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mm/nommu: don't use VM_MAYSHARE for MAP_PRIVATE mappings 

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@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
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 17:08:55 +01:00 committed by Andrew Morton
parent fc4f4be9b5
commit b6b7a8faf0
2 changed files with 37 additions and 23 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
include/linux/mm.h
View File

@ -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 */
@ -1358,7 +1363,7 @@ 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

53
mm/nommu.c
View File

@ -892,28 +892,35 @@ 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 {
/* 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;
}
} 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;
/* 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;
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 {
/* MAP_SHARED file mapping: NOMMU_MAP_DIRECT is set. */
vm_flags |= VM_SHARED | VM_MAYSHARE |
(capabilities & NOMMU_VMFLAGS);
}
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);