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Re: thinking out loud: wip-rtl, ELF, pages, and mmap
From: |
dsmich |
Subject: |
Re: thinking out loud: wip-rtl, ELF, pages, and mmap |
Date: |
Wed, 24 Apr 2013 18:20:27 -0400 |
---- Andy Wingo <address@hidden> wrote:
...
> Then I realized: why am I doing all of this if the kernel has a virtual
> memory system already that does all this for me?
>
> So I have a new plan, I think. I'll change the linker to always emit
> sections and segments that correspond exactly in their on-disk layout
> and in their in-memory layout. (In ELF terms: segments are contiguous,
> with p_memsz == p_filesz.) I'll put commonly needed things at the
> beginning, and debugging info and the section table at the end. Then
> I'll just map the whole thing with PROT_READ, and set PROT_WRITE on
> those page-aligned segments that need it. (Obviously in the future,
> PROT_EXEC as well.)
>
> Then I'll just record a list of ELF objects that have been loaded.
> Simple bisection will map IP -> ELF, and from there we have the section
> table in memory (lazily paged in by the virtual memory system) and can
> find the symtab and other debug info.
>
> So that's the plan. It's a significant change, and I wondered if folks
> had some experience or reactions.
>
> Note that we have a read()-based fallback if mmap is not available.
> This strategy also makes the read-based fallback easier.
>
> Thoughts?
Sounds like a great idea! I think it's good to use the facilities
that already exist instead of reinventing the wheel.
A thought was that this would artificially bloat the size of the process
when the extra info isn't being used. But so what? The *real* memory
won't be consumed. And it's not like we are running out of addressable
memory space anyway. It will be a bit ickier for systems without
virtual memory.
What about non-ELF systems? You are using your own ELF reader and mmap, not
relying on the system to load the files right? So that's not an issue?
-Dale
- thinking out loud: wip-rtl, ELF, pages, and mmap, Andy Wingo, 2013/04/24
- Re: thinking out loud: wip-rtl, ELF, pages, and mmap,
dsmich <=
- Re: thinking out loud: wip-rtl, ELF, pages, and mmap, Andy Wingo, 2013/04/28
- Re: thinking out loud: wip-rtl, ELF, pages, and mmap, Ludovic Courtès, 2013/04/28
- Re: thinking out loud: wip-rtl, ELF, pages, and mmap, Nala Ginrut, 2013/04/29