Support PIE (Position Independent Executables)

Closes #4503
Revives #3960
Merges branch 'pie' into master

1 files changed, 26 insertions, 26 deletions

diff --git a/lib/std/dynamic_library.zig b/lib/std/dynamic_library.zig
index aca6cf958c..0de4bed84d 100644
--- a/lib/std/dynamic_library.zig
+++ b/lib/std/dynamic_library.zig
@@ -59,48 +59,48 @@ const RDebug = extern struct {
     r_ldbase: usize,
 };
 
-fn elf_get_va_offset(phdrs: []elf.Phdr) !usize {
-    for (phdrs) |*phdr| {
-        if (phdr.p_type == elf.PT_LOAD) {
-            return @ptrToInt(phdr) - phdr.p_vaddr;
-        }
+// TODO: This should be weak (#1917)
+extern var _DYNAMIC: [128]elf.Dyn;
+
+comptime {
+    if (std.Target.current.os.tag == .linux) {
+        asm (
+            \\ .weak _DYNAMIC
+            \\ .hidden _DYNAMIC
+        );
     }
-    return error.InvalidExe;
 }
 
 pub fn linkmap_iterator(phdrs: []elf.Phdr) !LinkMap.Iterator {
-    const va_offset = try elf_get_va_offset(phdrs);
-
-    const dyn_table = init: {
-        for (phdrs) |*phdr| {
-            if (phdr.p_type == elf.PT_DYNAMIC) {
-                const ptr = @intToPtr([*]elf.Dyn, va_offset + phdr.p_vaddr);
-                break :init ptr[0 .. phdr.p_memsz / @sizeOf(elf.Dyn)];
-            }
-        }
+    if (@ptrToInt(&_DYNAMIC[0]) == 0) {
         // No PT_DYNAMIC means this is either a statically-linked program or a
         // badly corrupted one
         return LinkMap.Iterator{ .current = null };
-    };
+    }
 
     const link_map_ptr = init: {
-        for (dyn_table) |*dyn| {
-            switch (dyn.d_tag) {
+        var i: usize = 0;
+        while (_DYNAMIC[i].d_tag != elf.DT_NULL) : (i += 1) {
+            switch (_DYNAMIC[i].d_tag) {
                 elf.DT_DEBUG => {
-                    const r_debug = @intToPtr(*RDebug, dyn.d_val);
-                    if (r_debug.r_version != 1) return error.InvalidExe;
-                    break :init r_debug.r_map;
+                    const ptr = @intToPtr(?*RDebug, _DYNAMIC[i].d_val);
+                    if (ptr) |r_debug| {
+                        if (r_debug.r_version != 1) return error.InvalidExe;
+                        break :init r_debug.r_map;
+                    }
                 },
                 elf.DT_PLTGOT => {
-                    const got_table = @intToPtr([*]usize, dyn.d_val);
-                    // The address to the link_map structure is stored in the
-                    // second slot
-                    break :init @intToPtr(?*LinkMap, got_table[1]);
+                    const ptr = @intToPtr(?[*]usize, _DYNAMIC[i].d_val);
+                    if (ptr) |got_table| {
+                        // The address to the link_map structure is stored in
+                        // the second slot
+                        break :init @intToPtr(?*LinkMap, got_table[1]);
+                    }
                 },
                 else => {},
             }
         }
-        return error.InvalidExe;
+        return LinkMap.Iterator{ .current = null };
     };
 
     return LinkMap.Iterator{ .current = link_map_ptr };