Re: Looking for rationale for the minidump format

On Mon, 2021-11-22 at 09:47 -0800, John Baldwin wrote:
> On 11/21/21 6:42 AM, Michał Górny wrote:
> > Hi, everyone.
> > 
> > As part of the work contracted by the FreeBSD Foundation, I'm working
> > on adding explicit minidump support to LLDB.  When discussing
> > the options with upstream, I've been asked why FreeBSD created their own
> > minidump format.
> > 
> > I did a bit of digging but TBH all the rationale I could get was to
> > create partial memory dumps.  However, unless I'm mistaken the ELF
> > format is perfectly capable of that -- e.g. via creating an explicit
> > segment for every continuous active region.
> > 
> > Does anyone happen to know what the original rationale for creating
> > a custom file format was, or know where to find one?  Thanks in advance.
> 
> The direct map aliases pages mapped via kmem.  You'd be double dumping
> all the data mapped into kmem, once for the direct map and once for the
> non-direct mappings.
> 
> You can think of minidumps as being a dump of physical memory, whereas
> an ELF core for a virtually-mapped kernel wants to dump virtual memory,
> and there is the disconnect.
> 
> [...]
> 
> You could perhaps imagine something similar where you had an ELF core
> with physical memory for PT_LOAD instead of virtual and a way to hint that
> so that the debugger would handle all the virtual -> PA translation, but
> you'd still need some home-grown notes for some of the other metadata we
> pass along (like the message buffer, etc.).  Also, changing the format
> doesn't help with reading existing crash dumps.
> 

Thank you for your reply.  If I understand correctly, you're comparing
minidump with a "proper" (i.e. virtual memory-based) ELF core.  However,
the "full memory dump" ELF core also uses physical memory map model, is
that correct?  Does that mean that using a different core format makes
it clear that it's a physical memory dump and not virtual?

That said, please correct me if I'm mistaken but I think we should be
able to create a "virtual memory mapped" ELF core without too much
duplication.  We could creating multiple segments with different p_vaddr
values but the same file p_offset, correct (and maybe p_paddr)?  I'm not
advocating for changing the format, just trying to improve my knowledge.

-- 
Best regards,
Michał Górny