diff --git a/include/semihosting/guestfd.h b/include/semihosting/guestfd.h
index 5aa2722cb2..275b91b73a 100644
--- a/include/semihosting/guestfd.h
+++ b/include/semihosting/guestfd.h
@@ -41,4 +41,17 @@ GuestFD *get_guestfd(int guestfd);
 void associate_guestfd(int guestfd, int hostfd);
 void staticfile_guestfd(int guestfd, const uint8_t *data, size_t len);
 
+/*
+ * Syscall implementations for semi-hosting.  Argument loading from
+ * the guest is performed by the caller; results are returned via
+ * the 'complete' callback.  String operands are in address/len pairs.
+ * The len argument may be 0 (when the semihosting abi does not
+ * already provide the length), or non-zero (where it should include
+ * the terminating zero).
+ */
+
+void semihost_sys_open(CPUState *cs, gdb_syscall_complete_cb complete,
+                       target_ulong fname, target_ulong fname_len,
+                       int gdb_flags, int mode);
+
 #endif /* SEMIHOSTING_GUESTFD_H */
diff --git a/semihosting/arm-compat-semi.c b/semihosting/arm-compat-semi.c
index 49f976cbc5..e2decd54e4 100644
--- a/semihosting/arm-compat-semi.c
+++ b/semihosting/arm-compat-semi.c
@@ -35,9 +35,9 @@
 #include "semihosting/semihost.h"
 #include "semihosting/console.h"
 #include "semihosting/common-semi.h"
-#include "semihosting/guestfd.h"
 #include "qemu/timer.h"
 #include "exec/gdbstub.h"
+#include "semihosting/guestfd.h"
 
 #ifdef CONFIG_USER_ONLY
 #include "qemu.h"
@@ -98,21 +98,6 @@ static int gdb_open_modeflags[12] = {
     GDB_O_RDWR | GDB_O_CREAT | GDB_O_APPEND | GDB_O_BINARY
 };
 
-static int open_modeflags[12] = {
-    O_RDONLY,
-    O_RDONLY | O_BINARY,
-    O_RDWR,
-    O_RDWR | O_BINARY,
-    O_WRONLY | O_CREAT | O_TRUNC,
-    O_WRONLY | O_CREAT | O_TRUNC | O_BINARY,
-    O_RDWR | O_CREAT | O_TRUNC,
-    O_RDWR | O_CREAT | O_TRUNC | O_BINARY,
-    O_WRONLY | O_CREAT | O_APPEND,
-    O_WRONLY | O_CREAT | O_APPEND | O_BINARY,
-    O_RDWR | O_CREAT | O_APPEND,
-    O_RDWR | O_CREAT | O_APPEND | O_BINARY
-};
-
 #ifndef CONFIG_USER_ONLY
 
 /**
@@ -285,20 +270,6 @@ common_semi_flen_cb(CPUState *cs, target_ulong ret, target_ulong err)
     common_semi_set_ret(cs, size);
 }
 
-static int common_semi_open_guestfd;
-
-static void
-common_semi_open_cb(CPUState *cs, target_ulong ret, target_ulong err)
-{
-    if (ret == (target_ulong)-1) {
-        dealloc_guestfd(common_semi_open_guestfd);
-    } else {
-        associate_guestfd(common_semi_open_guestfd, ret);
-        ret = common_semi_open_guestfd;
-    }
-    common_semi_cb(cs, ret, err);
-}
-
 /*
  * Types for functions implementing various semihosting calls
  * for specific types of guest file descriptor. These must all
@@ -602,22 +573,10 @@ void do_common_semihosting(CPUState *cs)
                 staticfile_guestfd(ret, featurefile_data,
                                    sizeof(featurefile_data));
             }
-        } else if (use_gdb_syscalls()) {
-            unlock_user(s, arg0, 0);
-            common_semi_open_guestfd = alloc_guestfd();
-            gdb_do_syscall(common_semi_open_cb,
-                           "open,%s,%x,1a4", arg0, (int)arg2 + 1,
-                           gdb_open_modeflags[arg1]);
-            break;
         } else {
-            hostfd = open(s, open_modeflags[arg1], 0644);
-            if (hostfd < 0) {
-                ret = -1;
-                err = errno;
-            } else {
-                ret = alloc_guestfd();
-                associate_guestfd(ret, hostfd);
-            }
+            semihost_sys_open(cs, common_semi_cb, arg0, arg2 + 1,
+                              gdb_open_modeflags[arg1], 0644);
+            break;
         }
         unlock_user(s, arg0, 0);
         common_semi_cb(cs, ret, err);
diff --git a/semihosting/guestfd.c b/semihosting/guestfd.c
index b6405f5663..1393279333 100644
--- a/semihosting/guestfd.c
+++ b/semihosting/guestfd.c
@@ -11,6 +11,11 @@
 #include "qemu/osdep.h"
 #include "exec/gdbstub.h"
 #include "semihosting/guestfd.h"
+#ifdef CONFIG_USER_ONLY
+#include "qemu.h"
+#else
+#include "semihosting/softmmu-uaccess.h"
+#endif
 
 static GArray *guestfd_array;
 
@@ -116,3 +121,133 @@ void dealloc_guestfd(int guestfd)
     assert(gf);
     gf->type = GuestFDUnused;
 }
+
+/*
+ * Validate or compute the length of the string (including terminator).
+ */
+static int validate_strlen(CPUState *cs, target_ulong str, target_ulong tlen)
+{
+    CPUArchState *env G_GNUC_UNUSED = cs->env_ptr;
+    char c;
+
+    if (tlen == 0) {
+        ssize_t slen = target_strlen(str);
+
+        if (slen < 0) {
+            return -EFAULT;
+        }
+        if (slen >= INT32_MAX) {
+            return -ENAMETOOLONG;
+        }
+        return slen + 1;
+    }
+    if (tlen > INT32_MAX) {
+        return -ENAMETOOLONG;
+    }
+    if (get_user_u8(c, str + tlen - 1)) {
+        return -EFAULT;
+    }
+    if (c != 0) {
+        return -EINVAL;
+    }
+    return tlen;
+}
+
+static int validate_lock_user_string(char **pstr, CPUState *cs,
+                                     target_ulong tstr, target_ulong tlen)
+{
+    int ret = validate_strlen(cs, tstr, tlen);
+    CPUArchState *env G_GNUC_UNUSED = cs->env_ptr;
+    char *str = NULL;
+
+    if (ret > 0) {
+        str = lock_user(VERIFY_READ, tstr, ret, true);
+        ret = str ? 0 : -EFAULT;
+    }
+    *pstr = str;
+    return ret;
+}
+
+/*
+ * File-related semihosting syscall implementations.
+ */
+
+static gdb_syscall_complete_cb gdb_open_complete;
+
+static void gdb_open_cb(CPUState *cs, target_ulong ret, target_ulong err)
+{
+    if (!err) {
+        int guestfd = alloc_guestfd();
+        associate_guestfd(guestfd, ret);
+        ret = guestfd;
+    }
+    gdb_open_complete(cs, ret, err);
+}
+
+static void gdb_open(CPUState *cs, gdb_syscall_complete_cb complete,
+                     target_ulong fname, target_ulong fname_len,
+                     int gdb_flags, int mode)
+{
+    int len = validate_strlen(cs, fname, fname_len);
+    if (len < 0) {
+        complete(cs, -1, -len);
+        return;
+    }
+
+    gdb_open_complete = complete;
+    gdb_do_syscall(gdb_open_cb, "open,%s,%x,%x",
+                   fname, len, (target_ulong)gdb_flags, (target_ulong)mode);
+}
+
+static void host_open(CPUState *cs, gdb_syscall_complete_cb complete,
+                      target_ulong fname, target_ulong fname_len,
+                      int gdb_flags, int mode)
+{
+    CPUArchState *env G_GNUC_UNUSED = cs->env_ptr;
+    char *p;
+    int ret, host_flags;
+
+    ret = validate_lock_user_string(&p, cs, fname, fname_len);
+    if (ret < 0) {
+        complete(cs, -1, -ret);
+        return;
+    }
+
+    if (gdb_flags & GDB_O_WRONLY) {
+        host_flags = O_WRONLY;
+    } else if (gdb_flags & GDB_O_RDWR) {
+        host_flags = O_RDWR;
+    } else {
+        host_flags = O_RDONLY;
+    }
+    if (gdb_flags & GDB_O_CREAT) {
+        host_flags |= O_CREAT;
+    }
+    if (gdb_flags & GDB_O_TRUNC) {
+        host_flags |= O_TRUNC;
+    }
+    if (gdb_flags & GDB_O_EXCL) {
+        host_flags |= O_EXCL;
+    }
+
+    ret = open(p, host_flags, mode);
+    if (ret < 0) {
+        complete(cs, -1, errno);
+    } else {
+        int guestfd = alloc_guestfd();
+        associate_guestfd(guestfd, ret);
+        complete(cs, guestfd, 0);
+    }
+    unlock_user(p, fname, 0);
+}
+
+void semihost_sys_open(CPUState *cs, gdb_syscall_complete_cb complete,
+                       target_ulong fname, target_ulong fname_len,
+                       int gdb_flags, int mode)
+{
+    if (use_gdb_syscalls()) {
+        gdb_open(cs, complete, fname, fname_len, gdb_flags, mode);
+    } else {
+        host_open(cs, complete, fname, fname_len, gdb_flags, mode);
+    }
+}
