bugs in loader/i386/pc/multiboot.c

[Tom Bachmann]

Hi list,

first of all, please CC me in your replies, I'm not subscribed.

I have been using grub2 for a while now (it's just nicer than grub 
legacy to work with, especially as it can load my 64 bit elf files). I 
didn't realize, however, that you switched to svn (guess that says 
something about my attentiveness - and the usefulness of grub2), so I 
just today got a report from a codeveloper who said he couldn't boot my 
code with the newest grub2 build. I'm now running grub2 (latest svn) 
inside qemu-system-x86_64 (version 0.9.1) on x86_64 (debian).

Now, as it turns out, it doesn't work for me, either. After fiddling 
around with my kernel image for quite a while I convinced myself that 
this is a grub bug: even with _start: jmp _start (i.e. the entire kernel 
effectively consisting of a while(1)/*do nothing*/; loop), qemu either 
signals a triple-fault or the code loops at a completely different 
address. Using manually inserted calls to grub_printf, I found the 
following problems in loader/i386/pc/multiboot.c.

1) The entry address is overwritten.
At line 462 grub_multiboot_load_elf is called. This initializes the 
variable ENTRY as a side effect. However, at lines 478/484 this variable 
is overwritten with a (as far as I can tell) non-sensical value.
I'm really trying not to make this sound like a rant, but I cannot 
believe this code has ever worked in this constellation.

2) Reading the mmap doesn't work.
Now this could be a qemu artifact (would still be nice to fix), but even 
with the two above-mentioned lines removed the code doesn't seem to 
work. I still have to uncomment lines 466-468 which call 
grub_fill_multiboot_mmap. This call doesn't seem to return. Indeed my 
introspection printfs suggest that the inner hook of that function is 
called three times, but adding printfs to grub_machine_map_iterate (in 
kern/i386/pc/mmap.c I believe) doesn't seem to work (just crashes grub?) 
and so I had to stop looking around here.

I hope my description is helpful and I have included everything necessary.

Thanks,
Tom
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-ness-