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SubjectRe: MAP_FIXED for ELF mappings
On Wed 04-10-17 23:12:38, Baoquan He wrote:
> I made a clean up patch according to Oleg's suggestion. It's trying to
> get an map area to cover total_size, then do mmap for for the 1st
> program segment only. Not sure if this way is correct.
>
> >From 40f231bb78a74caebcb4a898089a9fa5323be05f Mon Sep 17 00:00:00 2001
> From: Baoquan He <bhe@redhat.com>
> Date: Fri, 29 Sep 2017 21:35:30 +0800
> Subject: [PATCH] binfmt_elf: Clean up the elf_map
>
> Oleg pointed out that it's really ugly to do mmap of the total_size, then
> unmap the region excluding the 1st segment. The right way should be search
> an unmapped area which can cover region of total_size, then map the 1st
> segment only.
>
> And also update the code comment accordingly. In below commit, the relevant
> code comment is not changed to cover the ELF binary image case.
> commit a87938b2e2 ("fs/binfmt_elf.c: fix bug in loading of PIE binaries")
>
> Signed-off-by: Baoquan He <bhe@redhat.com>
> ---
> fs/binfmt_elf.c | 27 +++++++++++++++------------
> 1 file changed, 15 insertions(+), 12 deletions(-)
>
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 72b7ecba7ead..43a47b2aa3f6 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -357,22 +357,25 @@ static unsigned long elf_map(struct file *filep, unsigned long addr,
> return addr;
>
> /*
> - * total_size is the size of the ELF (interpreter) image.
> - * The _first_ mmap needs to know the full size, otherwise
> - * randomization might put this image into an overlapping
> - * position with the ELF binary image. (since size < total_size)
> - * So we first map the 'big' image - and unmap the remainder at
> - * the end. (which unmap is needed for ELF images with holes.)
> + * total_size is the size of the ELF binary image or the ELF loader
> + * image. For loader image, the _first_ mmap needs to know the full
> + * size, otherwise randomization might put image into an overlapping
> + * position with the ELF binary image.(since size < total_size)
> + * So we use total_size to get an area to cover the whole loader image,
> + * then map the 1st progment segment only with its own size. For binary
> + * image, similarly, the _first_ mmap also needs to know the full size,
> + * otherwise randomization might put image above mm->mmap_base.
> */
> if (total_size) {
> total_size = ELF_PAGEALIGN(total_size);
> - map_addr = vm_mmap(filep, addr, total_size, prot, flags, off);
> - if (!BAD_ADDR(map_addr))
> - vm_munmap(map_addr+size, total_size-size);
> - } else
> - map_addr = vm_mmap(filep, addr, size, prot, flags, off);
> + addr = get_unmapped_area(file, addr, total_size, off, flags);

So how does this prevent clobbering an existing VMA when flags contains
MAP_FIXED?

> + if (offset_in_page(addr))
> + return addr;
> + }
> +
> + map_addr = vm_mmap(filep, addr, size, prot, flags, off);
>
> - return(map_addr);
> + return (map_addr);
> }
>
> #endif /* !elf_map */
> --
> 2.5.5

--
Michal Hocko
SUSE Labs

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