[PATCH v2 05/24] target/riscv: tracking indirect branches (fcfi) for zicfilp

[Deepak Gupta]

zicfilp protects forward control flow (if enabled) by enforcing all
indirect call and jmp must land on a landing pad instruction `lpad`. If
target of an indirect call or jmp is not `lpad` then cpu/hart must raise
a sw check exception with tval = 2.

This patch implements the mechanism using TCG. Target architecture branch
instruction must define the end of a TB. Using this property, during
translation of branch instruction, TB flag = FCFI_LP_EXPECTED can be set.
Translation of target TB can check if FCFI_LP_EXPECTED flag is set and a
flag (fcfi_lp_expected) can be set in DisasContext. If `lpad` gets
translated, fcfi_lp_expected flag in DisasContext can be cleared. Else
it'll fault.

This patch also also adds flag for forward cfi in DisasContext.

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
Co-developed-by: Jim Shu <jim.shu@sifive.com>
Co-developed-by: Andy Chiu <andy.chiu@sifive.com>
---
 target/riscv/cpu.h        |  2 ++
 target/riscv/cpu_bits.h   |  3 +++
 target/riscv/cpu_helper.c | 12 ++++++++++
 target/riscv/translate.c  | 48 +++++++++++++++++++++++++++++++++++++++
 4 files changed, 65 insertions(+)

diff --git a/target/riscv/cpu.h b/target/riscv/cpu.h
index 12334f9540..7fed5d2750 100644
--- a/target/riscv/cpu.h
+++ b/target/riscv/cpu.h
@@ -606,6 +606,8 @@ FIELD(TB_FLAGS, ITRIGGER, 22, 1)
 FIELD(TB_FLAGS, VIRT_ENABLED, 23, 1)
 FIELD(TB_FLAGS, PRIV, 24, 2)
 FIELD(TB_FLAGS, AXL, 26, 2)
+/* zicfilp needs a TB flag to track indirect branches */
+FIELD(TB_FLAGS, FCFI_LP_EXPECTED, 28, 1)
 
 #ifdef TARGET_RISCV32
 #define riscv_cpu_mxl(env)  ((void)(env), MXL_RV32)
diff --git a/target/riscv/cpu_bits.h b/target/riscv/cpu_bits.h
index 127f2179dc..477e24feaf 100644
--- a/target/riscv/cpu_bits.h
+++ b/target/riscv/cpu_bits.h
@@ -691,6 +691,9 @@ typedef enum RISCVException {
     RISCV_EXCP_SEMIHOST = 0x3f,
 } RISCVException;
 
+/* zicfilp defines lp violation results in sw check with tval = 2*/
+#define RISCV_EXCP_SW_CHECK_FCFI_TVAL      2
+
 #define RISCV_EXCP_INT_FLAG                0x80000000
 #define RISCV_EXCP_INT_MASK                0x7fffffff
 
diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c
index 41bc73ad60..2cb1d45467 100644
--- a/target/riscv/cpu_helper.c
+++ b/target/riscv/cpu_helper.c
@@ -134,6 +134,18 @@ void cpu_get_tb_cpu_state(CPURISCVState *env, vaddr *pc,
         flags = FIELD_DP32(flags, TB_FLAGS, VILL, 1);
     }
 
+    if (cpu_get_fcfien(env)) {
+        /*
+         * For Forward CFI, only the expectation of a lpcll at
+         * the start of the block is tracked (which can only happen
+         * when FCFI is enabled for the current processor mode). A jump
+         * or call at the end of the previous TB will have updated
+         * env->elp to indicate the expectation.
+         */
+        flags = FIELD_DP32(flags, TB_FLAGS, FCFI_LP_EXPECTED,
+                           env->elp != NO_LP_EXPECTED);
+    }
+
 #ifdef CONFIG_USER_ONLY
     fs = EXT_STATUS_DIRTY;
     vs = EXT_STATUS_DIRTY;
diff --git a/target/riscv/translate.c b/target/riscv/translate.c
index acba90f170..c746d7df08 100644
--- a/target/riscv/translate.c
+++ b/target/riscv/translate.c
@@ -20,6 +20,7 @@
 #include "qemu/log.h"
 #include "cpu.h"
 #include "tcg/tcg-op.h"
+#include "tcg/tcg-temp-internal.h"
 #include "exec/exec-all.h"
 #include "exec/helper-proto.h"
 #include "exec/helper-gen.h"
@@ -44,6 +45,7 @@ static TCGv load_val;
 /* globals for PM CSRs */
 static TCGv pm_mask;
 static TCGv pm_base;
+static TCGOp *cfi_lp_check;
 
 /*
  * If an operation is being performed on less than TARGET_LONG_BITS,
@@ -116,6 +118,9 @@ typedef struct DisasContext {
     bool frm_valid;
     bool insn_start_updated;
     const GPtrArray *decoders;
+    /* zicfilp extension. cfi enabled or not. lp expected or not */
+    bool fcfi_enabled;
+    bool fcfi_lp_expected;
 } DisasContext;
 
 static inline bool has_ext(DisasContext *ctx, uint32_t ext)
@@ -1238,6 +1243,8 @@ static void riscv_tr_init_disas_context(DisasContextBase 
*dcbase, CPUState *cs)
     ctx->pm_base_enabled = FIELD_EX32(tb_flags, TB_FLAGS, PM_BASE_ENABLED);
     ctx->ztso = cpu->cfg.ext_ztso;
     ctx->itrigger = FIELD_EX32(tb_flags, TB_FLAGS, ITRIGGER);
+    ctx->fcfi_lp_expected = FIELD_EX32(tb_flags, TB_FLAGS, FCFI_LP_EXPECTED);
+    ctx->fcfi_enabled = cpu_get_fcfien(env) && ctx->fcfi_lp_expected;
     ctx->zero = tcg_constant_tl(0);
     ctx->virt_inst_excp = false;
     ctx->decoders = cpu->decoders;
@@ -1245,6 +1252,39 @@ static void riscv_tr_init_disas_context(DisasContextBase 
*dcbase, CPUState *cs)
 
 static void riscv_tr_tb_start(DisasContextBase *db, CPUState *cpu)
 {
+    DisasContext *ctx = container_of(db, DisasContext, base);
+
+    if (ctx->fcfi_lp_expected) {
+        /*
+         * Since we can't look ahead to confirm that the first
+         * instruction is a legal landing pad instruction, emit
+         * compare-and-branch sequence that will be fixed-up in
+         * riscv_tr_tb_stop() to either statically hit or skip an
+         * illegal instruction exception depending on whether the
+         * flag was lowered by translation of a CJLP or JLP as
+         * the first instruction in the block.
+         */
+        TCGv_i32 immediate;
+        TCGLabel *l;
+        l = gen_new_label();
+        immediate = tcg_temp_new_i32();
+        tcg_gen_movi_i32(immediate, 0);
+        cfi_lp_check = tcg_last_op();
+        tcg_gen_brcondi_i32(TCG_COND_EQ, immediate, 0, l);
+        tcg_temp_free_i32(immediate);
+        tcg_gen_st_tl(
+            tcg_constant_tl(RISCV_EXCP_SW_CHECK_FCFI_TVAL),
+            tcg_env, offsetof(CPURISCVState, sw_check_code));
+        generate_exception(ctx, RISCV_EXCP_SW_CHECK);
+        gen_set_label(l);
+        /*
+         * Despite the use of gen_exception_illegal(), the rest of
+         * the TB needs to be generated. The TCG optimizer will
+         * clean things up depending on which path ends up being
+         * active.
+         */
+        ctx->base.is_jmp = DISAS_NEXT;
+    }
 }
 
 static void riscv_tr_insn_start(DisasContextBase *dcbase, CPUState *cpu)
@@ -1303,6 +1343,14 @@ static void riscv_tr_tb_stop(DisasContextBase *dcbase, 
CPUState *cpu)
     default:
         g_assert_not_reached();
     }
+
+    if (ctx->fcfi_lp_expected) {
+        /*
+         * If the "lp expected" flag is still up, the block needs to take an
+         * illegal instruction exception.
+         */
+        tcg_set_insn_param(cfi_lp_check, 1, tcgv_i32_arg(tcg_constant_i32(1)));
+    }
 }
 
 static const TranslatorOps riscv_tr_ops = {
-- 
2.44.0