linux-kernel - [PATCH] powerpc: Simplify module TOC handling

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Message-Id: <1453077867-23873-1-git-send-email-mpe@ellerman.id.au>
Date:	Mon, 18 Jan 2016 11:44:27 +1100
From:	Michael Ellerman <mpe@...erman.id.au>
To:	<linuxppc-dev@...abs.org>
Cc:	amodra@...il.com, Anton Blanchard <anton@...ba.org>,
	rusty@...tcorp.com.au, quentin.casasnovas@...cle.com,
	paul.gortmaker@...driver.com, <linux-kernel@...r.kernel.org>
Subject: [PATCH] powerpc: Simplify module TOC handling

From: Alan Modra <amodra@...il.com>

PowerPC64 uses the symbol .TOC. much as other targets use
_GLOBAL_OFFSET_TABLE_. It identifies the value of the GOT pointer (or in
powerpc parlance, the TOC pointer). Global offset tables are generally
local to an executable or shared library, or in the kernel, module. Thus
it does not make sense for a module to resolve a relocation against
.TOC. to the kernel's .TOC. value. A module has its own .TOC., and
indeed the powerpc64 module relocation processing ignores the kernel
value of .TOC. and instead calculates a module-local value.

This patch removes code involved in exporting the kernel .TOC., tweaks
modpost to ignore an undefined .TOC., and the module loader to twiddle
the section symbol so that .TOC. isn't seen as undefined.

Note that if the kernel was compiled with -msingle-pic-base then ELFv2
would not have function global entry code setting up r2. In that case
the module call stubs would need to be modified to set up r2 using the
kernel .TOC. value, requiring some of this code to be reinstated.

mpe: Furthermore a change in binutils master (not yet released) causes
the current way we handle the TOC to no longer work when building with
MODVERSIONS=y and RELOCATABLE=n. The symptom is that modules can not be
loaded due to there being no version found for TOC.

Cc: stable@...r.kernel.org # 3.16+
Signed-off-by: Alan Modra <amodra@...il.com>
Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
---
 arch/powerpc/kernel/misc_64.S   | 28 ----------------------------
 arch/powerpc/kernel/module_64.c | 12 +++++++++---
 scripts/mod/modpost.c           |  3 ++-
 3 files changed, 11 insertions(+), 32 deletions(-)

diff --git a/arch/powerpc/kernel/misc_64.S b/arch/powerpc/kernel/misc_64.S
index db475d41b57a..f28754c497e5 100644
--- a/arch/powerpc/kernel/misc_64.S
+++ b/arch/powerpc/kernel/misc_64.S
@@ -701,31 +701,3 @@ _GLOBAL(kexec_sequence)
 	li	r5,0
 	blr	/* image->start(physid, image->start, 0); */
 #endif /* CONFIG_KEXEC */
-
-#ifdef CONFIG_MODULES
-#if defined(_CALL_ELF) && _CALL_ELF == 2
-
-#ifdef CONFIG_MODVERSIONS
-.weak __crc_TOC.
-.section "___kcrctab+TOC.","a"
-.globl __kcrctab_TOC.
-__kcrctab_TOC.:
-	.llong	__crc_TOC.
-#endif
-
-/*
- * Export a fake .TOC. since both modpost and depmod will complain otherwise.
- * Both modpost and depmod strip the leading . so we do the same here.
- */
-.section "__ksymtab_strings","a"
-__kstrtab_TOC.:
-	.asciz "TOC."
-
-.section "___ksymtab+TOC.","a"
-/* This symbol name is important: it's used by modpost to find exported syms */
-.globl __ksymtab_TOC.
-__ksymtab_TOC.:
-	.llong 0 /* .value */
-	.llong __kstrtab_TOC.
-#endif /* ELFv2 */
-#endif /* MODULES */
diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
index 59663af9315f..ac64ffdb52c8 100644
--- a/arch/powerpc/kernel/module_64.c
+++ b/arch/powerpc/kernel/module_64.c
@@ -326,7 +326,10 @@ static void dedotify_versions(struct modversion_info *vers,
 		}
 }
 
-/* Undefined symbols which refer to .funcname, hack to funcname (or .TOC.) */
+/*
+ * Undefined symbols which refer to .funcname, hack to funcname. Make .TOC.
+ * seem to be defined (value set later).
+ */
 static void dedotify(Elf64_Sym *syms, unsigned int numsyms, char *strtab)
 {
 	unsigned int i;
@@ -334,8 +337,11 @@ static void dedotify(Elf64_Sym *syms, unsigned int numsyms, char *strtab)
 	for (i = 1; i < numsyms; i++) {
 		if (syms[i].st_shndx == SHN_UNDEF) {
 			char *name = strtab + syms[i].st_name;
-			if (name[0] == '.')
+			if (name[0] == '.') {
+				if (strcmp(name+1, "TOC.") == 0)
+					syms[i].st_shndx = SHN_ABS;
 				memmove(name, name+1, strlen(name));
+			}
 		}
 	}
 }
@@ -351,7 +357,7 @@ static Elf64_Sym *find_dot_toc(Elf64_Shdr *sechdrs,
 	numsyms = sechdrs[symindex].sh_size / sizeof(Elf64_Sym);
 
 	for (i = 1; i < numsyms; i++) {
-		if (syms[i].st_shndx == SHN_UNDEF
+		if (syms[i].st_shndx == SHN_ABS
 		    && strcmp(strtab + syms[i].st_name, "TOC.") == 0)
 			return &syms[i];
 	}
diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index e080746e1a6b..48958d3cec9e 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -594,7 +594,8 @@ static int ignore_undef_symbol(struct elf_info *info, const char *symname)
 		if (strncmp(symname, "_restgpr0_", sizeof("_restgpr0_") - 1) == 0 ||
 		    strncmp(symname, "_savegpr0_", sizeof("_savegpr0_") - 1) == 0 ||
 		    strncmp(symname, "_restvr_", sizeof("_restvr_") - 1) == 0 ||
-		    strncmp(symname, "_savevr_", sizeof("_savevr_") - 1) == 0)
+		    strncmp(symname, "_savevr_", sizeof("_savevr_") - 1) == 0 ||
+		    strcmp(symname, ".TOC.") == 0)
 			return 1;
 	/* Do not ignore this symbol */
 	return 0;
-- 
2.5.0