- Fix the vmlinux size check on 64-bit along with adding useful clarifications on the topic

(Arvind Sankar)
 
 - Remove -m16 workaround now that the GCC versions that need it are unsupported
 (Nick Desaulniers)
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Merge tag 'x86_build_for_v5.11' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 build updates from Borislav Petkov:
 "Two x86 build fixes:

   - Fix the vmlinux size check on 64-bit along with adding useful
     clarifications on the topic (Arvind Sankar)

   - Remove -m16 workaround now that the GCC versions that need it are
     unsupported (Nick Desaulniers)"

* tag 'x86_build_for_v5.11' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/build: Remove -m16 workaround for unsupported versions of GCC
  x86/build: Fix vmlinux size check on 64-bit

arch/x86/Makefile

@@ -24,14 +24,7 @@ endif
 
 # How to compile the 16-bit code.  Note we always compile for -march=i386;
 # that way we can complain to the user if the CPU is insufficient.
-#
+REALMODE_CFLAGS	:= -m16 -g -Os -DDISABLE_BRANCH_PROFILING \
-# The -m16 option is supported by GCC >= 4.9 and clang >= 3.5. For
-# older versions of GCC, include an *assembly* header to make sure that
-# gcc doesn't play any games behind our back.
-CODE16GCC_CFLAGS := -m32 -Wa,$(srctree)/arch/x86/boot/code16gcc.h
-M16_CFLAGS	 := $(call cc-option, -m16, $(CODE16GCC_CFLAGS))
-
-REALMODE_CFLAGS	:= $(M16_CFLAGS) -g -Os -DDISABLE_BRANCH_PROFILING \
 		   -Wall -Wstrict-prototypes -march=i386 -mregparm=3 \
 		   -fno-strict-aliasing -fomit-frame-pointer -fno-pic \
 		   -mno-mmx -mno-sse

arch/x86/boot/code16gcc.h

@@ -1,12 +0,0 @@
-/* SPDX-License-Identifier: GPL-2.0 */
-#
-# code16gcc.h
-#
-# This file is added to the assembler via -Wa when compiling 16-bit C code.
-# This is done this way instead via asm() to make sure gcc does not reorder
-# things around us.
-#
-# gcc 4.9+ has a real -m16 option so we can drop this hack long term.
-#
-
-	.code16gcc

arch/x86/include/asm/page_32_types.h

@@ -53,7 +53,13 @@
 #define STACK_TOP_MAX		STACK_TOP
 
 /*
- * Kernel image size is limited to 512 MB (see in arch/x86/kernel/head_32.S)
+ * In spite of the name, KERNEL_IMAGE_SIZE is a limit on the maximum virtual
+ * address for the kernel image, rather than the limit on the size itself. On
+ * 32-bit, this is not a strict limit, but this value is used to limit the
+ * link-time virtual address range of the kernel, and by KASLR to limit the
+ * randomized address from which the kernel is executed. A relocatable kernel
+ * can be loaded somewhat higher than KERNEL_IMAGE_SIZE as long as enough space
+ * remains for the vmalloc area.
  */
 #define KERNEL_IMAGE_SIZE	(512 * 1024 * 1024)
 

arch/x86/include/asm/page_64_types.h

@@ -98,8 +98,10 @@
 #define STACK_TOP_MAX		TASK_SIZE_MAX
 
 /*
- * Maximum kernel image size is limited to 1 GiB, due to the fixmap living
+ * In spite of the name, KERNEL_IMAGE_SIZE is a limit on the maximum virtual
- * in the next 1 GiB (see level2_kernel_pgt in arch/x86/kernel/head_64.S).
+ * address for the kernel image, rather than the limit on the size itself.
+ * This can be at most 1 GiB, due to the fixmap living in the next 1 GiB (see
+ * level2_kernel_pgt in arch/x86/kernel/head_64.S).
  *
  * On KASLR use 1 GiB by default, leaving 1 GiB for modules once the
  * page tables are fully set up.

arch/x86/include/asm/pgtable_32.h

@@ -57,19 +57,13 @@ do {						\
 #endif
 
 /*
- * This is how much memory in addition to the memory covered up to
+ * This is used to calculate the .brk reservation for initial pagetables.
- * and including _end we need mapped initially.
+ * Enough space is reserved to allocate pagetables sufficient to cover all
- * We need:
+ * of LOWMEM_PAGES, which is an upper bound on the size of the direct map of
- *     (KERNEL_IMAGE_SIZE/4096) / 1024 pages (worst case, non PAE)
+ * lowmem.
- *     (KERNEL_IMAGE_SIZE/4096) / 512 + 4 pages (worst case for PAE)
  *
- * Modulo rounding, each megabyte assigned here requires a kilobyte of
+ * With PAE paging (PTRS_PER_PMD > 1), we allocate PTRS_PER_PGD == 4 pages for
- * memory, which is currently unreclaimed.
+ * the PMD's in addition to the pages required for the last level pagetables.
- *
- * This should be a multiple of a page.
- *
- * KERNEL_IMAGE_SIZE should be greater than pa(_end)
- * and small than max_low_pfn, otherwise will waste some page table entries
  */
 #if PTRS_PER_PMD > 1
 #define PAGE_TABLE_SIZE(pages) (((pages) / PTRS_PER_PMD) + PTRS_PER_PGD)

arch/x86/kernel/head_64.S

@@ -531,21 +531,19 @@ SYM_DATA_END(level3_kernel_pgt)
 
 SYM_DATA_START_PAGE_ALIGNED(level2_kernel_pgt)
 	/*
-	 * 512 MB kernel mapping. We spend a full page on this pagetable
+	 * Kernel high mapping.
-	 * anyway.
 	 *
-	 * The kernel code+data+bss must not be bigger than that.
+	 * The kernel code+data+bss must be located below KERNEL_IMAGE_SIZE in
+	 * virtual address space, which is 1 GiB if RANDOMIZE_BASE is enabled,
+	 * 512 MiB otherwise.
 	 *
-	 * (NOTE: at +512MB starts the module area, see MODULES_VADDR.
+	 * (NOTE: after that starts the module area, see MODULES_VADDR.)
-	 *  If you want to increase this then increase MODULES_VADDR
-	 *  too.)
 	 *
-	 *  This table is eventually used by the kernel during normal
+	 * This table is eventually used by the kernel during normal runtime.
-	 *  runtime.  Care must be taken to clear out undesired bits
+	 * Care must be taken to clear out undesired bits later, like _PAGE_RW
-	 *  later, like _PAGE_RW or _PAGE_GLOBAL in some cases.
+	 * or _PAGE_GLOBAL in some cases.
 	 */
-	PMDS(0, __PAGE_KERNEL_LARGE_EXEC,
+	PMDS(0, __PAGE_KERNEL_LARGE_EXEC, KERNEL_IMAGE_SIZE/PMD_SIZE)
-		KERNEL_IMAGE_SIZE/PMD_SIZE)
 SYM_DATA_END(level2_kernel_pgt)
 
 SYM_DATA_START_PAGE_ALIGNED(level2_fixmap_pgt)

arch/x86/kernel/vmlinux.lds.S

@@ -454,13 +454,13 @@ 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 +470,12 @@ 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");
-
 #ifdef CONFIG_SMP
 . = ASSERT((fixed_percpu_data == 0),
            "fixed_percpu_data is not at start of per-cpu area");
 #endif
 
-#endif /* CONFIG_X86_32 */
+#endif /* CONFIG_X86_64 */
 
 #ifdef CONFIG_KEXEC_CORE
 #include <asm/kexec.h>