linux-kernel - Re: MAP_FIXED for ELF mappings

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Message-ID: <20171005163320.GA20848@redhat.com>
Date:   Thu, 5 Oct 2017 18:33:20 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Baoquan He <bhe@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Jiri Kosina <jkosina@...e.cz>,
        Al Viro <viro@...iv.linux.org.uk>, Ingo Molnar <mingo@...e.hu>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: MAP_FIXED for ELF mappings

On 10/04, Michal Hocko wrote:
>
> On Wed 04-10-17 23:12:38, Baoquan He wrote:
> >  	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?

I got lost...

this is just cleanup, it should not change the behaviour, with or without
MAP_FIXED. It just avoids the mmap(total_size) + munmap(extra_size).

Oleg.