Re: [PATCH 1/2] fs, elf: drop MAP_FIXED usage from elf_map

[James Hogan <james.hogan@mips.com>]

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On Mon, Oct 16, 2017 at 09:00:47PM +0200, Michal Hocko wrote:
> [CCing metag people for the metag elf_map implementation specific. The thread
> starts here http://lkml.kernel.org/r/20171016134446.19910-1-mhocko@kernel.org]
> 
> On Mon 16-10-17 09:39:14, Kees Cook wrote:
> > On Mon, Oct 16, 2017 at 6:44 AM, Michal Hocko <mhocko@kernel.org> wrote:
> > > +               return -EAGAIN;
> > > +       }
> > > +
> > > +       return map_addr;
> > > +}
> > > +
> > >  static unsigned long elf_map(struct file *filep, unsigned long addr,
> > >                 struct elf_phdr *eppnt, int prot, int type,
> > >                 unsigned long total_size)
> > > @@ -366,11 +389,11 @@ static unsigned long elf_map(struct file *filep, unsigned long addr,
> > 
> > elf_map is redirected on metag -- it should probably have its vm_mmap
> > calls adjust too.
> 
> Thanks for spotting this. I am not really familiar with metag. It seems
> to clear MAP_FIXED already
> 	tcm_tag = tcm_lookup_tag(addr);
> 
> 	if (tcm_tag != TCM_INVALID_TAG)
> 		type &= ~MAP_FIXED;
> 
> So if there is a tag the flag is cleared. I do not understand this code
> (and git log doesn't help) but why is this MAP_FIXED code really needed?

This function was added to the metag port in mid-2010 to support ELFs
with tightly coupled memory (TCM) segments, for example metag "core"
memories are at fixed virtual addresses and aren't MMU mappable (i.e.
globally accessible), and are outside of the usual userland address
range, but are as fast as cache. The commit message says this:

> Override the definition of the elf_map() function to special case
> sections that are loaded at the address of the internal memories.
> If we have such a section, map it at a different address and copy
> the contents of the section into the appropriate memory.

So yeh, it looks like if the section is meant to use TCM based on the
virtual address, it drops MAP_FIXED so that the vm_mmap can succeed
(because its outside the normally valid range), and then copies it
directly to the desired TCM so the program can use it.

Hope that helps add some context to understand whats needed.

There was some description of this in an ELCE-2010 talk by the original
author Will Newton that may also be of interest [1].

Cheers
James

[1] http://free-electrons.com/blog/elce-2010-videos/
    "Exploiting On-chip Memories in Embedded Linux Applications"
    See slides about "core memories".

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