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