target/arm: Define and use new load_cpu_field_low32()

In several places in the 32-bit Arm translate.c, we try to use
load_cpu_field() to load from a CPUARMState field into a TCGv_i32
where the field is actually 64-bit. This works on little-endian
hosts, but gives the wrong half of the register on big-endian.

Add a new load_cpu_field_low32() which loads the low 32 bits
of a 64-bit field into a TCGv_i32. The new macro includes a
compile-time check against accidentally using it on a field
of the wrong size. Use it to fix the two places in the code
where we were using load_cpu_field() on a 64-bit field.

This fixes a bug where on big-endian hosts the guest would
crash after executing an ERET instruction, and a more corner
case one where some UNDEFs for attempted accesses to MSR
banked registers from Secure EL1 might go to the wrong EL.

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230424153909.1419369-2-peter.maydell@linaro.org

target/arm/tcg/translate.c

@@ -2816,7 +2816,7 @@ static bool msr_banked_access_decode(DisasContext *s, int r, int sysm, int rn,
             if (arm_dc_feature(s, ARM_FEATURE_AARCH64) &&
                 dc_isar_feature(aa64_sel2, s)) {
                 /* Target EL is EL<3 minus SCR_EL3.EEL2> */
-                tcg_el = load_cpu_field(cp15.scr_el3);
+                tcg_el = load_cpu_field_low32(cp15.scr_el3);
                 tcg_gen_sextract_i32(tcg_el, tcg_el, ctz32(SCR_EEL2), 1);
                 tcg_gen_addi_i32(tcg_el, tcg_el, 3);
             } else {
@@ -6396,7 +6396,7 @@ static bool trans_ERET(DisasContext *s, arg_ERET *a)
     }
     if (s->current_el == 2) {
         /* ERET from Hyp uses ELR_Hyp, not LR */
-        tmp = load_cpu_field(elr_el[2]);
+        tmp = load_cpu_field_low32(elr_el[2]);
     } else {
         tmp = load_reg(s, 14);
     }

target/arm/translate-a32.h

@@ -61,6 +61,13 @@ static inline TCGv_i32 load_cpu_offset(int offset)
 
 #define load_cpu_field(name) load_cpu_offset(offsetof(CPUARMState, name))
 
+/* Load from the low half of a 64-bit field to a TCGv_i32 */
+#define load_cpu_field_low32(name)                                      \
+    ({                                                                  \
+        QEMU_BUILD_BUG_ON(sizeof_field(CPUARMState, name) != 8);        \
+        load_cpu_offset(offsetoflow32(CPUARMState, name));              \
+    })
+
 void store_cpu_offset(TCGv_i32 var, int offset, int size);
 
 #define store_cpu_field(var, name)                              \