Modify the value returned by the kern.usrstack sysctl to reflect the

user stack that Valgrind synthesizes for the guest. Without this change
the sysctl will return the stack of the Valgrind host. This manifested itself
as a problem on rust compiled binaries, which were trying to add an extra
guard page but were failing since Valgrind refused guest mmaps into what it
considered to be its own memory space.

coregrind/m_main.c

@@ -3843,6 +3843,13 @@ UWord voucher_mach_msg_set ( UWord arg1 )
 #endif
 
 
+Word VG_(get_usrstack)(void)
+{
+   return VG_PGROUNDDN(the_iicii.clstack_end - the_iifii.clstack_max_size);
+}
+
+
+
 /*--------------------------------------------------------------------*/
 /*--- end                                                          ---*/
 /*--------------------------------------------------------------------*/

coregrind/m_syswrap/syswrap-freebsd.c

@@ -1983,6 +1983,32 @@ PRE(sys___sysctl)
       }
    }
 
+   /*
+    * Special handling cases
+    *
+    * 1. kern.userstack
+    *    This sysctl returns the address of the bottom of the user stack
+    *    (that is the highest user stack address, since the stack grows
+    *    downwards). Without any special handling this would return the
+    *    address of the host userstack. We have created a stack for the
+    *    guest (in aspacemgr) and that is the one that we want the guest
+    *    to see. Aspacemgr is setup in m_main.c with the adresses and sizes
+    *    saved to file static variables in that file, so we call
+    *    VG_(get_usrstack)() to retrieve them from there.
+    */
+   if (SARG2 >= 2 && ML_(safe_to_deref)(name, 2*sizeof(int))) {
+      if (name[0] == 1 && name[1] == 33) {
+         // kern.userstack
+         Word tmp = VG_(get_usrstack)();
+         size_t* out = (size_t*)ARG3;
+         size_t* outlen = (size_t*)ARG4;
+         *out = tmp;
+         *outlen = sizeof(size_t);
+         SET_STATUS_Success(0);
+      }
+   }
+
+
    PRE_REG_READ6(int, "__sysctl", int *, name, vki_u_int32_t, namelen, void *, oldp,
                  vki_size_t *, oldlenp, void *, newp, vki_size_t, newlen);
 

coregrind/pub_core_aspacemgr.h

@@ -384,6 +384,10 @@ extern Bool VG_(am_search_for_new_segment)(Addr *start, SizeT *size,
                                            UInt *prot);
 #endif
 
+/* For kern.usrstack syscall on FreeBSD */
+extern Word VG_(get_usrstack)(void);
+
+
 #endif   // __PUB_CORE_ASPACEMGR_H
 
 /*--------------------------------------------------------------------*/