semihosting: Add target_strlen for softmmu-uaccess.h

Mirror the interface of the user-only function of the same name.
Use probe_access_flags for the common case of ram, and
cpu_memory_rw_debug for the uncommon case of mmio.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
v3: Use probe_access_flags (pmm)

include/semihosting/softmmu-uaccess.h

@@ -53,4 +53,7 @@ void softmmu_unlock_user(CPUArchState *env, void *p,
                          target_ulong addr, target_ulong len);
 #define unlock_user(s, args, len) softmmu_unlock_user(env, s, args, len)
 
+ssize_t softmmu_strlen_user(CPUArchState *env, target_ulong addr);
+#define target_strlen(p) softmmu_strlen_user(env, p)
+
 #endif /* SEMIHOSTING_SOFTMMU_UACCESS_H */

semihosting/uaccess.c

@@ -8,6 +8,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "exec/exec-all.h"
 #include "semihosting/softmmu-uaccess.h"
 
 void *softmmu_lock_user(CPUArchState *env, target_ulong addr,
@@ -23,6 +24,54 @@ void *softmmu_lock_user(CPUArchState *env, target_ulong addr,
     return p;
 }
 
+ssize_t softmmu_strlen_user(CPUArchState *env, target_ulong addr)
+{
+    int mmu_idx = cpu_mmu_index(env, false);
+    size_t len = 0;
+
+    while (1) {
+        size_t left_in_page;
+        int flags;
+        void *h;
+
+        /* Find the number of bytes remaining in the page. */
+        left_in_page = TARGET_PAGE_SIZE - (addr & ~TARGET_PAGE_MASK);
+
+        flags = probe_access_flags(env, addr, MMU_DATA_LOAD,
+                                   mmu_idx, true, &h, 0);
+        if (flags & TLB_INVALID_MASK) {
+            return -1;
+        }
+        if (flags & TLB_MMIO) {
+            do {
+                uint8_t c;
+                if (cpu_memory_rw_debug(env_cpu(env), addr, &c, 1, 0)) {
+                    return -1;
+                }
+                if (c == 0) {
+                    return len;
+                }
+                addr++;
+                len++;
+                if (len > INT32_MAX) {
+                    return -1;
+                }
+            } while (--left_in_page != 0);
+        } else {
+            char *p = memchr(h, 0, left_in_page);
+            if (p) {
+                len += p - (char *)h;
+                return len <= INT32_MAX ? (ssize_t)len : -1;
+            }
+            addr += left_in_page;
+            len += left_in_page;
+            if (len > INT32_MAX) {
+                return -1;
+            }
+        }
+    }
+}
+
 char *softmmu_lock_user_string(CPUArchState *env, target_ulong addr)
 {
     /* TODO: Make this something that isn't fixed size.  */