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Subject[PATCH mm-stable RFC 2/2] mm/nommu: don't use VM_MAYSHARE for MAP_PRIVATE mappings
Date
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.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
include/linux/mm.h | 7 ++++++-
mm/nommu.c | 51 +++++++++++++++++++++++++++-------------------
2 files changed, 36 insertions(+), 22 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 734d0bc7c7c6..c229eee6211c 100644
--- a/include/linux/mm.h
+++ b/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 */

@@ -1374,7 +1379,7 @@ 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

diff --git a/mm/nommu.c b/mm/nommu.c
index 6c4bdc07a776..5c628c868648 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -892,29 +892,36 @@ 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 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 (!(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 {
- /* 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;
+ /* MAP_SHARED file mapping: NOMMU_MAP_DIRECT is set. */
+ vm_flags |= VM_SHARED | VM_MAYSHARE |
+ (capabilities & NOMMU_VMFLAGS);
}

- /* 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;
-
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);
--
2.38.1
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