linux-user/hppa: Allocate page zero as a commpage

We're about to start validating PAGE_EXEC, which means that we've
got to mark page zero executable.  We had been special casing this
entirely within translate.

Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Tested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

linux-user/elfload.c

@@ -1646,6 +1646,34 @@ static inline void init_thread(struct target_pt_regs *regs,
     regs->gr[31] = infop->entry;
 }
 
+#define LO_COMMPAGE  0
+
+static bool init_guest_commpage(void)
+{
+    void *want = g2h_untagged(LO_COMMPAGE);
+    void *addr = mmap(want, qemu_host_page_size, PROT_NONE,
+                      MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED, -1, 0);
+
+    if (addr == MAP_FAILED) {
+        perror("Allocating guest commpage");
+        exit(EXIT_FAILURE);
+    }
+    if (addr != want) {
+        return false;
+    }
+
+    /*
+     * On Linux, page zero is normally marked execute only + gateway.
+     * Normal read or write is supposed to fail (thus PROT_NONE above),
+     * but specific offsets have kernel code mapped to raise permissions
+     * and implement syscalls.  Here, simply mark the page executable.
+     * Special case the entry points during translation (see do_page_zero).
+     */
+    page_set_flags(LO_COMMPAGE, LO_COMMPAGE + TARGET_PAGE_SIZE,
+                   PAGE_EXEC | PAGE_VALID);
+    return true;
+}
+
 #endif /* TARGET_HPPA */
 
 #ifdef TARGET_XTENSA
@@ -2326,12 +2354,12 @@ static abi_ulong create_elf_tables(abi_ulong p, int argc, int envc,
 }
 
 #if defined(HI_COMMPAGE)
-#define LO_COMMPAGE 0
+#define LO_COMMPAGE -1
 #elif defined(LO_COMMPAGE)
 #define HI_COMMPAGE 0
 #else
 #define HI_COMMPAGE 0
-#define LO_COMMPAGE 0
+#define LO_COMMPAGE -1
 #define init_guest_commpage() true
 #endif
 
@@ -2555,7 +2583,7 @@ static void pgb_static(const char *image_name, abi_ulong orig_loaddr,
         } else {
             offset = -(HI_COMMPAGE & -align);
         }
-    } else if (LO_COMMPAGE != 0) {
+    } else if (LO_COMMPAGE != -1) {
         loaddr = MIN(loaddr, LO_COMMPAGE & -align);
     }