Messages in this thread | | | Date | Tue, 27 Oct 2020 21:08:03 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH] x86/build: Fix vmlinux size check on 64-bit |
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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.
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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