On 1/31/22 17:38, Christophe Leroy wrote:

Le 31/01/2022 à 05:14, Alexey Kardashevskiy a écrit :

quoted

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:

According to relocs_check.sh, this is expected to happen only with
binutils < 2.19. Today minimum binutils version is 2.23

Have you observed this problem with newer version of binutils ?

Oh yeah. 2.36.1. And the toolchain folks explained internally that this 
is correct behavior and this was a ticking bomb which exploded now and 
the kernel has to deal with it.

quoted

\#pragma GCC push_options
\#pragma GCC optimize ("O0")

AFAIU Linux Kernel is always built with O2

Correct. Even O1 hides this.

Have you observed the problem with O2 ?

Yes, I see it once I enable CONFIG_PRINTK_INDEX (this is how it was 
spotted with my particular config, there is still a fair chance that 
this config option does not cause UADDR64 always) but I did not debug 
with it enabled as pretty much every single __func__ passed to printk 
caused unaligned relocation (tens of thousands). Note that this 
particular case can be fixed by removing __packed from "struct pi_entry" 
(== re-arm the bomb). Thanks,

quoted

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 <redacted>
---

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:

quoted

quoted

quoted

p *(unsigned long *) 0x1e804d0

$1 = 0xc000000001a804b0

quoted

quoted

quoted

p *(unsigned long *) 0x1e804dc

$2 = 0x0

After:

quoted

quoted

quoted

p *(unsigned long *) 0x1e804d0

$1 = 0x1e804b0

quoted

quoted

quoted

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