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Message-Id: <20220309055118.1551013-1-maskray@google.com>
Date: Tue, 8 Mar 2022 21:51:18 -0800
From: Fangrui Song <maskray@...gle.com>
To: Michael Ellerman <mpe@...erman.id.au>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Alexey Kardashevskiy <aik@...abs.ru>,
linuxppc-dev@...ts.ozlabs.org
Cc: Nathan Chancellor <nathan@...nel.org>,
linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
Fangrui Song <maskray@...gle.com>
Subject: [PATCH] powerpc: Replace ppc64 DT_RELACOUNT usage with DT_RELASZ/24
DT_RELACOUNT is an ELF dynamic tag inherited from SunOS indicating the
number of R_*_RELATIVE relocations. It is optional but {ld.lld,ld.lld}
-z combreloc always creates it (if non-zero) to slightly speed up glibc
ld.so relocation resolving by avoiding R_*R_PPC64_RELATIVE type
comparison. The tag is otherwise nearly unused in the wild and I'd
recommend that software avoids using it.
lld>=14.0.0 (since commit da0e5b885b25cf4ded0fa89b965dc6979ac02ca9)
underestimates DT_RELACOUNT for ppc64 when position-independent long
branch thunks are used. Correcting it needs non-trivial arch-specific
complexity which I'd prefer to avoid. Since our code always compares the
relocation type with R_PPC64_RELATIVE, replacing every occurrence of
DT_RELACOUNT with DT_RELASZ/sizeof(Elf64_Rela)=DT_RELASZ/24 is a correct
alternative.
DT_RELASZ is in practice bounded by an uint32_t. Dividing x by 24 can be
implemented as (uint32_t)(x*0xaaaaaaab) >> 4.
Link: https://github.com/ClangBuiltLinux/linux/issues/1581
Reported-by: Nathan Chancellor <nathan@...nel.org>
Signed-off-by: Fangrui Song <maskray@...gle.com>
---
arch/powerpc/boot/crt0.S | 28 +++++++++++++++++-----------
arch/powerpc/kernel/reloc_64.S | 15 +++++++++------
2 files changed, 26 insertions(+), 17 deletions(-)
diff --git a/arch/powerpc/boot/crt0.S b/arch/powerpc/boot/crt0.S
index feadee18e271..1c96ebe7ef1a 100644
--- a/arch/powerpc/boot/crt0.S
+++ b/arch/powerpc/boot/crt0.S
@@ -8,7 +8,7 @@
#include "ppc_asm.h"
RELA = 7
-RELACOUNT = 0x6ffffff9
+RELASZ = 8
.data
/* A procedure descriptor used when booting this as a COFF file.
@@ -65,7 +65,7 @@ p_base: mflr r10 /* r10 now points to runtime addr of p_base */
subf r11,r11,r12 /* runtime - linktime offset */
/* The dynamic section contains a series of tagged entries.
- * We need the RELA and RELACOUNT entries. */
+ * We need the RELA and RELASZ entries. */
li r9,0
li r0,0
9: lwz r8,0(r12) /* get tag */
@@ -75,18 +75,21 @@ p_base: mflr r10 /* r10 now points to runtime addr of p_base */
bne 11f
lwz r9,4(r12) /* get RELA pointer in r9 */
b 12f
-11: addis r8,r8,(-RELACOUNT)@ha
- cmpwi r8,RELACOUNT@l
+11: cmpwi r8,RELASZ
bne 12f
- lwz r0,4(r12) /* get RELACOUNT value in r0 */
+ lwz r0,4(r12) /* get RELASZ / 24 in r0 */
+ lis r8,0xaaaa
+ ori r8,r8,0xaaab
+ mulhwu r0,r0,r8
+ srwi r0,r0,4
12: addi r12,r12,8
b 9b
/* The relocation section contains a list of relocations.
* We now do the R_PPC_RELATIVE ones, which point to words
* which need to be initialized with addend + offset.
- * The R_PPC_RELATIVE ones come first and there are RELACOUNT
- * of them. */
+ * The R_PPC_RELATIVE ones come first and there are at most
+ * RELASZ/24 of them. */
10: /* skip relocation if we don't have both */
cmpwi r0,0
beq 3f
@@ -160,14 +163,17 @@ p_base: mflr r10 /* r10 now points to runtime addr of p_base */
bne 10f
ld r13,8(r11) /* get RELA pointer in r13 */
b 11f
-10: addis r12,r12,(-RELACOUNT)@ha
- cmpdi r12,RELACOUNT@l
+10: cmpdi r12,RELASZ
bne 11f
- ld r8,8(r11) /* get RELACOUNT value in r8 */
+ ld r8,8(r11) /* get RELASZ / 24 in r8 */
+ lis r0,0xaaaa
+ ori r0,r0,0xaaab
+ mulhwu r8,r8,r0
+ srwi r8,r8,4
11: addi r11,r11,16
b 9b
12:
- cmpdi r13,0 /* check we have both RELA and RELACOUNT */
+ cmpdi r13,0 /* check we have both RELA and RELASZ */
cmpdi cr1,r8,0
beq 3f
beq cr1,3f
diff --git a/arch/powerpc/kernel/reloc_64.S b/arch/powerpc/kernel/reloc_64.S
index 02d4719bf43a..362be759609f 100644
--- a/arch/powerpc/kernel/reloc_64.S
+++ b/arch/powerpc/kernel/reloc_64.S
@@ -8,7 +8,7 @@
#include <asm/ppc_asm.h>
RELA = 7
-RELACOUNT = 0x6ffffff9
+RELASZ = 8
R_PPC64_RELATIVE = 22
/*
@@ -27,7 +27,7 @@ _GLOBAL(relocate)
add r10,r10,r12 /* r10 has runtime addr of _stext */
/*
- * Scan the dynamic section for the RELA and RELACOUNT entries.
+ * Scan the dynamic section for the RELA and RELASZ entries.
*/
li r7,0
li r8,0
@@ -38,13 +38,16 @@ _GLOBAL(relocate)
bne 2f
ld r7,8(r11) /* get RELA pointer in r7 */
b 3f
-2: addis r6,r6,(-RELACOUNT)@ha
- cmpdi r6,RELACOUNT@l
+2: cmpdi r6,RELASZ
bne 3f
- ld r8,8(r11) /* get RELACOUNT value in r8 */
+ ld r8,8(r11) /* get RELA / 24 in r8 */
+ lis r0,0xaaaa
+ ori r0,r0,0xaaab
+ mulhwu r8,r8,r0
+ srwi r8,r8,4
3: addi r11,r11,16
b 1b
-4: cmpdi r7,0 /* check we have both RELA and RELACOUNT */
+4: cmpdi r7,0 /* check we have both RELA and RELASZ */
cmpdi cr1,r8,0
beq 6f
beq cr1,6f
--
2.35.1.616.g0bdcbb4464-goog
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