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.le...@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