On 10/07/14 21:36, Matt Fleming wrote:
On Thu, 10 Jul, at 04:59:23PM, Michael Brown wrote:
The kernel will align itself to the nearest boundary specified by the
kernel_alignment field in the bzImage header. If the kernel is loaded
to an address which is not sufficiently aligned, it will therefore use
memory beyond that indicated solely by the init_size field.
The PE/COFF headers now include a .bss section to describe the
requirements of the init_size field, but do not currently expose the
alignment requirement. Consequently, a kernel loaded via the PE entry
point may still end up overwriting unexpected areas of memory.
Is this actually true? There is code within the EFI boot stub to
allocate space for the kernel image and perform the relocation if it's
not already suitably aligned.
Or is the above paragraph referring to the previously merged patch?
The "...headers now include..." part was referring to the previously
merged patch to add the .bss section.
I haven't actually looked at the code which performs the alignment; I
was going on hpa's concern that merely exposing init_size would be
insufficient due to the potential for alignment. My understanding
(possibly incorrect) was that the alignment was carried out using
something simple along the lines of:
new_kernel_start = align ( kernel_start, kernel_alignment );
memmove ( new_kernel_start, kernel_start, kernel_len );
i.e. that the memory used for alignment was not explicitly allocated.
If the EFI boot stub instead allocates space for the aligned kernel
using AllocatePages() (and allocates enough space for the whole of
init_size), then the problem I described does not exist.
Michael
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