Messages in this thread | | | From | Arvind Sankar <> | Date | Wed, 28 Oct 2020 12:45:51 -0400 | Subject | Re: [PATCH] x86/build: Fix vmlinux size check on 64-bit |
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On Wed, Oct 28, 2020 at 02:39:09PM +0100, Borislav Petkov wrote: > On Tue, Oct 27, 2020 at 05:14:22PM -0400, Arvind Sankar wrote: > > This is indeed just a small correctness fixlet, but I'm not following > > the rest of your comments. > > I'm just trying to make sense of that house of cards we have here. > > > PHYSICAL_START has an effect independent of the setting of > > RELOCATABLE. > > Theoretically you can set PHYSICAL_START to 0x0: > > config PHYSICAL_START > hex "Physical address where the kernel is loaded" if (EXPERT || CRASH_DUMP) > default "0x1000000" > help > This gives the physical address where the kernel is loaded. > > If kernel is a not relocatable (CONFIG_RELOCATABLE=n) then > bzImage will decompress itself to above physical address and > run from there. > ^^^^^^^^^^^^^^ > > and disable RELOCATABLE: > > CONFIG_PHYSICAL_START=0x0 > # CONFIG_RELOCATABLE is not set > > but then you hit this: > > ld: per-CPU data too large - increase CONFIG_PHYSICAL_START > > full output at the end of the mail.
You don't want to try to run the kernel from physical address 0 in any case. The default is set to 16MiB to avoid low memory, historically to avoid the 24-bit ISA DMA range.
> > That said, AFAICT, RELOCATABLE and PHYSICAL_START look like historical > > artifacts at this point: RELOCATABLE should be completely irrelevant for > > the 64-bit kernel, and there's really no reason to be able to configure > > the start VA of the kernel, that should just be constant independent of > > PHYSICAL_START. > > See the CONFIG_PHYSICAL_START help text. Apparently there has been a > use case where one can set PHYSICAL_START to the region where a kdump > kernel is going to be loaded and that kdump kernel is a vmlinux and not > a bzImage and thus not relocatable.
This doesn't matter for the 64-bit kernel, which can be run from any physical address independent of the RELOCATABLE/PHYSICAL_START settings. It only matters on 32-bit, where VA and PA are tied together by VA == __PAGE_OFFSET + PA On 64-bit, the kernel's location in VA space and physical space can be independently moved around, so a kernel that starts at 16MiB in VA space can be loaded anywhere above 16MiB in physical space.
> > Going back to the question at hand, if you think about it, the kernel > image *is* between _text or _stext and _end. And KERNEL_IMAGE_SIZE is > exactly what it is - the size of the kernel image. > > Now, if you were talking about a kernel *mapping* size, then I'd > understand but this check is for the kernel *image* size. >
KERNEL_IMAGE_SIZE is _not_ the size of the kernel image, the name is misleading. It is the maximum VA that the kernel can occupy, it is used to prepopulate the PMD-level pagetable for initial boot (level2_kernel_pgt) and is also used to define MODULES_VADDR, so it _is_ talking about mappings. If you have a 30MiB kernel that is placed at a starting VA of 510MiB when KERNEL_IMAGE_SIZE is 512MiB, it won't boot.
> But reading that commit message again: > > these build-time and link-time checks would have prevented the > vmlinux size regression. > > this *is* talking about vmlinux size and that starts at _text... >
Increasing vmlinux size can trigger the problem by pushing _end beyond KERNEL_IMAGE_SIZE, but the problem occurs once _end - __START_KERNEL_map exceeds KERNEL_IMAGE_SIZE, not when _end - _text exceeds it, hence this patch.
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