Re: [RFC][PATCH] Randomize kernel base address on boot

[H. Peter Anvin]

On 05/25/2011 07:03 AM, Dan Rosenberg wrote:
> 
> My current idea is to use int 0x15, eax = 0xe801 (which seems to be
> nearly universally supported) and use bx/dx to determine the amount of
> contiguous, usable memory above 16 MB, which seems to be exactly what we
> want to know.  If the BIOS does not support this function I'll be sure
> to catch that and skip the randomization.  Likewise, if the amount of
> returned memory seems insufficient or otherwise confusing, I'll skip the
> randomization.
> 

No, sorry.  This has been wrong for over 10 years; there is no
substitute for the full (e820) memory map.  *Furthermore*, based on
where in the bootup sequence you are doing this, you also have to
consider any other memory structures that the kernel needs to be aware
of (initramfs, any chunks in the linked list, the command line, EFI
handover structures, etc.)  This is in fact an arbitrarily complex
operation... we have *finally* gotten the kernel to the point where (a)
the boot loader can actually do the right thing in all cases and (b) the
kernel will reserve or copy all the auxiliary memory chunks it needs at
a very early point.

Sorry, this cannot be short-circuited.

> Given this information, do you have a conservative guess for how close
> to the top of available memory we can put the kernel?  As in, let's say
> we have an XYZ MB chunk of contiguous, free memory, how should I
> calculate the highest, safe place to put the kernel in that region?
> 
> I'm going to continue to enforce the requirement that 16 MB is the
> lowest address we can safely load the kernel, and I'd still appreciate
> any information on why 2/4 MB default alignment might cause problems.

The problem with all of that was backwards compatibility with existing
relocating bootloaders.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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