target/arm: Implement secure function return

Secure function return happens when a non-secure function has been
called using BLXNS and so has a particular magic LR value (either
0xfefffffe or 0xfeffffff). The function return via BX behaves
specially when the new PC value is this magic value, in the same
way that exception returns are handled.

Adjust our BX excret guards so that they recognize the function
return magic number as well, and perform the function-return
unstacking in do_v7m_exception_exit().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1507556919-24992-5-git-send-email-peter.maydell@linaro.org

1 files changed, 12 insertions, 2 deletions

diff --git a/target/arm/translate.c b/target/arm/translate.c
index caf0d5849a..5c6f9fea1b 100644
--- a/target/arm/translate.c
+++ b/target/arm/translate.c
@@ -964,7 +964,8 @@ static inline void gen_bx_excret(DisasContext *s, TCGv_i32 var)
      * s->base.is_jmp that we need to do the rest of the work later.
      */
     gen_bx(s, var);
-    if (s->v7m_handler_mode && arm_dc_feature(s, ARM_FEATURE_M)) {
+    if (arm_dc_feature(s, ARM_FEATURE_M_SECURITY) ||
+        (s->v7m_handler_mode && arm_dc_feature(s, ARM_FEATURE_M))) {
         s->base.is_jmp = DISAS_BX_EXCRET;
     }
 }
@@ -973,9 +974,18 @@ static inline void gen_bx_excret_final_code(DisasContext *s)
 {
     /* Generate the code to finish possible exception return and end the TB */
     TCGLabel *excret_label = gen_new_label();
+    uint32_t min_magic;
+
+    if (arm_dc_feature(s, ARM_FEATURE_M_SECURITY)) {
+        /* Covers FNC_RETURN and EXC_RETURN magic */
+        min_magic = FNC_RETURN_MIN_MAGIC;
+    } else {
+        /* EXC_RETURN magic only */
+        min_magic = EXC_RETURN_MIN_MAGIC;
+    }
 
     /* Is the new PC value in the magic range indicating exception return? */
-    tcg_gen_brcondi_i32(TCG_COND_GEU, cpu_R[15], 0xff000000, excret_label);
+    tcg_gen_brcondi_i32(TCG_COND_GEU, cpu_R[15], min_magic, excret_label);
     /* No: end the TB as we would for a DISAS_JMP */
     if (is_singlestepping(s)) {
         gen_singlestep_exception(s);