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SubjectRe: MAP_FIXED for ELF mappings
On 10/04/17 at 05:17pm, Michal Hocko wrote:
> 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.

Oh, no, here, it won't include the PIE binary case. Since it must be
from ELF_ET_DYN_BASE. Here should be the "ld.so program" case.

> > */
> > 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?

Earlier flush_old_exec() is called to clean all old VMAs. Here if
total_size is non-zero only if it's the 1st prgoram segment of dynamic
loader, either from load_elf_interp() or the "ld.so program" case. With
my understanding, it can't be meeting an existing VMA. Not sure if
it's correct. Currently, it haven't passed to ld.so to link other .so.

>
> > + 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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