target/ppc: don't byte swap ELFv2 signal handler

Previously, the signal handler would be byte swapped if the target and
host CPU used different endianness. This would cause a SIGSEGV when
attempting to translate the opcode pointed to by the swapped address.

 Thread 1 "qemu-ppc64" received signal SIGSEGV, Segmentation fault.
 0x00000000600a9257 in ldl_he_p (ptr=0x4c2c061000000000) at qemu/include/qemu/bswap.h:351
 351        __builtin_memcpy(&r, ptr, sizeof(r));

 #0  0x00000000600a9257 in ldl_he_p (ptr=0x4c2c061000000000) at qemu/include/qemu/bswap.h:351
 #1  0x00000000600a92fe in ldl_be_p (ptr=0x4c2c061000000000) at qemu/include/qemu/bswap.h:449
 #2  0x00000000600c0790 in translator_ldl_swap at qemu/include/exec/translator.h:201
 #3  0x000000006011c1ab in ppc_tr_translate_insn at qemu/target/ppc/translate.c:7856
 #4  0x000000006005ae70 in translator_loop at qemu/accel/tcg/translator.c:102

The signal handler will be byte swapped as a result of the __get_user()
call in sigaction() if it is necessary, no additional swap is required.

Signed-off-by: Vincent Fazio <vfazio@gmail.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200319133244.8818-1-vfazio@xes-inc.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

linux-user/ppc/signal.c

@@ -567,10 +567,8 @@ void setup_rt_frame(int sig, struct target_sigaction *ka,
         env->nip = tswapl(handler->entry);
         env->gpr[2] = tswapl(handler->toc);
     } else {
-        /* ELFv2 PPC64 function pointers are entry points, but R12
-         * must also be set */
-        env->nip = tswapl((target_ulong) ka->_sa_handler);
-        env->gpr[12] = env->nip;
+        /* ELFv2 PPC64 function pointers are entry points. R12 must also be set. */
+        env->gpr[12] = env->nip = ka->_sa_handler;
     }
 #else
     env->nip = (target_ulong) ka->_sa_handler;