tools/libxc: misc: Mark const the parameter 'keys' of xc_send_debug_keys()

OCaml is using a string to describe the parameter 'keys' of
xc_send_debug_keys(). Since Ocaml 4.06.01, String_val() will return a
const char * when using -safe-string. This will result to a build
failure because xc_send_debug_keys() expects a char *.

The function should never modify the parameter 'keys' and therefore the
parameter should be const. Unfortunately, this is not directly possible
because DECLARE_HYPERCALL_BOUNCE() is expecting a non-const variable.

A new macro DECLARE_HYPERCALL_BOUNCE_IN() is introduced and will take
care of const parameter. The first user will be xc_send_debug_keys() but
this can be used in more place in the future.

Reported-by: Dario Faggioli <dfaggioli@suse.com>
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Ian Jackson <ian.jackson@eu.citrix.com>
(cherry picked from commit 2b8079610e)

tools/libxc/include/xenctrl.h

@@ -1225,7 +1225,7 @@ int xc_readconsolering(xc_interface *xch,
                        unsigned int *pnr_chars,
                        int clear, int incremental, uint32_t *pindex);
 
-int xc_send_debug_keys(xc_interface *xch, char *keys);
+int xc_send_debug_keys(xc_interface *xch, const char *keys);
 int xc_set_parameters(xc_interface *xch, char *params);
 
 typedef struct xen_sysctl_physinfo xc_physinfo_t;

tools/libxc/xc_misc.c

@@ -167,11 +167,11 @@ int xc_readconsolering(xc_interface *xch,
     return ret;
 }
 
-int xc_send_debug_keys(xc_interface *xch, char *keys)
+int xc_send_debug_keys(xc_interface *xch, const char *keys)
 {
     int ret, len = strlen(keys);
     DECLARE_SYSCTL;
-    DECLARE_HYPERCALL_BOUNCE(keys, len, XC_HYPERCALL_BUFFER_BOUNCE_IN);
+    DECLARE_HYPERCALL_BOUNCE_IN(keys, len);
 
     if ( xc_hypercall_bounce_pre(xch, keys) )
         return -1;

tools/libxc/xc_private.h

@@ -181,6 +181,14 @@ enum {
  */
 #define DECLARE_HYPERCALL_BOUNCE(_ubuf, _sz, _dir) DECLARE_NAMED_HYPERCALL_BOUNCE(_ubuf, _ubuf, _sz, _dir)
 
+/*
+ * Declare a bounce buffer shadowing the named user data pointer that
+ * cannot be modified.
+ */
+#define DECLARE_HYPERCALL_BOUNCE_IN(_ubuf, _sz)                     \
+    DECLARE_NAMED_HYPERCALL_BOUNCE(_ubuf, (void *)(_ubuf), _sz,     \
+                                   XC_HYPERCALL_BUFFER_BOUNCE_IN)
+
 /*
  * Set the size of data to bounce. Useful when the size is not known
  * when the bounce buffer is declared.