[src/trunk]: src Add a note about PIE for amd64 requiring clean objdir.

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/634df7e5bd28
branches:  trunk
changeset: 344757:634df7e5bd28
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Sat Apr 16 18:28:22 2016 +0000

description:
Add a note about PIE for amd64 requiring clean objdir.

diffstat:

 UPDATING |  21 ++++++++++++++++++++-
 1 files changed, 20 insertions(+), 1 deletions(-)

diffs (35 lines):

diff -r 4397b1ca9adf -r 634df7e5bd28 UPDATING
--- a/UPDATING  Sat Apr 16 16:57:06 2016 +0000
+++ b/UPDATING  Sat Apr 16 18:28:22 2016 +0000
@@ -1,4 +1,4 @@
-$NetBSD: UPDATING,v 1.267 2016/04/03 06:16:02 mrg Exp $
+$NetBSD: UPDATING,v 1.268 2016/04/16 18:28:22 riastradh Exp $
 
 This file (UPDATING) is intended to be a brief reference to recent
 changes that might cause problems in the build process, and a guide for
@@ -19,6 +19,25 @@
 Recent changes:
 ^^^^^^^^^^^^^^^
 
+20160410:
+
+       amd64 needs full "make cleandir" or deletion of objdir now that
+       PIE has been enabled for amd64.  PIE, or position-independent
+       executables, means all code, including executables and not just
+       shared libraries, is position-independent and hence able to be
+       relocated by ASLR, address space layout randomization.
+
+       The change was made in Makefile variables for compiler and
+       linker flags, for which make(1) does not record dependencies,
+       hence it is unable to detect that all .o files need rebuilding.
+
+       Partial rebuilds with some modified source files will likely
+       fail when linking executables, since the linker refuses to mix
+       position-independent code with position-dependent code in
+       position-independent executables:
+
+               .../x86_64--netbsd/bin/ld: foo.o: relocation R_X86_64_32 against `...' cannot be used when making a shared object; recompile with -fPIC
+
 20160401:
        Ports switching to GCC 5.3 will need a full "make cleandir".