[PATCH 8/9] RISC-V: Support RISCV_ALIGN relocations in modules.

[Palmer Dabbelt]

On Thu, 22 Feb 2018 19:12:17 PST (-0800), shea at shealevy.com wrote:
> While legal, this noop implementation may be inefficient.
>
> Signed-off-by: Shea Levy <shea at shealevy.com>
> ---
>  arch/riscv/kernel/module.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/arch/riscv/kernel/module.c b/arch/riscv/kernel/module.c
> index 4f3e15e7995b..827cf1211360 100644
> --- a/arch/riscv/kernel/module.c
> +++ b/arch/riscv/kernel/module.c
> @@ -150,8 +150,8 @@ static int apply_r_riscv_call_rela(struct module *me, u32 *location,
>  	return 0;
>  }
>
> -static int apply_r_riscv_relax_rela(struct module *me, u32 *location,
> -				    Elf_Addr v)
> +static int apply_r_riscv_noop_rela(struct module *me, u32 *location,
> +				   Elf_Addr v)
>  {
>  	return 0;
>  }
> @@ -181,7 +181,8 @@ static int (*reloc_handlers_rela[]) (struct module *me, u32 *location,
>  	[R_RISCV_PCREL_LO12_I]		= apply_r_riscv_pcrel_lo12_i_rela,
>  	[R_RISCV_PCREL_LO12_S]		= apply_r_riscv_pcrel_lo12_s_rela,
>  	[R_RISCV_CALL]			= apply_r_riscv_call_rela,
> -	[R_RISCV_RELAX]			= apply_r_riscv_relax_rela,
> +	[R_RISCV_RELAX]			= apply_r_riscv_noop_rela,
> +	[R_RISCV_ALIGN]			= apply_r_riscv_noop_rela,
>  	[R_RISCV_ADD32]			= apply_r_riscv_add32_rela,
>  	[R_RISCV_SUB32]			= apply_r_riscv_sub32_rela,
>  };

The output of most RISC-V toolchains requires a relaxation pass for alignment 
for correctness.  Even if you're enforcing this somewhere in your module build 
flow you should at least check for alignment when loading modules so there 
aren't silent breakages, if R_RISCV_ALIGN is escaping to the kernel's loader 
then it probably needs to be handled.

For example, I did this (not the best example, but there should be some way to 
generate this from C I was lazy):

    diff --git a/crypto/authenc.c b/crypto/authenc.c
    index d3d6d72fe649..4715d7f63f3e 100644
    --- a/crypto/authenc.c
    +++ b/crypto/authenc.c
    @@ -42,6 +42,17 @@ struct authenc_request_ctx {
            char tail[];
     };
    
    +__asm__ (
    +".align 2"
    +"\n\t.global align2"
    +"\n\talign2:"
    +"\n\tret"
    +"\n\t.align 3"
    +"\n\t.global align3"
    +"\n\talign3:"
    +"\n\tret"
    +);
    +
     static void authenc_request_complete(struct aead_request *req, int err)
     {
            if (err != -EINPROGRESS)

and ended up with some misaligned stuff:

    $ riscv64-unknown-elf-objdump -d -r crypto/authenc.ko | head -n25
    
    crypto/authenc.ko:     file format elf64-littleriscv
    
    
    Disassembly of section .text:
    
    0000000000000000 <align2-0x2>:
       0:	0001                	nop
    			0: R_RISCV_ALIGN	*ABS*+0x2
    
    0000000000000002 <align2>:
       2:	8082                	ret
       4:	0001                	nop
    			4: R_RISCV_ALIGN	*ABS*+0x6
       6:	00000013          	nop
    
    000000000000000a <align3>:
       a:	8082                	ret
    
    000000000000000c <crypto_authenc_extractkeys>:
       c:	1141                	addi	sp,sp,-16
       e:	e422                	sd	s0,8(sp)
      10:	0800                	addi	s0,sp,16
      12:	470d                	li	a4,3
      14:	08c77063          	bleu	a2,a4,94 <.L4>