While testing the 86-DOS lDebug [1] booting from GRUB2, newer versions of the
debugger would fail to load when booted using GRUB's freedos command. The
behaviour observed in a qemu i386 machine was that the ROM-BIOS's boot load
would start anew, instead of loading the selected debugger as kernel.
It came to light that there was a size limit: Kernel files that were 58880
bytes (E600h) long or shorter succeeded to boot, while files that were 64000
bytes or longer failed in the manner described.
Eventually it turned out that the relocator16 stub succeeded whenever it was
placed completely within the first 64 KiB of the Low Memory Area. The chunk
for the relocator is allocated with a minimum address of 0x8010 and a maximum
address just below 0xA0000 [2]. That means if the kernel is, for instance,
E600h bytes long, then the kernel will be allocated memory starting at 00600h
(the fixed FreeDOS kernel load address) up to E600h + 00600h = 0EC00h, which
leaves 1400h (5120) bytes for the relocator to stay in the first 64 KiB.
If the kernel is 64000 bytes (FA00h) long, then the relocator must go to
FA00h + 00600h = 10000h at least which is outside the first 64 KiB.
The problem is that the relocator16 initialises the DS register with a
"pseudo real mode" descriptor, which is defined with a segment limit of
64 KiB and a segment base of zero. After that, the relocator addressed
parts of itself (implicitly) using the DS register, with an offset from
ESI, which holds the linear address of the relocator's base [3]. With the
larger kernel files this would lead to accessing data beyond the 64 KiB
segment limit, presumably leading to a fault and perhaps a subsequent
triple-fault or such.
This patch fixes the relocator to set the segment base of the descriptors
to the base address of the relocator; then, the subsequent accesses to
the relocator's variables are done without the ESI register as an index.
This does not interfere with the relocator's or its target's normal
operation; the segment limits are still loaded with 64 KiB and all the
segment bases are subsequently reset by the relocator anyway.
Current versions of the debugger to test are uploaded to [4]. The file
ldebugnh.com (LZ4-compressed and built with -D_EXTHELP=0) at 58368 bytes
loads successfully, whereas ldebug.com at 64000 bytes fails. Loading one
of these files requires setting root to a FAT FS partition and using the
freedos command to specify the file as kernel:
set root='(hd0,msdos1)'
freedos /ldebug.com
boot
Booting the file using the multiboot command (which uses a WIP entrypoint
of the debugger) works, as it does not use GRUB's relocator16 but instead
includes a loader in the kernel itself, which drops it back to 86 Mode.
[1]: https://hg.ulukai.org/ecm/ldebug
[2]: http://git.savannah.gnu.org/cgit/grub.git/tree/grub-core/lib/i386/relocator.c?id=495781f5ed1b48bf27f16c53940d6700c181c74c#n127
[3]: http://git.savannah.gnu.org/cgit/grub.git/tree/grub-core/lib/i386/relocator16.S?id=495781f5ed1b48bf27f16c53940d6700c181c74c#n97
[4]: https://ulukai.org/ecm/lDebug-5479a7988d21-nohelp.zip
Signed-off-by: C. Masloch <pushbx@ulukai.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
In order to maintain the coding style consistency, it was requested to
replace the methods that use "__asm__ __volatile__" with "asm volatile".
Signed-off-by: Jesús Diéguez Fernández <jesusdf@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Commit 0ba90a7f01 ("efi: Move grub_reboot() into kernel") broke
the build on i386-efi - genmoddep.awk bails out with message
grub_reboot in reboot is duplicated in kernel
This is because both lib/i386/reset.c and kern/efi/efi.c now provide
this function.
Rather than explicitly list each i386 platform variant in
Makefile.core.def, include the contents of lib/i386/reset.c only when
GRUB_MACHINE_EFI is not set.
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Unified Extensible Firmware Interface Specification, Version 2.6,
section 2.3.4, x64 Platforms, boot services, says among others:
The stack must be 16-byte aligned. So, do it. Otherwise OS may
boot only by chance as it happens right now.
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Modify the page table construction to allow multiple virtual regions
to be mapped. This is done as preparation for removing the p2m list
from the initial kernel mapping in order to support huge pv domains.
This allows a cleaner approach for mapping the relocator page by
using this capability.
The interface to the assembler level of the relocator has to be changed
in order to be able to process multiple page table areas.
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Add grub_relocator64_efi relocator. It will be used on EFI 64-bit platforms
when multiboot2 compatible image requests MULTIBOOT_TAG_TYPE_EFI_BS. Relocator
will set lower parts of %rax and %rbx accordingly to multiboot2 specification.
On the other hand processor mode, just before jumping into loaded image, will
be set accordingly to Unified Extensible Firmware Interface Specification,
Version 2.4 Errata B, section 2.3.4, x64 Platforms, boot services. This way
loaded image will be able to use EFI boot services without any issues.
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Vladimir Serbinenko <phcoder@gmail.com>
* include/grub/i386/relocator.h (grub_relocator16_state):
New member ebp.
* grub-core/lib/i386/relocator.c (grub_relocator16_ebp): New extern
variable.
(grub_relocator16_boot): Handle %ebp.
* grub-core/lib/i386/relocator16.S: Likewise.
* grub-core/loader/i386/pc/freedos.c:
Load BPB to pass kernel which partition to load from.
Check that kernel file is not too large.
Set register dl to BIOS unit number as well.
* include/grub/i386/pc/int.h (grub_i386_idt): New struct.
(grub_realidt): New var.
* grub-core/lib/i386/relocator16.S (grub_relocator16_idt): New variable
Load idt.
* grub-core/lib/i386/relocator.c (grub_relocator16_idt):
New declaration.
(grub_relocator16_boot): Set grub_relocator16_idt.
* grub-core/kern/i386/realmode.S (realidt): Renamed to ...
(LOCAL(realidt)): ... this.
* grub-core/boot/i386/pc/startup_raw.S: Pass pointer to realidt in eax.
* grub-core/kern/i386/pc/startup.S: Save pointer to realidt.
(grub_realidt): New variable.
part. Instead setup the correct stack in RM.
* grub-core/lib/i386/relocator.c (grub_relocator16_boot): Reserve place
for stack.
* include/grub/i386/relocator_private.h: New file.
* grub-core/lib/i386/relocator16.S (grub_relocator16_start): Move
grub_relocator16_sp to %esp rather than %ss, and zero-extend it.
Fixes Ubuntu bug #683904.
