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Subject[PATCH 1/3] powerpc/modules: Load modules closer to kernel text
Date
On book3s/32, when STRICT_KERNEL_RWX is selected, modules are
allocated on the segment just before kernel text, ie on the
0xb0000000-0xbfffffff when PAGE_OFFSET is 0xc0000000.

On the 8xx, TASK_SIZE is 0x80000000. The space between TASK_SIZE and
PAGE_OFFSET is not used and could be used for modules.

The idea comes from ARM architecture.

Having modules just below PAGE_OFFSET offers an opportunity to
minimise the distance between kernel text and modules and avoid
trampolines in modules to access kernel functions or other module
functions.

When MODULES_VADDR is defined, powerpc has it's own module_alloc()
function. In that function, first try to allocate the module
above the limit defined by '_etext - 32M'. Then if the allocation
fails, fallback to the entire MODULES area.

DEBUG logs in module_32.c without the patch:

[ 1572.588822] module_32: Applying ADD relocate section 13 to 12
[ 1572.588891] module_32: Doing plt for call to 0xc00671a4 at 0xcae04024
[ 1572.588964] module_32: Initialized plt for 0xc00671a4 at cae04000
[ 1572.589037] module_32: REL24 value = CAE04000. location = CAE04024
[ 1572.589110] module_32: Location before: 48000001.
[ 1572.589171] module_32: Location after: 4BFFFFDD.
[ 1572.589231] module_32: ie. jump to 03FFFFDC+CAE04024 = CEE04000
[ 1572.589317] module_32: Applying ADD relocate section 15 to 14
[ 1572.589386] module_32: Doing plt for call to 0xc00671a4 at 0xcadfc018
[ 1572.589457] module_32: Initialized plt for 0xc00671a4 at cadfc000
[ 1572.589529] module_32: REL24 value = CADFC000. location = CADFC018
[ 1572.589601] module_32: Location before: 48000000.
[ 1572.589661] module_32: Location after: 4BFFFFE8.
[ 1572.589723] module_32: ie. jump to 03FFFFE8+CADFC018 = CEDFC000

With the patch:

[ 279.404671] module_32: Applying ADD relocate section 13 to 12
[ 279.404741] module_32: REL24 value = C00671B4. location = BF808024
[ 279.404814] module_32: Location before: 48000001.
[ 279.404874] module_32: Location after: 4885F191.
[ 279.404933] module_32: ie. jump to 0085F190+BF808024 = C00671B4
[ 279.405016] module_32: Applying ADD relocate section 15 to 14
[ 279.405085] module_32: REL24 value = C00671B4. location = BF800018
[ 279.405156] module_32: Location before: 48000000.
[ 279.405215] module_32: Location after: 4886719C.
[ 279.405275] module_32: ie. jump to 0086719C+BF800018 = C00671B4

We see that with the patch, no plt entries are set.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
arch/powerpc/kernel/module.c | 23 ++++++++++++++++++++---
1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/kernel/module.c b/arch/powerpc/kernel/module.c
index a211b0253cdb..fab84024650c 100644
--- a/arch/powerpc/kernel/module.c
+++ b/arch/powerpc/kernel/module.c
@@ -14,6 +14,7 @@
#include <asm/firmware.h>
#include <linux/sort.h>
#include <asm/setup.h>
+#include <asm/sections.h>

static LIST_HEAD(module_bug_list);

@@ -88,12 +89,28 @@ int module_finalize(const Elf_Ehdr *hdr,
}

#ifdef MODULES_VADDR
+static __always_inline void *
+__module_alloc(unsigned long size, unsigned long start, unsigned long end)
+{
+ return __vmalloc_node_range(size, 1, start, end, GFP_KERNEL,
+ PAGE_KERNEL_EXEC, VM_FLUSH_RESET_PERMS, NUMA_NO_NODE,
+ __builtin_return_address(0));
+}
+
void *module_alloc(unsigned long size)
{
+ unsigned long limit = (unsigned long)_etext - SZ_32M;
+ void *ptr = NULL;
+
BUILD_BUG_ON(TASK_SIZE > MODULES_VADDR);

- return __vmalloc_node_range(size, 1, MODULES_VADDR, MODULES_END, GFP_KERNEL,
- PAGE_KERNEL_EXEC, VM_FLUSH_RESET_PERMS, NUMA_NO_NODE,
- __builtin_return_address(0));
+ /* First try within 32M limit from _etext to avoid branch trampolines */
+ if (MODULES_VADDR < PAGE_OFFSET && MODULES_END > limit)
+ ptr = __module_alloc(size, limit, MODULES_END);
+
+ if (!ptr)
+ ptr = __module_alloc(size, MODULES_VADDR, MODULES_END);
+
+ return ptr;
}
#endif
--
2.25.0
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