On Sat, Apr 2, 2016 at 10:17 PM, Piotr Kwapulinski
<kwapulinski.piotr@xxxxxxxxx> wrote:
> Currently the mmap(MAP_FIXED) discards the overlapping part of the
> existing VMA(s).
> Introduce the new MAP_DONTUNMAP flag which forces the mmap to fail
> with ENOMEM whenever the overlapping occurs and MAP_FIXED is set.
> No existing mapping(s) is discarded.
How userspace is supposed to use this and handle failure?
For now you can get the same behavior in couple syscalls:
mmap without MAP_FIXED if resulting address differs unmmap and handle error.
Twice slower but this is error-path so you anyway have to some extra actions.
> The implementation tests the MAP_DONTUNMAP flag right before unmapping
> the VMA. The tile arch is the dependency of mmap_flags.
>
> I did the isolated tests and also tested it with Gentoo full
> installation.
>
> Signed-off-by: Piotr Kwapulinski <kwapulinski.piotr@xxxxxxxxx>
> ---
> arch/tile/mm/elf.c | 1 +
> include/linux/mm.h | 3 ++-
> include/uapi/asm-generic/mman-common.h | 1 +
> mm/mmap.c | 10 +++++++---
> 4 files changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/arch/tile/mm/elf.c b/arch/tile/mm/elf.c
> index 6225cc9..dae4b33 100644
> --- a/arch/tile/mm/elf.c
> +++ b/arch/tile/mm/elf.c
> @@ -142,6 +142,7 @@ int arch_setup_additional_pages(struct linux_binprm *bprm,
> if (!retval) {
> unsigned long addr = MEM_USER_INTRPT;
> addr = mmap_region(NULL, addr, INTRPT_SIZE,
> + MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE,
> VM_READ|VM_EXEC|
> VM_MAYREAD|VM_MAYWRITE|VM_MAYEXEC, 0);
> if (addr > (unsigned long) -PAGE_SIZE)
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ed6407d..31dcdfb 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2048,7 +2048,8 @@ extern int install_special_mapping(struct mm_struct *mm,
> extern unsigned long get_unmapped_area(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
>
> extern unsigned long mmap_region(struct file *file, unsigned long addr,
> - unsigned long len, vm_flags_t vm_flags, unsigned long pgoff);
> + unsigned long len, unsigned long mmap_flags,
> + vm_flags_t vm_flags, unsigned long pgoff);
> extern unsigned long do_mmap(struct file *file, unsigned long addr,
> unsigned long len, unsigned long prot, unsigned long flags,
> vm_flags_t vm_flags, unsigned long pgoff, unsigned long *populate);
> diff --git a/include/uapi/asm-generic/mman-common.h b/include/uapi/asm-generic/mman-common.h
> index 5827438..3655be3 100644
> --- a/include/uapi/asm-generic/mman-common.h
> +++ b/include/uapi/asm-generic/mman-common.h
> @@ -19,6 +19,7 @@
> #define MAP_TYPE 0x0f /* Mask for type of mapping */
> #define MAP_FIXED 0x10 /* Interpret addr exactly */
> #define MAP_ANONYMOUS 0x20 /* don't use a file */
> +#define MAP_DONTUNMAP 0x40 /* don't unmap overlapping VMA */
> #ifdef CONFIG_MMAP_ALLOW_UNINITIALIZED
> # define MAP_UNINITIALIZED 0x4000000 /* For anonymous mmap, memory could be uninitialized */
> #else
> diff --git a/mm/mmap.c b/mm/mmap.c
> index bd2e1a53..ab429c3 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1286,7 +1286,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
> vm_flags |= VM_NORESERVE;
> }
>
> - addr = mmap_region(file, addr, len, vm_flags, pgoff);
> + addr = mmap_region(file, addr, len, flags, vm_flags, pgoff);
> if (!IS_ERR_VALUE(addr) &&
> ((vm_flags & VM_LOCKED) ||
> (flags & (MAP_POPULATE | MAP_NONBLOCK)) == MAP_POPULATE))
> @@ -1422,7 +1422,8 @@ static inline int accountable_mapping(struct file *file, vm_flags_t vm_flags)
> }
>
> unsigned long mmap_region(struct file *file, unsigned long addr,
> - unsigned long len, vm_flags_t vm_flags, unsigned long pgoff)
> + unsigned long len, unsigned long mmap_flags,
> + vm_flags_t vm_flags, unsigned long pgoff)
> {
> struct mm_struct *mm = current->mm;
> struct vm_area_struct *vma, *prev;
> @@ -1448,7 +1449,10 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> /* Clear old maps */
> while (find_vma_links(mm, addr, addr + len, &prev, &rb_link,
> &rb_parent)) {
> - if (do_munmap(mm, addr, len))
> + const bool dont_unmap =
> + (mmap_flags & (MAP_DONTUNMAP | MAP_FIXED))
> + == (MAP_DONTUNMAP | MAP_FIXED);
> + if (dont_unmap || do_munmap(mm, addr, len))
> return -ENOMEM;
> }
>
> --
> 2.7.4
>
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