm_flags |= VM_SHARED | VM_MAYSHARE |

			    (capabilities & NOMMU_VMFLAGS);

	}



	return vm_flags;

}
@@ -952,9 +959,11 @@ static int do_mmap_private(struct vm_area_struct *vma, 
	void *base;

	int ret, order;



	/* invoke the file's mapping function so that it can keep track of

	 * shared mappings on devices or memory

	 * - VM_MAYSHARE will be set if it may attempt to share

	/*

	 * Invoke the file's mapping function so that it can keep track of

	 * shared mappings on devices or memory. VM_MAYOVERLAY will be set if

	 * it may attempt to share, which will make is_nommu_shared_mapping()

	 * happy.

	 */

	if (capabilities & NOMMU_MAP_DIRECT) {

		ret = call_mmap(vma->vm_file, vma);