Messages in this thread | | | From | Arvind Sankar <> | Date | Tue, 27 Oct 2020 17:14:22 -0400 | Subject | Re: [PATCH] x86/build: Fix vmlinux size check on 64-bit |
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On Tue, Oct 27, 2020 at 09:08:03PM +0100, Borislav Petkov wrote: > On Mon, Oct 05, 2020 at 11:15:39AM -0400, Arvind Sankar wrote: > > Commit b4e0409a36f4 ("x86: check vmlinux limits, 64-bit") added a check > > that the size of the 64-bit kernel is less than KERNEL_IMAGE_SIZE. > > > > The check uses (_end - _text), but this is not enough. The initial PMD > > used in startup_64() (level2_kernel_pgt) can only map upto > > KERNEL_IMAGE_SIZE from __START_KERNEL_map, not from _text. > > > > The correct check is the same as for 32-bit, since LOAD_OFFSET is > > defined appropriately for the two architectures. Just check > > (_end - LOAD_OFFSET) against KERNEL_IMAGE_SIZE unconditionally. > > > > Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu> > > --- > > arch/x86/kernel/vmlinux.lds.S | 11 ++--------- > > 1 file changed, 2 insertions(+), 9 deletions(-) > > > > diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S > > index bf9e0adb5b7e..b38832821b98 100644 > > --- a/arch/x86/kernel/vmlinux.lds.S > > +++ b/arch/x86/kernel/vmlinux.lds.S > > @@ -454,13 +454,12 @@ SECTIONS > > ASSERT(SIZEOF(.rela.dyn) == 0, "Unexpected run-time relocations (.rela) detected!") > > } > > > > -#ifdef CONFIG_X86_32 > > /* > > * The ASSERT() sink to . is intentional, for binutils 2.14 compatibility: > > */ > > . = ASSERT((_end - LOAD_OFFSET <= KERNEL_IMAGE_SIZE), > > "kernel image bigger than KERNEL_IMAGE_SIZE"); > > -#else > > +#ifdef CONFIG_X86_64 > > /* > > * Per-cpu symbols which need to be offset from __per_cpu_load > > * for the boot processor. > > @@ -470,18 +469,12 @@ INIT_PER_CPU(gdt_page); > > INIT_PER_CPU(fixed_percpu_data); > > INIT_PER_CPU(irq_stack_backing_store); > > > > -/* > > - * Build-time check on the image size: > > - */ > > -. = ASSERT((_end - _text <= KERNEL_IMAGE_SIZE), > > - "kernel image bigger than KERNEL_IMAGE_SIZE"); > > So we have this: > > SECTIONS > { > #ifdef CONFIG_X86_32 > . = LOAD_OFFSET + LOAD_PHYSICAL_ADDR; > phys_startup_32 = ABSOLUTE(startup_32 - LOAD_OFFSET); > #else > . = __START_KERNEL; > ^^^^^^^^^^ > > which sets the location counter to > > #define __START_KERNEL (__START_KERNEL_map + __PHYSICAL_START) > > which is 0xffffffff80000000 + ALIGN(CONFIG_PHYSICAL_START, CONFIG_PHYSICAL_ALIGN) > > and that second term after the '+' has effect only when > CONFIG_RELOCATABLE=n and that's not really used on modern kernel configs > as RELOCATABLE is selected by EFI_STUB and RANDOMIZE_BASE depends on at > and and ... > > So IOW, in a usual .config we have: > > __START_KERNEL_map at 0xffffffff80000000 > _text at 0xffffffff81000000 > > So practically and for the majority of configs, the kernel image really > does start at _text and not at __START_KERNEL_map and we map 16Mb which > is 4 PMDs of unused pages. So basically you're correcting that here - > that the number tested against KERNEL_IMAGE_SIZE is 16Mb more. > > Yes, no? > > Or am I missing some more important aspect and this is more than just a > small correctness fixlet? > > Thx. >
This is indeed just a small correctness fixlet, but I'm not following the rest of your comments. PHYSICAL_START has an effect independent of the setting of RELOCATABLE. It's where the kernel image starts in virtual address space, as shown by the 16MiB difference between __START_KERNEL_map and _text in the usual .config situation. In all configs, not just majority, the kernel image itself starts at _text. The 16MiB gap below _text is not actually mapped, but the important point is that the way the initial construction of pagetables is currently setup, the code cannot map anything above __START_KERNEL_map + KERNEL_IMAGE_SIZE, so _end needs to be below that.
If KASLR was disabled (either at build-time or run-time), these link-time addresses are where the kernel actually lives (in VA space); and if it was enabled, it will make sure to place the _end of the kernel below KERNEL_IMAGE_SIZE when choosing a random virtual location.
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.
Thanks.
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