[PATCH] arm: Avoid UB in elf_machine_rel()

[Palmer Dabbelt]

This recently came up during a cleanup to remove misaligned accesses
from the RISC-V port.

Link: https://sourceware.org/pipermail/libc-alpha/2022-June/139961.html
Suggested-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

--

I haven't build this one, as I'm just doing it locally and don't have
everything setup here.
---
 sysdeps/arm/dl-machine.h | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/sysdeps/arm/dl-machine.h b/sysdeps/arm/dl-machine.h
index 68dcb96d9d..6bb3917340 100644
--- a/sysdeps/arm/dl-machine.h
+++ b/sysdeps/arm/dl-machine.h
@@ -402,10 +402,7 @@ elf_machine_rel (struct link_map *map, struct r_scope_elem *scope[],
 	  break;
 	case R_ARM_ABS32:
 	  {
-	    struct unaligned
-	      {
-		Elf32_Addr x;
-	      } __attribute__ ((packed, may_alias));
+	    ElfW(Addr) tmp;
 # ifndef RTLD_BOOTSTRAP
 	   /* This is defined in rtld.c, but nowhere in the static
 	      libc.a; make the reference weak so static programs can
@@ -425,7 +422,9 @@ elf_machine_rel (struct link_map *map, struct r_scope_elem *scope[],
 	      value -= SYMBOL_ADDRESS (map, refsym, true);
 # endif
 	    /* Support relocations on mis-aligned offsets.  */
-	    ((struct unaligned *) reloc_addr)->x += value;
+	    memcpy(&tmp, reloc_addr, sizeof tmp);
+	    tmp += value;
+	    memcpy (reloc_addr, &tmp, sizeof tmp);
 	    break;
 	  }
 	case R_ARM_TLS_DESC:
-- 
2.34.1