Re: [PATCH v4 2/3] powerpc/reloc64: add support for 32-bit CRC pseudo-symbols

[Ard Biesheuvel]

On 17 January 2017 at 19:26, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
> In preparation of modifying the core modversions code to emit the CRCs
> as 32-bit quantities, ensure that 64-bit PowerPC will be able to deal
> with this when CONFIG_RELOCATABLE=y, in which case the CRCs will be
> emitted into the final ELF binary as R_PPC64_ADDR32 relocations.
>
> Since 32-bit relocations cannot be used to relocate memory addresses on
> 64-bit architectures, and since the CRC pseudo-symbol references are
> emitted as anonymous relocations (i.e., against the NULL symbol in the
> .dynsym section) with the final value recorded in the addend (*), we
> can disregard any relocations where the symbol index != 0.
>
> * Note that unsatisfied CRC pseudo-symbol references are emitted as
>   R_PPC64_ADDR32 relocations against named symbols that are typed as
>   weak undefined in the .dynsym symbol table. These can simply be
>   ignored (as before), considering that zero CRCs are interpreted as
>   missing, and the module code deals with that accordingly.
>
> As it turns out, binutils for powerpc does not account for any relocations
> beyond R_PPC64_RELATIVE ones in the RELACOUNT field of the .dynamic section,
> which is unfortunate, since we need to do extra work to figure out the size
> of the relocation array. So with a little help from the linker scripts,
> grab an end pointer rather than a count, and iterate over the entire section.
>

While this is the case of RELACOUNT, RELASZ appears to behave as
expected. For instance, on a random vmlinux file (with patch 3/3
applied), I get

Dynamic Section:
  HASH                 0xc000000000cf5a40
  STRTAB               0xc000000000cf4d80
  SYMTAB               0xc000000000cf3e98
  STRSZ                0x0000000000000bae
  SYMENT               0x0000000000000018
  DEBUG                0x0000000000000000
  RELA                 0xc000000000cf5ee8
  RELASZ               0x00000000003308b8
  RELAENT              0x0000000000000018
  TEXTREL              0x0000000000000000
  FLAGS_1              0x0000000008000000
  RELACOUNT            0x0000000000020444

where RELACOUNT is the number of just the R_PPC64_RELATIVE
relocations, whereas RELASZ covers all of them (which is why RELASZ /
RELAENT != RELACOUNT)

So if preferred, I can respin this patch to retrieve RELASZ from the
dynamic section rather than the end of the .rela section from the
linker script, which would reduce the delta of this patch somewhat.


> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> ---
>  arch/powerpc/kernel/reloc_64.S    | 60 ++++++++++++--------
>  arch/powerpc/kernel/vmlinux.lds.S |  1 +
>  arch/powerpc/relocs_check.sh      |  5 +-
>  3 files changed, 40 insertions(+), 26 deletions(-)
>
> diff --git a/arch/powerpc/kernel/reloc_64.S b/arch/powerpc/kernel/reloc_64.S
> index d88736fbece6..e50f5d778ea2 100644
> --- a/arch/powerpc/kernel/reloc_64.S
> +++ b/arch/powerpc/kernel/reloc_64.S
> @@ -12,8 +12,8 @@
>  #include <asm/ppc_asm.h>
>
>  RELA = 7
> -RELACOUNT = 0x6ffffff9
>  R_PPC64_RELATIVE = 22
> +R_PPC64_ADDR32 = 1
>
>  /*
>   * r3 = desired final address of kernel
> @@ -29,29 +29,27 @@ _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      r8,(p_rela_end - 0b)(r12)
> +       add     r8,r8,r12       /* r8 has addr of end of .rela.dyn section */
>
>         /*
> -        * Scan the dynamic section for the RELA and RELACOUNT entries.
> +        * Scan the dynamic section for the RELA entry.
> +        * NOTE: the RELACOUNT entry only covers R_PPC64_RELATIVE relocations,
> +        *       so we cannot use it here.
>          */
>         li      r7,0
> -       li      r8,0
>  1:     ld      r6,0(r11)       /* get tag */
>         cmpdi   r6,0
> -       beq     4f              /* end of list */
> +       beq     3f              /* end of list */
>         cmpdi   r6,RELA
> -       bne     2f
> -       ld      r7,8(r11)       /* get RELA pointer in r7 */
> -       b       3f
> -2:     addis   r6,r6,(-RELACOUNT)@ha
> -       cmpdi   r6,RELACOUNT@l
> -       bne     3f
> -       ld      r8,8(r11)       /* get RELACOUNT value in r8 */
> -3:     addi    r11,r11,16
> +       beq     2f
> +       addi    r11,r11,16
>         b       1b
> -4:     cmpdi   r7,0            /* check we have both RELA and RELACOUNT */
> -       cmpdi   cr1,r8,0
> -       beq     6f
> -       beq     cr1,6f
> +2:     ld      r7,8(r11)       /* get RELA pointer in r7 */
> +3:     cmpdi   r7,0            /* check we have both RELA and a non-empty */
> +       cmpd    cr1,r8,r9       /* .rela.dyn section                       */
> +       beq     7f
> +       beq     cr1,7f
>
>         /*
>          * Work out linktime address of _stext and hence the
> @@ -63,26 +61,40 @@ _GLOBAL(relocate)
>         subf    r7,r7,r9        /* cur_offset */
>         subf    r10,r7,r10
>         subf    r3,r10,r3       /* final_offset */
> +       b       4f
>
>         /*
>          * Run through the list of relocations and process the
> -        * R_PPC64_RELATIVE ones.
> +        * R_PPC64_RELATIVE and R_PPC64_ADDR32 ones.
>          */
> -       mtctr   r8
> -5:     ld      r0,8(9)         /* ELF64_R_TYPE(reloc->r_info) */
> +3:     addi    r9,r9,24
> +4:     cmpd    r9,r8
> +       beq     7f
> +5:     ld      r0,8(9)         /* reloc->r_info (type *and* symbol index) */
> +       ld      r6,0(r9)        /* reloc->r_offset */
>         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
>         stdx    r0,r7,r6
> -       addi    r9,r9,24
> -       bdnz    5b
> +       b       3b
> +
> +       /*
> +        * CRCs of exported symbols are emitted as 32-bit relocations against
> +        * the NULL .dynsym entry, with the CRC value recorded in the addend.
> +        */
> +6:     cmpdi   r0,R_PPC64_ADDR32
> +       bne     3b
> +       ld      r0,16(r9)       /* reloc->r_addend */
> +       stwx    r0,r7,r6
> +       b       3b
> +
> +7:     blr
>
> -6:     blr
>
>  .balign 8
>  p_dyn: .llong  __dynamic_start - 0b
>  p_rela:        .llong  __rela_dyn_start - 0b
> +p_rela_end:
> +       .llong  __rela_dyn_end - 0b
>  p_st:  .llong  _stext - 0b
> -
> diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
> index 7394b770ae1f..654728fc860d 100644
> --- a/arch/powerpc/kernel/vmlinux.lds.S
> +++ b/arch/powerpc/kernel/vmlinux.lds.S
> @@ -241,6 +241,7 @@ SECTIONS
>         {
>                 __rela_dyn_start = .;
>                 *(.rela*)
> +               __rela_dyn_end = .;
>         }
>  #endif
>         /* .exit.data is discarded at runtime, not link time,
> diff --git a/arch/powerpc/relocs_check.sh b/arch/powerpc/relocs_check.sh
> index ec2d5c835170..f9636b4e4548 100755
> --- a/arch/powerpc/relocs_check.sh
> +++ b/arch/powerpc/relocs_check.sh
> @@ -30,7 +30,7 @@ bad_relocs=$(
>         # On PPC64:
>         #       R_PPC64_RELATIVE, R_PPC64_NONE
>         #       R_PPC64_ADDR64 mach_<name>
> -       #       R_PPC64_ADDR64 __crc_<name>
> +       #       R_PPC64_ADDR32 __crc_<name>
>         # On PPC:
>         #       R_PPC_RELATIVE, R_PPC_ADDR16_HI,
>         #       R_PPC_ADDR16_HA,R_PPC_ADDR16_LO,
> @@ -43,7 +43,8 @@ R_PPC_ADDR16_HA
>  R_PPC_RELATIVE
>  R_PPC_NONE' |
>         grep -E -v '\<R_PPC64_ADDR64[[:space:]]+mach_' |
> -       grep -E -v '\<R_PPC64_ADDR64[[:space:]]+__crc_'
> +       grep -E -v '\<R_PPC64_ADDR32[[:space:]]+__crc_' |
> +       grep -E -v '\<R_PPC64_ADDR32[[:space:]]+\*ABS\*'
>  )
>
>  if [ -z "$bad_relocs" ]; then
> --
> 2.7.4
>