powerpc/64: modules support building with PCREL addresing

Build modules using PCREL addressing when CONFIG_PPC_KERNEL_PCREL=y.

- The module loader must handle several new relocation types:

  * R_PPC64_REL24_NOTOC is a function call handled like R_PPC_REL24, but
    does not restore r2 upon return. The external function call stub is
    changed to use pcrel addressing to load the function pointer rather
    than based on the module TOC.

  * R_PPC64_GOT_PCREL34 is a reference to external data. A GOT table
    must be built by hand, because the linker adds this during the final
    link (which is not done for kernel modules). The GOT table is built
    similarly to the way the external function call stub table is. This
    section is called .mygot because .got has a special meaning for the
    linker and can become upset.

  * R_PPC64_PCREL34 is used for local data addressing, but there is a
    special case where the percpu section is moved at load-time to the
    percpu area which is out of range of this relocation. This requires
    the PCREL34 relocations are converted to use GOT_PCREL34 addressing.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Some coding style & formatting fixups]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230408021752.862660-7-npiggin@gmail.com

arch/powerpc/Makefile

@@ -107,9 +107,7 @@ LDFLAGS_vmlinux-$(CONFIG_RELOCATABLE) += -z notext
 LDFLAGS_vmlinux	:= $(LDFLAGS_vmlinux-y)
 
 ifdef CONFIG_PPC64
-ifdef CONFIG_PPC_KERNEL_PCREL
-	KBUILD_CFLAGS_MODULE += $(call cc-option,-mno-pcrel)
-endif
+ifndef CONFIG_PPC_KERNEL_PCREL
 ifeq ($(call cc-option-yn,-mcmodel=medium),y)
 	# -mcmodel=medium breaks modules because it uses 32bit offsets from
 	# the TOC pointer to create pointers where possible. Pointers into the
@@ -124,6 +122,7 @@ else
 	export NO_MINIMAL_TOC := -mno-minimal-toc
 endif
 endif
+endif
 
 CFLAGS-$(CONFIG_PPC64)	:= $(call cc-option,-mtraceback=no)
 ifdef CONFIG_PPC64_ELF_ABI_V2

arch/powerpc/include/asm/module.h

@@ -27,8 +27,13 @@ struct ppc_plt_entry {
 struct mod_arch_specific {
 #ifdef __powerpc64__
 	unsigned int stubs_section;	/* Index of stubs section in module */
+#ifdef CONFIG_PPC_KERNEL_PCREL
+	unsigned int got_section;	/* What section is the GOT? */
+	unsigned int pcpu_section;	/* .data..percpu section */
+#else
 	unsigned int toc_section;	/* What section is the TOC? */
 	bool toc_fixed;			/* Have we fixed up .TOC.? */
+#endif
 
 	/* For module function descriptor dereference */
 	unsigned long start_opd;
@@ -52,12 +57,15 @@ struct mod_arch_specific {
 
 /*
  * Select ELF headers.
- * Make empty section for module_frob_arch_sections to expand.
+ * Make empty sections for module_frob_arch_sections to expand.
  */
 
 #ifdef __powerpc64__
 #    ifdef MODULE
 	asm(".section .stubs,\"ax\",@nobits; .align 3; .previous");
+#        ifdef CONFIG_PPC_KERNEL_PCREL
+	    asm(".section .mygot,\"a\",@nobits; .align 3; .previous");
+#        endif
 #    endif
 #else
 #    ifdef MODULE

arch/powerpc/include/asm/ppc_asm.h

@@ -183,7 +183,7 @@
 /*
  * Used to name C functions called from asm
  */
-#if defined(CONFIG_PPC_KERNEL_PCREL) && !defined(MODULE)
+#ifdef CONFIG_PPC_KERNEL_PCREL
 #define CFUNC(name) name@notoc
 #else
 #define CFUNC(name) name
@@ -216,7 +216,7 @@
 	.globl name; \
 name:
 
-#if defined(CONFIG_PPC_KERNEL_PCREL) && !defined(MODULE)
+#ifdef CONFIG_PPC_KERNEL_PCREL
 #define _GLOBAL_TOC _GLOBAL
 #else
 #define _GLOBAL_TOC(name) \
@@ -379,7 +379,7 @@ GLUE(.,name):
 	ori	reg, reg, (expr)@l;		\
 	rldimi	reg, tmp, 32, 0
 
-#if defined(CONFIG_PPC_KERNEL_PCREL) && !defined(MODULE)
+#ifdef CONFIG_PPC_KERNEL_PCREL
 #define LOAD_REG_ADDR(reg,name)			\
 	pla	reg,name@pcrel
 

arch/powerpc/include/uapi/asm/elf.h

@@ -279,8 +279,12 @@ typedef elf_fpreg_t elf_vsrreghalf_t32[ELF_NVSRHALFREG];
 #define R_PPC64_TLSLD		108
 #define R_PPC64_TOCSAVE		109
 
+#define R_PPC64_REL24_NOTOC	116
 #define R_PPC64_ENTRY		118
 
+#define R_PPC64_PCREL34		132
+#define R_PPC64_GOT_PCREL34	133
+
 #define R_PPC64_REL16		249
 #define R_PPC64_REL16_LO	250
 #define R_PPC64_REL16_HI	251