Re: [PATCH 5/6] refactor and simplify ELF loader in objcodes.c

[Andy Wingo]

On Thu 23 May 2013 22:35, address@hidden (Ludovic Courtès) writes:

>> The best is to use mmap.  But if you fall back, malloc is OK, and we can
>> manually align it.  In any case the loader ends up adding part of the
>> image to the GC roots, so it's probably best that the memory not be
>> managed by the GC itself.
>
> But malloc’d and mmap’d regions are not scanned by default, so here they
> would need to be scanned, or the GC-managed data they refer to would
> need to be registered as root, no?
>
> (This reminds me of
> <http://thread.gmane.org/gmane.comp.programming.garbage-collection.boehmgc/2576>,
> except that with Guile’s loader GNU_RELRO regions could refer to
> GC-managed data.)

See the patch :)  There is a part of the dynamic section that specifies
the GC roots, which are added using GC_add_roots.

>>> Since .go are only produced by Guile, can it really happen to have .go
>>> files without 8-byte alignment?
>>
>> Yes!  If the system supports mmap, .go files should be produced with
>> 4096-byte alignment so that the whole thing can be mapped read-only, and
>> write permissions only need be given to the writable pages.  (Mprotect
>> only works on a page level.)
>
> Oh, I’m lost.  Is it 8B or 4KiB?  Can’t .go files always be
> page-aligned?

In 2.0 the alignment is 8 bytes.  In 2.2, it can be any multiple of 8
bytes -- including 4096.

>From linker.scm:

    ;;; By default, the ELF files created by the linker include some padding
    ;;; so that different parts of the file can be loaded in with different
    ;;; permissions.  For example, some parts of the file are read-only and
    ;;; thus can be shared between processes.  Some parts of the file don't
    ;;; need to be loaded at all.  However this padding can be too much for
    ;;; interactive compilation, when the code is never written out to disk;
    ;;; in that case, pass #:page-aligned? #f to `link-elf'.

And:

    If @var{page-aligned?} is true, read-only and writable data are
    separated so that only those writable parts of the image need be mapped
    with writable permissions.  This is suitable for situations where you
    would write a file out to disk and read it in with @code{mmap}.
    Otherwise if @var{page-aligned?} is false, sections default to 8-byte
    alignment.

Andy
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