Re: [PATCH] docs: procs -- Describe /proc/<pid>/map_files entry

[Cyrill Gorcunov]

On Tue, Jan 27, 2015 at 11:50:49AM -0800, Kees Cook wrote:
> > +
> > +The main purpose of map_files directory is to be able to retrieve a set of
> > +memory mapped files in a fast way instead of parsing /proc/<pid>/maps or
> > +/proc/<pid>/smaps which contain a way more records. Same time one can open(2)
> > +mappings from the listings of two processes and comparing inodes figure out
> > +which anonymous memory areas are actually shared.
> 
> Thanks for details! I still don't understand how this is used for
> checkpoint/restore when the mmap offset isn't shown. Can't a process
> map, say 4K of a file, from different offsets, and it would show up
> as:
> 
> 400000-401000 -> /some/file
> 401000-402000 -> /some/file
> 
> but there'd be no way to know how to restore that mapping?

In criu we use a few sources of information (ie we scan not only
map_files, but have to use /proc/pid/smaps as well which has
offset for mapping). So at the end we have all picture under
our hands.

> Are these symlinks "regular" symlinks, or are they something more
> special that bypasses VFS? If it bypasses VFS, I think adding and open
> check with PTRACE_ATTACH is needed, since now you're able to _modify_
> the memory space of the target process instead of just reading it.

Opening them goes same way as open of /proc/pid/fd/ entries as
far as I can tell. This should be enough, or I miss something
obvious here? Otherwise opening /proc/pid/fd/ should use
PTRACE_ATTACH instead of PTRACE_MODE_READ (as in proc_fd_access_allowed).
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