[PATCH kernel] powerpc/64: Add UADDR64 relocation support
Christophe Leroy
christophe.leroy at csgroup.eu
Mon Jan 31 17:38:03 AEDT 2022
Le 31/01/2022 à 05:14, Alexey Kardashevskiy a écrit :
> 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 ?
>
> \#pragma GCC push_options
> \#pragma GCC optimize ("O0")
AFAIU Linux Kernel is always built with O2
Have you observed the problem with O2 ?
> 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
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