[syslinux] [PATCH 0/2] Fixes for gcc 5

[Gene Cumm]

On Tue, Sep 29, 2015 at 12:46 AM, celelibi--- via Syslinux
<syslinux at zytor.com> wrote:
> From: Sylvain Gault <sylvain.gault at gmail.com>
>
> TL;DR: The section aligment in linker scripts messed-up the memory mapping
> needed for the compression / decompression to work.
>
> The bug with gcc 5 is not trivial, I'll do my best to explain it here.
>
> Basically, there are two memory mappings of the code. One in "virtual memory",
> and one in "load memory". The one in virtual memory may use the 4G of address
> space, while the one in "load memory" is more compact.
>
> The "load memory" mapping is actually used in syslinux to compress most of the
> code. The early stages of syslinux decompress the code at the correct virtual
> address. So both mappings must have the same data layout.
>
> The matching between both mappings is done through the ld script by specifying
> the LMA (load memory address) which is computed from on the VMA (virtual memory
> address).
>
> However, the way the LMA was computed included adding symbols just before the
> sections to get the VMA. Since the sections are aligned (depending on the
> alignment of its content), those symbols had to be aligned by hand to avoid to
> leave a gap.
>
> With gcc 5, the .data and .rodata sections got sometime a larger alignment (up
> to 32 bytes), which is greater than the 16 bytes that were recorded in the
> linker script. Thus introducing a gap between the real section's VMA and its
> assumed VMA.
>
> The first of these two patches just fix the aligment.
> The second remove the hand-alignment and compute the LMA of the sections so
> that the mappings will always match regardless of the aligment the sections
> need.
>
>
> On a side 2 questions:
> - Is core/syslinux.ld is still used? it looks like a forgotten file.
> - Is there really a bios version for x86_64? Looks like the boot files are
>   i386-specific.
>
> Tests and comments are welcome.
>
>
> Sylvain Gault
>
>  core/i386/syslinux.ld   | 65 ++++++++++---------------------------------------
>  core/x86_64/syslinux.ld | 65 ++++++++++---------------------------------------
>  2 files changed, 26 insertions(+), 104 deletions(-)

Excellent job!  Issues like this are always a challenge.  Thank you.

-- 
-Gene