plugins: try and make plugin_insn_append more ergonomic

Currently we make the assumption that the guest frontend loads all
op code bytes sequentially. This mostly holds up for regular fixed
encodings but some architectures like s390x like to re-read the
instruction which causes weirdness to occur. Rather than changing the
frontends make the plugin API a little more ergonomic and able to
handle the re-read case.

Stuff will still get strange if we read ahead of the opcode but so far
no front ends have done that and this patch asserts the case so we can
catch it early if they do.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211026102234.3961636-21-alex.bennee@linaro.org>

1 files changed, 10 insertions, 2 deletions

diff --git a/include/exec/plugin-gen.h b/include/exec/plugin-gen.h
index b1b72b5..f92f169 100644
--- a/include/exec/plugin-gen.h
+++ b/include/exec/plugin-gen.h
@@ -27,13 +27,21 @@ void plugin_gen_insn_end(void);
 void plugin_gen_disable_mem_helpers(void);
 void plugin_gen_empty_mem_callback(TCGv addr, uint32_t info);
 
-static inline void plugin_insn_append(const void *from, size_t size)
+static inline void plugin_insn_append(abi_ptr pc, const void *from, size_t size)
 {
     struct qemu_plugin_insn *insn = tcg_ctx->plugin_insn;
+    abi_ptr off;
 
     if (insn == NULL) {
         return;
     }
+    off = pc - insn->vaddr;
+    if (off < insn->data->len) {
+        g_byte_array_set_size(insn->data, off);
+    } else if (off > insn->data->len) {
+        /* we have an unexpected gap */
+        g_assert_not_reached();
+    }
 
     insn->data = g_byte_array_append(insn->data, from, size);
 }
@@ -62,7 +70,7 @@ static inline void plugin_gen_disable_mem_helpers(void)
 static inline void plugin_gen_empty_mem_callback(TCGv addr, uint32_t info)
 { }
 
-static inline void plugin_insn_append(const void *from, size_t size)
+static inline void plugin_insn_append(abi_ptr pc, const void *from, size_t size)
 { }
 
 #endif /* CONFIG_PLUGIN */