Don't set external __sysinfo in runtime

This removes the assumption in the runtime that a __vsyscall references
to a virtual syscall handler must be set by the runtime in an external
libc.

To ensure that musl can still initialise, there is a reference made to
the vsyscall handler in the stack of the root task if one exists
(otherwise the stack references a dummy).

Signed-off-by: Curtis Millar <curtis@curtism.me>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index b9bf3bc..3296e25 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -95,6 +95,7 @@
         src/init.c
         src/memset.c
         src/memcpy.c
+        src/vsyscall.c
 )
 if(("${KernelSel4Arch}" STREQUAL "aarch32") OR ("${KernelSel4Arch}" STREQUAL "arm_hyp"))
     list(

src/env.c

diff --git a/src/env.c b/src/env.c
index 16124fb..ed627f8 100644
--- a/src/env.c
+++ b/src/env.c
@@ -24,9 +24,6 @@
 #define MIN_ALIGN_BYTES 16
 #define MIN_ALIGNED __attribute__((aligned (MIN_ALIGN_BYTES)))
 
-// Global vsyscall handler (defined in musllibc)
-extern sel4runtime_size_t __sysinfo;
-
 // Static TLS for initial thread.
 static char static_tls[CONFIG_SEL4RUNTIME_STATIC_TLS] MIN_ALIGNED = {};
 
@@ -304,10 +301,6 @@
             env.program_header.headers = auxv[i].a_un.a_ptr;
             break;
         }
-        case AT_SYSINFO: {
-            __sysinfo = (sel4runtime_size_t)(auxv[i].a_un.a_ptr);
-            break;
-        }
         case AT_SEL4_BOOT_INFO: {
             seL4_BootInfo *bootinfo = auxv[i].a_un.a_ptr;
             if (bootinfo == SEL4RUNTIME_NULL) {

src/vsyscall.c

diff --git a/src/vsyscall.c b/src/vsyscall.c
new file mode 100644
index 0000000..de07ad9
--- /dev/null
+++ b/src/vsyscall.c
@@ -0,0 +1,19 @@
+/*
+ * Copyright 2020, Data61
+ * Commonwealth Scientific and Industrial Research Organisation (CSIRO)
+ * ABN 41 687 119 230.
+ *
+ * This software may be distributed and modified according to the terms of
+ * the BSD 2-Clause license. Note that NO WARRANTY is provided.
+ * See "LICENSE_BSD2.txt" for details.
+ *
+ * @TAG(DATA61_BSD)
+ */
+
+/* If no vsyscall implementation is provided, this dummy one is used. */
+long __sel4runtime_vsyscall(long sysnum, ...)
+{
+    return -1;
+}
+
+long __attribute__((weak, alias("__sel4runtime_vsyscall"))) sel4_vsyscall(long sysnum, ...);