Re: [Qemu-devel] [PATCH] Remove restriction that prevents bootimg elf64 images

[Anatol Pomozov]

+reply-all

On Thu, Jun 8, 2017 at 1:41 PM, Adam Lackorzynski
<address@hidden> wrote:
>
> On Tue Jun 06, 2017 at 21:41:48 -0700, Anatol Pomozov wrote:
>> It is possible to create a 64 bit elf image that has valid multiboot header.
>> qemu should be able to boot such images.
>
> But this 64bit image actually starts with 32bit code, right?

Correct. The very first part of the startup code has to be 32bit.
After it sets "long mode" it can use 64bit instructions. To make sure
that the preamble has only 32bit instructions one have to use asm
directive such as ".code32".

Here is an example from LitleKernel sturtup code:

https://github.com/littlekernel/lk/blob/master/arch/x86/64/start.S#L50

.code32 tells assembler to treat following text as 32 bit code. And
later when it jumps into "long mode"

https://github.com/littlekernel/lk/blob/master/arch/x86/64/start.S#L214
one can use 64bit code.

> So it's a 32bit program and the check verifies that this is the case.

While preamble have to contain 32 only instructions the rest of the
image can perfectly contain 64bit code. Right now 64bit binary cannot
be run with "qemu-system-x86_64 -kernel". But the same binary runs
fine if packed with GRUB as iso.

I tried to hack around this restriction by adding
"OUTPUT_FORMAT(elf32-i386)" to the linker file and compiling project
with 64bit support. But GNU ld program crashed at Ubuntu 14.04. It
means not that many people use this code path. GNU ld compiled from
HEAD does not have this problem but now GDB is confused by the fact
that ELF contains 64bit code while header reports i386.

Practically there is no reason for this check as it prevents running
64bit binaries with "qemu-system-x86_64 -kernel".