Syslinux - Jul 2013 - [PATCH 1/1] core: Add a check at ldlinux.sys build time.

On 07/26/2013 08:10 AM, Matt Fleming wrote:
> On Wed, 24 Jul, at 08:05:16AM, Raphael S.Carvalho wrote:
>> Check if ldlinux.sys is larger than 64k at build time.
>>
>> Signed-off-by: Raphael S.Carvalho <raphael.scarv at gmail.com>
>> ---
>>  core/Makefile         |    5 +++--
>>  core/ldlinux_limit.pl |   31 +++++++++++++++++++++++++++++++
>>  2 files changed, 34 insertions(+), 2 deletions(-)
>>  create mode 100644 core/ldlinux_limit.pl
> 
> Thanks, this looks good, though the patch was whitespace damaged. I also
> expanded on the commit message to explain *why* 64K is the limit we use.
> 
> I'll push this out once I've shrunk ldlinux.sys down to size.
> 
The test is wrong: we need it to fit into 64K *including* the boot
sector and two copies of the ADV.  Currently the ADV is 512 bytes, but
we probably need to make it 4K.

	-hpa

On Fri, Jul 26, 2013 at 4:58 PM, H. Peter Anvin <hpa at zytor.com> wrote:
> On 07/26/2013 08:10 AM, Matt Fleming wrote:
> > On Wed, 24 Jul, at 08:05:16AM, Raphael S.Carvalho wrote:
> >> Check if ldlinux.sys is larger than 64k at build time.
> >>
> >> Signed-off-by: Raphael S.Carvalho <raphael.scarv at
gmail.com>
> >> ---
> >>  core/Makefile         |    5 +++--
> >>  core/ldlinux_limit.pl |   31 +++++++++++++++++++++++++++++++
> >>  2 files changed, 34 insertions(+), 2 deletions(-)
> >>  create mode 100644 core/ldlinux_limit.pl
> >
> > Thanks, this looks good, though the patch was whitespace damaged. I
also
> > expanded on the commit message to explain *why* 64K is the limit we
use.
> >
> > I'll push this out once I've shrunk ldlinux.sys down to size.
> >
>
> The test is wrong: we need it to fit into 64K *including* the boot
> sector and two copies of the ADV.  Currently the ADV is 512 bytes, but
> we probably need to make it 4K.
>
>         -hpa
>
>
Bootsector is installed into the 0-512 range, whereas 2 copies of ADV into
(65536 - 2 * ADV_SIZE).
Then basically ldlinux.sys must fit between the bootsector and two copies
of ADV whose size may vary.

The range 0-64k is laid out something like this, right?!
[0](bootsector)[512](ldlinux.sys)[65536 - 2 * ADV_SIZE](2 copies of
ADV)[65536]

If so, the size of ldlinux.sys can be at most: 65536 - 2 * ADV_SIZE - 512.
If ADV_SIZE change from 512 to 4k, it will be difficult to install
ldlinux.sys at the 0-64k range of a BTRFS image.

Regards,
Raphael S. Carvalho.

On 07/26/2013 01:36 PM, Raphael S Carvalho wrote:
> 
> Bootsector is installed into the 0-512 range, whereas 2 copies of ADV
> into (65536 - 2 * ADV_SIZE).
> Then basically ldlinux.sys must fit between the bootsector and two
> copies of ADV whose size may vary.
> 
> The range 0-64k is laid out something like this, right?!
> [0](bootsector)[512](ldlinux.sys)[65536 - 2 * ADV_SIZE](2 copies of
> ADV)[65536]
> 
> If so, the size of ldlinux.sys can be at most: 65536 - 2 * ADV_SIZE - 512.
> If ADV_SIZE change from 512 to 4k, it will be difficult to install
> ldlinux.sys at the 0-64k range of a BTRFS image.
> 
That is correct.  We may have to do something like Grub's "stage
1.5", a
totally stripped down version of ldlinux.sys containing just enough to
load the real ldlinux.sys from the filesystem.  Sigh.

	-hpa