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Message-ID: <CAKv+Gu9Q1LCe4Rjxt-veA68p+sXVH82tGrrDFGumrbcMax6ZpQ@mail.gmail.com>
Date: Wed, 18 Jan 2017 15:30:07 +0000
From: Ard Biesheuvel <ard.biesheuvel@...aro.org>
To: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Michael Ellerman <mpe@...erman.id.au>,
Jessica Yu <jeyu@...hat.com>
Cc: Rusty Russell <rusty@...tcorp.com.au>,
"Suzuki K. Poulose" <suzuki.poulose@....com>,
Will Deacon <will.deacon@....com>,
Andrew Morton <akpm@...ux-foundation.org>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
"paulus@...ba.org" <paulus@...ba.org>,
Arnd Bergmann <arnd@...db.de>,
Linus Torvalds <torvalds@...ux-foundation.org>,
"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [PATCH v4 2/3] powerpc/reloc64: add support for 32-bit CRC pseudo-symbols
On 17 January 2017 at 19:26, Ard Biesheuvel <ard.biesheuvel@...aro.org> 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@...aro.org>
> ---
> 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
>
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