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: David 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: Andrew Morton <akpm@linux-foundation.org>

include/linux/mm.h

@@ -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

@@ -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;
-		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;
-	}
+	} 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;
 
-	/* 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 ((flags & MAP_PRIVATE) && current->ptrace)
-		vm_flags &= ~VM_MAYSHARE;
+		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.
+			 */
+			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);