Capstone assumes any s390x unknown instruction is 2 bytes.
Instead, use the ilen field in the first two bits of
the instruction to stay in sync with the insn stream.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 disas.c | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/disas.c b/disas.c
index 845c40fca8..1095bad049 100644
--- a/disas.c
+++ b/disas.c
@@ -178,6 +178,39 @@ static int print_insn_od_target(bfd_vma pc, disassemble_info *info)
    to share this across calls and across host vs target disassembly.  */
 static __thread cs_insn *cap_insn;
 
+/*
+ * The capstone library always skips 2 bytes for S390X.
+ * This is less than ideal, since we can tell from the first two bits
+ * the size of the insn and thus stay in sync with the insn stream.
+ */
+static size_t CAPSTONE_API
+cap_skipdata_s390x_cb(const uint8_t *code, size_t code_size,
+                      size_t offset, void *user_data)
+{
+    size_t ilen;
+
+    /* See get_ilen() in target/s390x/internal.h.  */
+    switch (code[offset] >> 6) {
+    case 0:
+        ilen = 2;
+        break;
+    case 1:
+    case 2:
+        ilen = 4;
+        break;
+    default:
+        ilen = 6;
+        break;
+    }
+
+    return ilen;
+}
+
+static const cs_opt_skipdata cap_skipdata_s390x = {
+    .mnemonic = ".byte",
+    .callback = cap_skipdata_s390x_cb
+};
+
 /* Initialize the Capstone library.  */
 /* ??? It would be nice to cache this.  We would need one handle for the
    host and one for the target.  For most targets we can reset specific
@@ -208,6 +241,10 @@ static cs_err cap_disas_start(disassemble_info *info, csh *handle)
 
     /* "Disassemble" unknown insns as ".byte W,X,Y,Z".  */
     cs_option(*handle, CS_OPT_SKIPDATA, CS_OPT_ON);
+    if (info->cap_arch == CS_ARCH_SYSZ) {
+        cs_option(*handle, CS_OPT_SKIPDATA_SETUP,
+                  (uintptr_t)&cap_skipdata_s390x);
+    }
 
     /* Allocate temp space for cs_disasm_iter.  */
     if (cap_insn == NULL) {
-- 
2.20.1

On 1/3/20 10:25 PM, Richard Henderson wrote:
> Capstone assumes any s390x unknown instruction is 2 bytes.
> Instead, use the ilen field in the first two bits of
> the instruction to stay in sync with the insn stream.
> 
> Reviewed-by: Thomas Huth <thuth@redhat.com>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>

> ---
>   disas.c | 37 +++++++++++++++++++++++++++++++++++++
>   1 file changed, 37 insertions(+)
> 
> diff --git a/disas.c b/disas.c
> index 845c40fca8..1095bad049 100644
> --- a/disas.c
> +++ b/disas.c
> @@ -178,6 +178,39 @@ static int print_insn_od_target(bfd_vma pc, disassemble_info *info)
>      to share this across calls and across host vs target disassembly.  */
>   static __thread cs_insn *cap_insn;
>   
> +/*
> + * The capstone library always skips 2 bytes for S390X.
> + * This is less than ideal, since we can tell from the first two bits
> + * the size of the insn and thus stay in sync with the insn stream.
> + */
> +static size_t CAPSTONE_API
> +cap_skipdata_s390x_cb(const uint8_t *code, size_t code_size,
> +                      size_t offset, void *user_data)
> +{
> +    size_t ilen;
> +
> +    /* See get_ilen() in target/s390x/internal.h.  */
> +    switch (code[offset] >> 6) {
> +    case 0:
> +        ilen = 2;
> +        break;
> +    case 1:
> +    case 2:
> +        ilen = 4;
> +        break;
> +    default:
> +        ilen = 6;
> +        break;
> +    }
> +
> +    return ilen;
> +}
> +
> +static const cs_opt_skipdata cap_skipdata_s390x = {
> +    .mnemonic = ".byte",
> +    .callback = cap_skipdata_s390x_cb
> +};
> +
>   /* Initialize the Capstone library.  */
>   /* ??? It would be nice to cache this.  We would need one handle for the
>      host and one for the target.  For most targets we can reset specific
> @@ -208,6 +241,10 @@ static cs_err cap_disas_start(disassemble_info *info, csh *handle)
>   
>       /* "Disassemble" unknown insns as ".byte W,X,Y,Z".  */
>       cs_option(*handle, CS_OPT_SKIPDATA, CS_OPT_ON);
> +    if (info->cap_arch == CS_ARCH_SYSZ) {
> +        cs_option(*handle, CS_OPT_SKIPDATA_SETUP,
> +                  (uintptr_t)&cap_skipdata_s390x);
> +    }
>   
>       /* Allocate temp space for cs_disasm_iter.  */
>       if (cap_insn == NULL) {
>