The patch titled
Subject: mm/nommu: don't use VM_MAYSHARE for MAP_PRIVATE mappings
has been added to the -mm mm-unstable branch. Its filename is
mm-nommu-dont-use-vm_mayshare-for-map_private-mappings.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-nommu-dont-use-vm_mayshare-for-map_private-mappings.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: David Hildenbrand <david@xxxxxxxxxx>
Subject: mm/nommu: don't use VM_MAYSHARE for MAP_PRIVATE mappings
Date: Mon, 2 Jan 2023 17:08:55 +0100
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@xxxxxxxxxx
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Nicolas Pitre <nico@xxxxxxxxxxx>
Cc: Pavel Begunkov <asml.silence@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/mm.h | 7 +++++
mm/nommu.c | 51 +++++++++++++++++++++++++------------------
2 files changed, 36 insertions(+), 22 deletions(-)
--- a/include/linux/mm.h~mm-nommu-dont-use-vm_mayshare-for-map_private-mappings
+++ a/include/linux/mm.h
@@ -276,7 +276,12 @@ extern unsigned int kobjsize(const void
#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 */
@@ -1396,7 +1401,7 @@ static inline bool is_nommu_shared_mappi
* 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
--- a/mm/nommu.c~mm-nommu-dont-use-vm_mayshare-for-map_private-mappings
+++ a/mm/nommu.c
@@ -892,29 +892,36 @@ static unsigned long determine_vm_flags(
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_are
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);
_
Patches currently in -mm which might be from david@xxxxxxxxxx are
mm-hugetlb-fix-pte-marker-handling-in-hugetlb_change_protection.patch
mm-hugetlb-fix-uffd-wp-handling-for-migration-entries-in-hugetlb_change_protection.patch
mm-userfaultfd-enable-writenotify-while-userfaultfd-wp-is-enabled-for-a-vma.patch
mm-userfaultfd-rely-on-vma-vm_page_prot-in-uffd_wp_range.patch
mm-userfaultfd-rely-on-vma-vm_page_prot-in-uffd_wp_range-fix.patch
mm-mprotect-drop-pgprot_t-parameter-from-change_protection.patch
mm-mprotect-drop-pgprot_t-parameter-from-change_protection-fix.patch
selftests-vm-cow-add-cow-tests-for-collapsing-of-pte-mapped-anon-thp.patch
mm-nommu-factor-out-check-for-nommu-shared-mappings-into-is_nommu_shared_mapping.patch
mm-nommu-dont-use-vm_mayshare-for-map_private-mappings.patch
drivers-misc-open-dice-dont-touch-vm_mayshare.patch
