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Subject[PATCH 2/3] mm/mremap.c: don't unmap the overlapping VMA(s)
Date
Currently the
mremap(new_size, MREMAP_MAYMOVE | MREMAP_FIXED, new_address)
discards the part of existing VMA(s) if it overlaps the memory region
specified by new_address and new_size.
Introduce the new MREMAP_DONTUNMAP flag which forces the mremap to
fail with ENOMEM whenever the overlapping occurs. No existing
mapping(s) is discarded.
The implementation tests the MAP_DONTUNMAP flag and scans the AS for
the overlapping VMA(s) right before unmapping the area.

I did the isolated tests and also tested it with Gentoo full
installation.

Signed-off-by: Piotr Kwapulinski <kwapulinski.piotr@gmail.com>
---
include/uapi/linux/mman.h | 5 +++--
mm/mremap.c | 23 +++++++++++++++++------
2 files changed, 20 insertions(+), 8 deletions(-)

diff --git a/include/uapi/linux/mman.h b/include/uapi/linux/mman.h
index ade4acd..bc6478e 100644
--- a/include/uapi/linux/mman.h
+++ b/include/uapi/linux/mman.h
@@ -3,8 +3,9 @@

#include <asm/mman.h>

-#define MREMAP_MAYMOVE 1
-#define MREMAP_FIXED 2
+#define MREMAP_MAYMOVE 1
+#define MREMAP_FIXED 2
+#define MREMAP_DONTUNMAP 4

#define OVERCOMMIT_GUESS 0
#define OVERCOMMIT_ALWAYS 1
diff --git a/mm/mremap.c b/mm/mremap.c
index 3fa0a467..f57d396 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -397,7 +397,8 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
}

static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
- unsigned long new_addr, unsigned long new_len, bool *locked)
+ unsigned long new_addr, unsigned long new_len,
+ unsigned long flags, bool *locked)
{
struct mm_struct *mm = current->mm;
struct vm_area_struct *vma;
@@ -415,9 +416,16 @@ static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
if (addr + old_len > new_addr && new_addr + new_len > addr)
goto out;

- ret = do_munmap(mm, new_addr, new_len);
- if (ret)
- goto out;
+ if (flags & MREMAP_DONTUNMAP) {
+ if (find_vma_intersection(mm, new_addr, new_len)) {
+ ret = -ENOMEM;
+ goto out;
+ }
+ } else {
+ ret = do_munmap(mm, new_addr, new_len);
+ if (ret)
+ goto out;
+ }

if (old_len >= new_len) {
ret = do_munmap(mm, addr+new_len, old_len - new_len);
@@ -482,12 +490,15 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
unsigned long charged = 0;
bool locked = false;

- if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE))
+ if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE | MREMAP_DONTUNMAP))
return ret;

if (flags & MREMAP_FIXED && !(flags & MREMAP_MAYMOVE))
return ret;

+ if (flags & MREMAP_DONTUNMAP && !(flags & MREMAP_FIXED))
+ return ret;
+
if (offset_in_page(addr))
return ret;

@@ -505,7 +516,7 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
down_write(&current->mm->mmap_sem);

if (flags & MREMAP_FIXED) {
- ret = mremap_to(addr, old_len, new_addr, new_len,
+ ret = mremap_to(addr, old_len, new_addr, new_len, flags,
&locked);
goto out;
}
--
2.7.4
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