linux-kernel - [PATCH] Replace __toc_start + 0x8000 with .TOC.

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Message-Id: <20210307051433.2756645-1-maskray@google.com>
Date:   Sat,  6 Mar 2021 21:14:33 -0800
From:   Fangrui Song <maskray@...gle.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        clang-built-linux <clang-built-linux@...glegroups.com>
Cc:     Fangrui Song <maskray@...gle.com>,
        Alexey Kardashevskiy <aik@...abs.ru>
Subject: [PATCH] Replace __toc_start + 0x8000 with .TOC.

TOC relocations are like GOT relocations on other architectures.
However, unlike other architectures, GNU ld's ppc64 port defines .TOC.
relative to the .got output section instead of the linker synthesized
.got input section. LLD defines .TOC. as the .got input section plus
0x8000. When CONFIG_PPC_OF_BOOT_TRAMPOLINE=y,
arch/powerpc/kernel/prom_init.o is built, and LLD computed .TOC. can be
different from __toc_start defined by the linker script.

Simplify kernel_toc_addr with asm label .TOC. so that we can get rid of
__toc_start.

With this change, powernv_defconfig with CONFIG_PPC_OF_BOOT_TRAMPOLINE=y
is bootable with LLD. There is still an untriaged issue with Alexey's
configuration.

Link: https://github.com/ClangBuiltLinux/linux/issues/1318
Reported-by: Alexey Kardashevskiy <aik@...abs.ru>
Signed-off-by: Fangrui Song <maskray@...gle.com>
---
 arch/powerpc/boot/crt0.S            |  2 +-
 arch/powerpc/boot/zImage.lds.S      |  1 -
 arch/powerpc/include/asm/sections.h | 10 ++--------
 arch/powerpc/kernel/head_64.S       |  2 +-
 arch/powerpc/kernel/vmlinux.lds.S   |  1 -
 5 files changed, 4 insertions(+), 12 deletions(-)

diff --git a/arch/powerpc/boot/crt0.S b/arch/powerpc/boot/crt0.S
index 1d83966f5ef6..e45907fe468f 100644
--- a/arch/powerpc/boot/crt0.S
+++ b/arch/powerpc/boot/crt0.S
@@ -28,7 +28,7 @@ p_etext:	.8byte	_etext
 p_bss_start:	.8byte	__bss_start
 p_end:		.8byte	_end
 
-p_toc:		.8byte	__toc_start + 0x8000 - p_base
+p_toc:		.8byte	.TOC. - p_base
 p_dyn:		.8byte	__dynamic_start - p_base
 p_rela:		.8byte	__rela_dyn_start - p_base
 p_prom:		.8byte	0
diff --git a/arch/powerpc/boot/zImage.lds.S b/arch/powerpc/boot/zImage.lds.S
index d6f072865627..32cf7816292f 100644
--- a/arch/powerpc/boot/zImage.lds.S
+++ b/arch/powerpc/boot/zImage.lds.S
@@ -39,7 +39,6 @@ SECTIONS
   . = ALIGN(256);
   .got :
   {
-    __toc_start = .;
     *(.got)
     *(.toc)
   }
diff --git a/arch/powerpc/include/asm/sections.h b/arch/powerpc/include/asm/sections.h
index 324d7b298ec3..bd22ca0b5eca 100644
--- a/arch/powerpc/include/asm/sections.h
+++ b/arch/powerpc/include/asm/sections.h
@@ -48,14 +48,8 @@ static inline int in_kernel_text(unsigned long addr)
 
 static inline unsigned long kernel_toc_addr(void)
 {
-	/* Defined by the linker, see vmlinux.lds.S */
-	extern unsigned long __toc_start;
-
-	/*
-	 * The TOC register (r2) points 32kB into the TOC, so that 64kB of
-	 * the TOC can be addressed using a single machine instruction.
-	 */
-	return (unsigned long)(&__toc_start) + 0x8000UL;
+	extern unsigned long toc asm(".TOC.");
+	return (unsigned long)(&toc);
 }
 
 static inline int overlaps_interrupt_vector_text(unsigned long start,
diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
index ece7f97bafff..9542d03b2efe 100644
--- a/arch/powerpc/kernel/head_64.S
+++ b/arch/powerpc/kernel/head_64.S
@@ -899,7 +899,7 @@ _GLOBAL(relative_toc)
 	blr
 
 .balign 8
-p_toc:	.8byte	__toc_start + 0x8000 - 0b
+p_toc:	.8byte	.TOC. - 0b
 
 /*
  * This is where the main kernel code starts.
diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
index 72fa3c00229a..c28f4e5bae3f 100644
--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S
@@ -328,7 +328,6 @@ SECTIONS
 
 	. = ALIGN(256);
 	.got : AT(ADDR(.got) - LOAD_OFFSET) {
-		__toc_start = .;
 #ifndef CONFIG_RELOCATABLE
 		__prom_init_toc_start = .;
 		arch/powerpc/kernel/prom_init.o*(.toc .got)
-- 
2.30.1.766.gb4fecdf3b7-goog