[PATCH mm-stable RFC 1/2] mm/nommu: factor out check for NOMMU shared mappings into is_nommu_shared_mapping()

[David Hildenbrand <david@redhat.com>]

We want to stop using VM_MAYSHARE in private mappings to pave the way
for clarifying the semantics of VM_MAYSHARE vs. VM_SHARED and reduce
the confusion. While CONFIG_MMU uses VM_MAYSHARE to represent MAP_SHARED,
!CONFIG_MMU also sets VM_MAYSHARE for selected R/O private file mappings
that are an effective overlay of a file mapping.

Let's factor out all relevant VM_MAYSHARE checks in !CONFIG_MMU code into
is_nommu_shared_mapping() first.

Note that whenever VM_SHARED is set, VM_MAYSHARE must be set as well
(unless there is a serious BUG). So there is not need to test for VM_SHARED
manually.

No functional change intended.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 drivers/char/mem.c    |  2 +-
 fs/cramfs/inode.c     |  2 +-
 fs/proc/task_nommu.c  |  2 +-
 fs/ramfs/file-nommu.c |  2 +-
 fs/romfs/mmap-nommu.c |  2 +-
 include/linux/mm.h    | 15 +++++++++++++++
 io_uring/io_uring.c   |  2 +-
 mm/nommu.c            | 11 ++++++-----
 8 files changed, 27 insertions(+), 11 deletions(-)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 83bf2a4dcb57..ffb101d349f0 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -343,7 +343,7 @@ static unsigned zero_mmap_capabilities(struct file *file)
 /* can't do an in-place private mapping if there's no MMU */
 static inline int private_mapping_ok(struct vm_area_struct *vma)
 {
-	return vma->vm_flags & VM_MAYSHARE;
+	return is_nommu_shared_mapping(vma->vm_flags);
 }
 #else
 
diff --git a/fs/cramfs/inode.c b/fs/cramfs/inode.c
index 61ccf7722fc3..50e4e060db68 100644
--- a/fs/cramfs/inode.c
+++ b/fs/cramfs/inode.c
@@ -437,7 +437,7 @@ static int cramfs_physmem_mmap(struct file *file, struct vm_area_struct *vma)
 
 static int cramfs_physmem_mmap(struct file *file, struct vm_area_struct *vma)
 {
-	return vma->vm_flags & (VM_SHARED | VM_MAYSHARE) ? 0 : -ENOSYS;
+	return is_nommu_shared_mapping(vma->vm_flags) ? 0 : -ENOSYS;
 }
 
 static unsigned long cramfs_physmem_get_unmapped_area(struct file *file,
diff --git a/fs/proc/task_nommu.c b/fs/proc/task_nommu.c
index 2fd06f52b6a4..0ec35072a8e5 100644
--- a/fs/proc/task_nommu.c
+++ b/fs/proc/task_nommu.c
@@ -38,7 +38,7 @@ void task_mem(struct seq_file *m, struct mm_struct *mm)
 		}
 
 		if (atomic_read(&mm->mm_count) > 1 ||
-		    vma->vm_flags & VM_MAYSHARE) {
+		    is_nommu_shared_mapping(vma->vm_flags)) {
 			sbytes += size;
 		} else {
 			bytes += size;
diff --git a/fs/ramfs/file-nommu.c b/fs/ramfs/file-nommu.c
index cb240eac5036..cd4537692751 100644
--- a/fs/ramfs/file-nommu.c
+++ b/fs/ramfs/file-nommu.c
@@ -264,7 +264,7 @@ static unsigned long ramfs_nommu_get_unmapped_area(struct file *file,
  */
 static int ramfs_nommu_mmap(struct file *file, struct vm_area_struct *vma)
 {
-	if (!(vma->vm_flags & (VM_SHARED | VM_MAYSHARE)))
+	if (!is_nommu_shared_mapping(vma->vm_flags))
 		return -ENOSYS;
 
 	file_accessed(file);
diff --git a/fs/romfs/mmap-nommu.c b/fs/romfs/mmap-nommu.c
index 2c4a23113fb5..4578dc45e50a 100644
--- a/fs/romfs/mmap-nommu.c
+++ b/fs/romfs/mmap-nommu.c
@@ -63,7 +63,7 @@ static unsigned long romfs_get_unmapped_area(struct file *file,
  */
 static int romfs_mmap(struct file *file, struct vm_area_struct *vma)
 {
-	return vma->vm_flags & (VM_SHARED | VM_MAYSHARE) ? 0 : -ENOSYS;
+	return is_nommu_shared_mapping(vma->vm_flags) ? 0 : -ENOSYS;
 }
 
 static unsigned romfs_mmap_capabilities(struct file *file)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index f3f196e4d66d..734d0bc7c7c6 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1363,6 +1363,21 @@ static inline bool is_cow_mapping(vm_flags_t flags)
 	return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
 }
 
+#ifndef CONFIG_MMU
+static inline bool is_nommu_shared_mapping(vm_flags_t flags)
+{
+	/*
+	 * NOMMU shared mappings are ordinary MAP_SHARED mappings and selected
+	 * R/O MAP_PRIVATE file mappings that are an effective R/O overlay of
+	 * a file mapping. R/O MAP_PRIVATE mappings might still modify
+	 * underlying memory if ptrace is active, so this is only possible if
+	 * ptrace does not apply. Note that there is no mprotect() to upgrade
+	 * write permissions later.
+	 */
+	return flags & VM_MAYSHARE;
+}
+#endif
+
 #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
 #define SECTION_IN_PAGE_FLAGS
 #endif
diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index b521186efa5c..84fa23b1f782 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -3174,7 +3174,7 @@ static __cold int io_uring_mmap(struct file *file, struct vm_area_struct *vma)
 
 static int io_uring_mmap(struct file *file, struct vm_area_struct *vma)
 {
-	return vma->vm_flags & (VM_SHARED | VM_MAYSHARE) ? 0 : -EINVAL;
+	return is_nommu_shared_mapping(vma->vm_flags) ? 0 : -EINVAL;
 }
 
 static unsigned int io_uring_nommu_mmap_capabilities(struct file *file)
diff --git a/mm/nommu.c b/mm/nommu.c
index 214c70e1d059..6c4bdc07a776 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -958,9 +958,10 @@ static int do_mmap_private(struct vm_area_struct *vma,
 	 */
 	if (capabilities & NOMMU_MAP_DIRECT) {
 		ret = call_mmap(vma->vm_file, vma);
+		/* shouldn't return success if we're not sharing */
+		if (WARN_ON_ONCE(!is_nommu_shared_mapping(vma->vm_flags)))
+			ret = -ENOSYS;
 		if (ret == 0) {
-			/* shouldn't return success if we're not sharing */
-			BUG_ON(!(vma->vm_flags & VM_MAYSHARE));
 			vma->vm_region->vm_top = vma->vm_region->vm_end;
 			return 0;
 		}
@@ -1106,7 +1107,7 @@ unsigned long do_mmap(struct file *file,
 	 *   these cases, sharing is handled in the driver or filesystem rather
 	 *   than here
 	 */
-	if (vm_flags & VM_MAYSHARE) {
+	if (is_nommu_shared_mapping(vm_flags)) {
 		struct vm_region *pregion;
 		unsigned long pglen, rpglen, pgend, rpgend, start;
 
@@ -1116,7 +1117,7 @@ unsigned long do_mmap(struct file *file,
 		for (rb = rb_first(&nommu_region_tree); rb; rb = rb_next(rb)) {
 			pregion = rb_entry(rb, struct vm_region, vm_rb);
 
-			if (!(pregion->vm_flags & VM_MAYSHARE))
+			if (!is_nommu_shared_mapping(pregion->vm_flags))
 				continue;
 
 			/* search for overlapping mappings on the same file */
@@ -1597,7 +1598,7 @@ static unsigned long do_mremap(unsigned long addr,
 	if (vma->vm_end != vma->vm_start + old_len)
 		return (unsigned long) -EFAULT;
 
-	if (vma->vm_flags & VM_MAYSHARE)
+	if (is_nommu_shared_mapping(vma->vm_flags))
 		return (unsigned long) -EPERM;
 
 	if (new_len > vma->vm_region->vm_end - vma->vm_region->vm_start)
-- 
2.38.1