Add an option so the module section is just after the mapped kernel. It
will ensure position independent modules are always at the right
distance from the kernel and do not require mcmodule=large. It also
optimize the available size for modules by getting rid of the empty
space on kernel randomization range.

Signed-off-by: Thomas Garnier <thgar...@google.com>
---
 Documentation/x86/x86_64/mm.txt         | 3 +++
 arch/x86/Kconfig                        | 4 ++++
 arch/x86/include/asm/pgtable_64_types.h | 6 ++++++
 arch/x86/kernel/head64.c                | 5 ++++-
 arch/x86/mm/dump_pagetables.c           | 3 ++-
 5 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/Documentation/x86/x86_64/mm.txt b/Documentation/x86/x86_64/mm.txt
index ea91cb61a602..ec1dfe4c3cfe 100644
--- a/Documentation/x86/x86_64/mm.txt
+++ b/Documentation/x86/x86_64/mm.txt
@@ -77,3 +77,6 @@ Their order is preserved but their base will be offset early 
at boot time.
 Be very careful vs. KASLR when changing anything here. The KASLR address
 range must not overlap with anything except the KASAN shadow area, which is
 correct as KASAN disables KASLR.
+
+If CONFIG_DYNAMIC_MODULE_BASE is enabled, the module section follows the end of
+the mapped kernel.
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 0cb1ae187c3e..df4134fd3247 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2236,6 +2236,10 @@ config RANDOMIZE_MEMORY_PHYSICAL_PADDING
 
           If unsure, leave at the default value.
 
+# Module section starts just after the end of the kernel module
+config DYNAMIC_MODULE_BASE
+       bool
+
 config X86_GLOBAL_STACKPROTECTOR
        bool "Stack cookie using a global variable"
        depends on CC_STACKPROTECTOR_AUTO
diff --git a/arch/x86/include/asm/pgtable_64_types.h 
b/arch/x86/include/asm/pgtable_64_types.h
index d5c21a382475..d4d3b21d5b3d 100644
--- a/arch/x86/include/asm/pgtable_64_types.h
+++ b/arch/x86/include/asm/pgtable_64_types.h
@@ -7,6 +7,7 @@
 #ifndef __ASSEMBLY__
 #include <linux/types.h>
 #include <asm/kaslr.h>
+#include <asm/sections.h>
 
 /*
  * These are used to make use of C type-checking..
@@ -126,7 +127,12 @@ extern unsigned int ptrs_per_p4d;
 
 #define VMALLOC_END            (VMALLOC_START + (VMALLOC_SIZE_TB << 40) - 1)
 
+#ifdef CONFIG_DYNAMIC_MODULE_BASE
+#define MODULES_VADDR          ALIGN(((unsigned long)_end + PAGE_SIZE), 
PMD_SIZE)
+#else
 #define MODULES_VADDR          (__START_KERNEL_map + KERNEL_IMAGE_SIZE)
+#endif
+
 /* The module sections ends with the start of the fixmap */
 #define MODULES_END            _AC(0xffffffffff000000, UL)
 #define MODULES_LEN            (MODULES_END - MODULES_VADDR)
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 2fe60e661227..ea4c498369d8 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -384,12 +384,15 @@ asmlinkage __visible void __init x86_64_start_kernel(char 
* real_mode_data)
         * Build-time sanity checks on the kernel image and module
         * area mappings. (these are purely build-time and produce no code)
         */
+#ifndef CONFIG_DYNAMIC_MODULE_BASE
        BUILD_BUG_ON(MODULES_VADDR < __START_KERNEL_map);
        BUILD_BUG_ON(MODULES_VADDR - __START_KERNEL_map < KERNEL_IMAGE_SIZE);
-       BUILD_BUG_ON(MODULES_LEN + KERNEL_IMAGE_SIZE > 2*PUD_SIZE);
+       BUILD_BUG_ON(!IS_ENABLED(CONFIG_RANDOMIZE_BASE_LARGE) &&
+                    MODULES_LEN + KERNEL_IMAGE_SIZE > 2*PUD_SIZE);
        BUILD_BUG_ON((__START_KERNEL_map & ~PMD_MASK) != 0);
        BUILD_BUG_ON((MODULES_VADDR & ~PMD_MASK) != 0);
        BUILD_BUG_ON(!(MODULES_VADDR > __START_KERNEL));
+#endif
        MAYBE_BUILD_BUG_ON(!(((MODULES_END - 1) & PGDIR_MASK) ==
                                (__START_KERNEL & PGDIR_MASK)));
        BUILD_BUG_ON(__fix_to_virt(__end_of_fixed_addresses) <= MODULES_END);
diff --git a/arch/x86/mm/dump_pagetables.c b/arch/x86/mm/dump_pagetables.c
index 62a7e9f65dec..6f0b1fa2a71a 100644
--- a/arch/x86/mm/dump_pagetables.c
+++ b/arch/x86/mm/dump_pagetables.c
@@ -104,7 +104,7 @@ static struct addr_marker address_markers[] = {
        [EFI_END_NR]            = { EFI_VA_END,         "EFI Runtime Services" 
},
 #endif
        [HIGH_KERNEL_NR]        = { __START_KERNEL_map, "High Kernel Mapping" },
-       [MODULES_VADDR_NR]      = { MODULES_VADDR,      "Modules" },
+       [MODULES_VADDR_NR]      = { 0/*MODULES_VADDR*/, "Modules" },
        [MODULES_END_NR]        = { MODULES_END,        "End Modules" },
        [FIXADDR_START_NR]      = { FIXADDR_START,      "Fixmap Area" },
        [END_OF_SPACE_NR]       = { -1,                 NULL }
@@ -599,6 +599,7 @@ static int __init pt_dump_init(void)
        address_markers[KASAN_SHADOW_START_NR].start_address = 
KASAN_SHADOW_START;
        address_markers[KASAN_SHADOW_END_NR].start_address = KASAN_SHADOW_END;
 #endif
+       address_markers[MODULES_VADDR_NR].start_address = MODULES_VADDR;
 #endif
 #ifdef CONFIG_X86_32
        address_markers[VMALLOC_START_NR].start_address = VMALLOC_START;
-- 
2.16.2.660.g709887971b-goog

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