Syslinux - Jan 2014 - chain.c32 Insane primary (MBR) partition

The following boot entries:
#
COM32 chain.c32
APPEND fs ntldr=/bootmgr
#

or:
#
COM32 chain.c32
APPEND hd0,1 ntldr=/bootmgr
#

might result in the following error:

#
Insane primary (MBR) partition.
Can't find myself on the drive I booted from.
#

when using chain.c32 v4.07.

I have seen similar error reports when using chain.c32 v4.06 too.

It works correctly with chain.c32 v4.05, even when originally booting 
with SYSLINUX 4.07 (i.e. by *only* replacing chain.c32 v4.07 with the 
one from v4.05; then it works).

I don't know if chain.c32 versions newer than 4.05 are "less 
tolerant" with the partitioning scheme (or LBA/CHS?) or with the 
exact boot entry code or whether this is something else.

Is this really a bug? A regression? Other?

Is this a problem related to parsing paths? Relative paths? "/" vs. 
"\"?

Suggestions?

TIA,
Ady

PS: Just by chance, could the following commit be somehow related?
 
http://git.zytor.com/?p=syslinux/syslinux.git;a=commit;h=b82f635b7ef05
0a1c7106b213c292134aa8e6c55

On 2014-01-03 08:07, Ady wrote:
> The following boot entries:
> #
> COM32 chain.c32
> APPEND fs ntldr=/bootmgr
> #
>
> or:
> #
> COM32 chain.c32
> APPEND hd0,1 ntldr=/bootmgr
> #
>
> might result in the following error:
>
> #
> Insane primary (MBR) partition.
> Can't find myself on the drive I booted from.
> #
The first line has been adjusted in later patches, but they are in 
elflink and firmware branches only. Currently it would report:

Primary (MBR) with invalid offset and/or length.

This often happens if the disk/image is used on different computers 
(e.g. on usb stick) and bios reports different disk sizes (usually +/- 1 
sector). This is a bios bug (potentially nasty one, imagine GPT which 
stores backup data at the end of the disk as well).

As current version of chain loader supports a good few potentially 
dangerous operations - mostly related to fixing chs geometry to match 
the one reported by bios - it's very strict at testing if everything is 
OK before proceeding.

Current version also supports 'relax' option which turns off most sanity
checks, but it was added a good while after the v4 syslinuxes.

v4.05 works fine, as it was before merging new version of chain.

> On 2014-01-03 08:07, Ady wrote:
> > The following boot entries:
> > #
> > COM32 chain.c32
> > APPEND fs ntldr=/bootmgr
> > #
> >
> > or:
> > #
> > COM32 chain.c32
> > APPEND hd0,1 ntldr=/bootmgr
> > #
> >
> > might result in the following error:
> >
> > #
> > Insane primary (MBR) partition.
> > Can't find myself on the drive I booted from.
> > #
> 
> The first line has been adjusted in later patches, but they are in 
> elflink and firmware branches only. Currently it would report:
> 
> Primary (MBR) with invalid offset and/or length.
> 
> This often happens if the disk/image is used on different computers 
> (e.g. on usb stick) and bios reports different disk sizes (usually +/- 1 
> sector). This is a bios bug (potentially nasty one, imagine GPT which 
> stores backup data at the end of the disk as well).
> 
> As current version of chain loader supports a good few potentially 
> dangerous operations - mostly related to fixing chs geometry to match 
> the one reported by bios - it's very strict at testing if everything is
> OK before proceeding.
> 
> Current version also supports 'relax' option which turns off most
sanity
> checks, but it was added a good while after the v4 syslinuxes.
> 
> v4.05 works fine, as it was before merging new version of chain.
> 
 
I am not sure to which release(s) you are referring to when you say 
"current" chain.c32, but that's just a detail.

Indeed the patches for chain.c32 of 2013Feb14 were applied to the 
elflink and firmware branches, but not to the 4.xx branch.

Simultaneously, chain.c32 included in 4.06-pre2 (just before the 
application of the patches in the other branches) still works, and 
the one included in 4.06-pre3 (just after the patches in the other 
branches) fails as I previously described.

Performing a web search of this issue, the "solution" is always the 
same: to replace the chain.c32 file with the one from v.4.05, even 
when originally booting with Syslinux 4.06 or 4.07.

Probably, there are patches (not only for com32/chain) that were 
originally prepared for the 4.xx branch but were re-based for the 
elflink branch and applied to the latter. So perhaps some patches 
should be applied to the 4.xx branch too.

TIA,
Ady.