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

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

[Palmer Dabbelt]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=c22d2021a9f9bdea62398976eea4f0e6ef668b7d

commit c22d2021a9f9bdea62398976eea4f0e6ef668b7d
Author: Kito Cheng <kito.cheng@sifive.com>
Date:   Tue Jun 28 21:52:19 2022 +0800

    riscv: Use memcpy to handle unaligned access when fixing R_RISCV_RELATIVE
    
    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
    
    Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

Diff:
---
 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