[PATCH v2] riscv: Use memcpy to handle unaligned access when fixing R_RISCV_RELATIVE

[PATCH v2] riscv: Use memcpy to handle unaligned access when fixing R_RISCV_RELATIVE

[Kito Cheng]

Although RISC-V Linux will enable the unaligned memory access handler by
default, that is quite expensive in general, using memcpy will be much cheaper
- just break down that into several load/store byte instructions.

ARM and MIPS has similar issue:

ARM: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51456
MIPS: https://gcc.gnu.org/legacy-ml/gcc-help/2005-07/msg00325.html

Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

---

V2 Changes:

- Fix minor typo
---
 sysdeps/riscv/dl-machine.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/sysdeps/riscv/dl-machine.h b/sysdeps/riscv/dl-machine.h
index 4bb858adaa..dfedc9801d 100644
--- a/sysdeps/riscv/dl-machine.h
+++ b/sysdeps/riscv/dl-machine.h
@@ -157,7 +157,10 @@ __attribute__ ((always_inline))
 elf_machine_rela_relative (ElfW(Addr) l_addr, const ElfW(Rela) *reloc,
 			  void *const reloc_addr)
 {
-  *(ElfW(Addr) *) reloc_addr = l_addr + reloc->r_addend;
+  /* R_RISCV_RELATIVE might located in debug info section which might not
+     aligned to XLEN bytes.  Also support relocations on unaligned offsets.  */
+  ElfW(Addr) value = l_addr + reloc->r_addend;
+  memcpy (reloc_addr, &value, sizeof value);
 }
 
 /* Perform a relocation described by R_INFO at the location pointed to
-- 
2.34.0

Re: [PATCH v2] riscv: Use memcpy to handle unaligned access when fixing R_RISCV_RELATIVE

[Palmer Dabbelt]

On Tue, 28 Jun 2022 06:52:19 PDT (-0700), kito.cheng@sifive.com wrote:
> Although RISC-V Linux will enable the unaligned memory access handler by
> default, that is quite expensive in general, using memcpy will be much cheaper
> - just break down that into several load/store byte instructions.
>
> ARM and MIPS has similar issue:
>
> ARM: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51456
> MIPS: https://gcc.gnu.org/legacy-ml/gcc-help/2005-07/msg00325.html
>
> Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
> Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
>
> ---
>
> V2 Changes:
>
> - Fix minor typo
> ---
>  sysdeps/riscv/dl-machine.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/sysdeps/riscv/dl-machine.h b/sysdeps/riscv/dl-machine.h
> index 4bb858adaa..dfedc9801d 100644
> --- a/sysdeps/riscv/dl-machine.h
> +++ b/sysdeps/riscv/dl-machine.h
> @@ -157,7 +157,10 @@ __attribute__ ((always_inline))
>  elf_machine_rela_relative (ElfW(Addr) l_addr, const ElfW(Rela) *reloc,
>  			  void *const reloc_addr)
>  {
> -  *(ElfW(Addr) *) reloc_addr = l_addr + reloc->r_addend;
> +  /* R_RISCV_RELATIVE might located in debug info section which might not
> +     aligned to XLEN bytes.  Also support relocations on unaligned offsets.  */
> +  ElfW(Addr) value = l_addr + reloc->r_addend;
> +  memcpy (reloc_addr, &value, sizeof value);
>  }
>
>  /* Perform a relocation described by R_INFO at the location pointed to

Committed.  Thanks!