[PATCH kernel] powerpc/64: Add UADDR64 relocation support

[Alexey Kardashevskiy]

When ld detects unaligned relocations, it emits R_PPC64_UADDR64
relocations instead of R_PPC64_RELATIVE. Currently R_PPC64_UADDR64 are
detected by arch/powerpc/tools/relocs_check.sh and expected not to work.
Below is a simple chunk to trigger this behaviour:

\#pragma GCC push_options
\#pragma GCC optimize ("O0")
struct entry {
        const char *file;
        int line;
} __attribute__((packed));
static const struct entry e1 = { .file = __FILE__, .line = __LINE__ };
static const struct entry e2 = { .file = __FILE__, .line = __LINE__ };
...
prom_printf("e1=%s %lx %lx\n", e1.file, (unsigned long) e1.file, mfmsr());
prom_printf("e2=%s %lx\n", e2.file, (unsigned long) e2.file);
\#pragma GCC pop_options


This adds support for UADDR64 for 64bit. This reuses __dynamic_symtab
from the 32bit which supports more relocation types already.

This adds a workaround for the number of relocations as the DT_RELACOUNT
ELF Dynamic Array Tag does not include relocations other than
R_PPC64_RELATIVE. This instead iterates over the entire .rela.dyn section.

Signed-off-by: Alexey Kardashevskiy <aik at ozlabs.ru>
---

Tested via qemu gdb stub (the kernel is loaded at 0x400000).

Disasm:

c000000001a804d0 <e1>:
c000000001a804d0:       b0 04 a8 01     .long 0x1a804b0
                        c000000001a804d0: R_PPC64_RELATIVE      *ABS*-0x3ffffffffe57fb50
c000000001a804d4:       00 00 00 c0     lfs     f0,0(0)
c000000001a804d8:       fa 08 00 00     .long 0x8fa

c000000001a804dc <e2>:
        ...
                        c000000001a804dc: R_PPC64_UADDR64       .rodata+0x4b0

Before relocation:

>>> p *(unsigned long *) 0x1e804d0
$1 = 0xc000000001a804b0
>>> p *(unsigned long *) 0x1e804dc
$2 = 0x0

After:
>>> p *(unsigned long *) 0x1e804d0
$1 = 0x1e804b0
>>> p *(unsigned long *) 0x1e804dc
$2 = 0x1e804b0
---
 arch/powerpc/kernel/reloc_64.S     | 47 +++++++++++++++++++++++++-----
 arch/powerpc/kernel/vmlinux.lds.S  |  3 +-
 arch/powerpc/tools/relocs_check.sh |  6 ----
 3 files changed, 41 insertions(+), 15 deletions(-)

diff --git a/arch/powerpc/kernel/reloc_64.S b/arch/powerpc/kernel/reloc_64.S
index 02d4719bf43a..a91175723d9d 100644
--- a/arch/powerpc/kernel/reloc_64.S
+++ b/arch/powerpc/kernel/reloc_64.S
@@ -10,6 +10,7 @@
 RELA = 7
 RELACOUNT = 0x6ffffff9
 R_PPC64_RELATIVE = 22
+R_PPC64_UADDR64 = 43
 
 /*
  * r3 = desired final address of kernel
@@ -25,6 +26,8 @@ _GLOBAL(relocate)
 	add	r9,r9,r12	/* r9 has runtime addr of .rela.dyn section */
 	ld	r10,(p_st - 0b)(r12)
 	add	r10,r10,r12	/* r10 has runtime addr of _stext */
+	ld	r13,(p_sym - 0b)(r12)
+	add	r13,r13,r12	/* r13 has runtime addr of .dynsym */
 
 	/*
 	 * Scan the dynamic section for the RELA and RELACOUNT entries.
@@ -46,8 +49,8 @@ _GLOBAL(relocate)
 	b	1b
 4:	cmpdi	r7,0		/* check we have both RELA and RELACOUNT */
 	cmpdi	cr1,r8,0
-	beq	6f
-	beq	cr1,6f
+	beq	9f
+	beq	cr1,9f
 
 	/*
 	 * Work out linktime address of _stext and hence the
@@ -60,25 +63,55 @@ _GLOBAL(relocate)
 	subf	r10,r7,r10
 	subf	r3,r10,r3	/* final_offset */
 
+	/*
+	 * FIXME
+	 * Here r8 is a number of relocations in .rela.dyn.
+	 * When ld issues UADDR64 relocations, they end up at the end
+	 * of the .rela.dyn section. However RELACOUNT does not include
+	 * them so the loop below is going to finish after the last
+	 * R_PPC64_RELATIVE as they normally go first.
+	 * Work out the size of .rela.dyn at compile time.
+	 */
+	ld	r8,(p_rela_end - 0b)(r12)
+	ld	r18,(p_rela - 0b)(r12)
+	sub	r8,r8,r18
+	li      r18,24		/* 24 == sizeof(elf64_rela) */
+	divd	r8,r8,r18
+
 	/*
 	 * Run through the list of relocations and process the
-	 * R_PPC64_RELATIVE ones.
+	 * R_PPC64_RELATIVE and R_PPC64_UADDR64 ones.
 	 */
 	mtctr	r8
-5:	ld	r0,8(9)		/* ELF64_R_TYPE(reloc->r_info) */
+5:	lwa	r0,8(r9)	/* ELF64_R_TYPE(reloc->r_info) */
 	cmpdi	r0,R_PPC64_RELATIVE
 	bne	6f
 	ld	r6,0(r9)	/* reloc->r_offset */
 	ld	r0,16(r9)	/* reloc->r_addend */
-	add	r0,r0,r3
+	b	7f
+
+6:	cmpdi	r0,R_PPC64_UADDR64
+	bne	8f
+	ld	r6,0(r9)
+	ld	r0,16(r9)
+	lwa	r14,12(r9) 	/* ELF64_R_SYM(reloc->r_info) */
+	mulli	r14,r14,24	/* 24 == sizeof(elf64_sym) */
+	add	r14,r14,r13	/* elf64_sym[ELF64_R_SYM] */
+	ld	r14,8(r14)
+	add	r0,r0,r14
+
+7:	add	r0,r0,r3
 	stdx	r0,r7,r6
-	addi	r9,r9,24
+
+8:	addi	r9,r9,24
 	bdnz	5b
 
-6:	blr
+9:	blr
 
 .balign 8
 p_dyn:	.8byte	__dynamic_start - 0b
 p_rela:	.8byte	__rela_dyn_start - 0b
+p_rela_end:	.8byte __rela_dyn_end - 0b
+p_sym:		.8byte __dynamic_symtab - 0b
 p_st:	.8byte	_stext - 0b
 
diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
index 2bcca818136a..e9d9bda3ffaf 100644
--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S
@@ -281,9 +281,7 @@ SECTIONS
 	. = ALIGN(8);
 	.dynsym : AT(ADDR(.dynsym) - LOAD_OFFSET)
 	{
-#ifdef CONFIG_PPC32
 		__dynamic_symtab = .;
-#endif
 		*(.dynsym)
 	}
 	.dynstr : AT(ADDR(.dynstr) - LOAD_OFFSET) { *(.dynstr) }
@@ -299,6 +297,7 @@ SECTIONS
 	{
 		__rela_dyn_start = .;
 		*(.rela*)
+		__rela_dyn_end = .;
 	}
 #endif
 	/* .exit.data is discarded at runtime, not link time,
diff --git a/arch/powerpc/tools/relocs_check.sh b/arch/powerpc/tools/relocs_check.sh
index 014e00e74d2b..956b9e236a60 100755
--- a/arch/powerpc/tools/relocs_check.sh
+++ b/arch/powerpc/tools/relocs_check.sh
@@ -54,9 +54,3 @@ fi
 num_bad=$(echo "$bad_relocs" | wc -l)
 echo "WARNING: $num_bad bad relocations"
 echo "$bad_relocs"
-
-# If we see this type of relocation it's an idication that
-# we /may/ be using an old version of binutils.
-if echo "$bad_relocs" | grep -q -F -w R_PPC64_UADDR64; then
-	echo "WARNING: You need at least binutils >= 2.19 to build a CONFIG_RELOCATABLE kernel"
-fi
-- 
2.30.2