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Date:   Tue,  8 Mar 2022 21:51:18 -0800
Message-Id: <20220309055118.1551013-1-maskray@google.com>
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Subject: [PATCH] powerpc: Replace ppc64 DT_RELACOUNT usage with DT_RELASZ/24
From:   Fangrui Song <maskray@google.com>
To:     Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Alexey Kardashevskiy <aik@ozlabs.ru>,
        linuxppc-dev@lists.ozlabs.org
Cc:     Nathan Chancellor <nathan@kernel.org>,
        linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
        Fangrui Song <maskray@google.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

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@kernel.org>
Signed-off-by: Fangrui Song <maskray@google.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