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mm/nommu: factor out check for NOMMU shared mappings into is_nommu_shared_mapping()

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Patch series "mm/nommu: don't use VM_MAYSHARE for MAP_PRIVATE mappings".

Trying to reduce the confusion around VM_SHARED and VM_MAYSHARE first
requires !CONFIG_MMU to stop using VM_MAYSHARE for MAP_PRIVATE mappings. 
CONFIG_MMU only sets VM_MAYSHARE for MAP_SHARED mappings.

This paves the way for further VM_MAYSHARE and VM_SHARED cleanups: for
example, renaming VM_MAYSHARED to VM_MAP_SHARED to make it cleaner what is
actually means.

Let's first get the weird case out of the way and not use VM_MAYSHARE in
MAP_PRIVATE mappings, using a new VM_MAYOVERLAY flag instead.


This patch (of 3):

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.

Link: https://lkml.kernel.org/r/20230102160856.500584-1-david@redhat.com
Link: https://lkml.kernel.org/r/20230102160856.500584-2-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: David Hildenbrand <david@redhat.com>
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>
This commit is contained in:
David Hildenbrand 2023-01-02 17:08:54 +01:00 committed by Andrew Morton
parent da0618c146
commit fc4f4be9b5
8 changed files with 27 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
drivers/char/mem.c
View File

@ -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

2
fs/cramfs/inode.c
View File

@ -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,

2
fs/proc/task_nommu.c
View File

@ -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;

2
fs/ramfs/file-nommu.c
View File

@ -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);

2
fs/romfs/mmap-nommu.c
View File

@ -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)

15
include/linux/mm.h
View File

@ -1347,6 +1347,21 @@ 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

2
io_uring/io_uring.c
View File

@ -3206,7 +3206,7 @@ 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)

11
mm/nommu.c
View File

@ -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 */
@ -1600,7 +1601,7 @@ 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)