[Xen-devel] [PATCH v3 07/16] x86/boot: create *.lnk files with linker script

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

Newer GCC (e.g. gcc version 5.1.1 20150618 (Red Hat 5.1.1-4) (GCC)) does
some code optimizations by creating data sections (e.g. jump addresses
for C switch/case are calculated using data in .rodata section). This
thing is not accepted by *.lnk build recipe which requires that only .text
section lives in output. Potentially we can inhibit this GCC behavior by
using special options, e.g. -fno-tree-switch-conversion. However, this
does not guarantee that in the future new similar optimizations or anything
which creates not accepted sections will not break our build recipes again.
Additionally, I do not mention that probably this is not good idea to just
disable random optimizations. So, take over full control on *.lnk linking
process by using linker script and merge all text and data sections into
one .text section.

Additionally, remove .got.plt section which is not used in our final code.

Signed-off-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
---
 xen/arch/x86/boot/build32.lds |   49 +++++++++++++++++++++++++++++++++++++++++
 xen/arch/x86/boot/build32.mk  |   10 ++++++---
 2 files changed, 56 insertions(+), 3 deletions(-)
 create mode 100644 xen/arch/x86/boot/build32.lds

diff --git a/xen/arch/x86/boot/build32.lds b/xen/arch/x86/boot/build32.lds
new file mode 100644
index 0000000..47db9c4
--- /dev/null
+++ b/xen/arch/x86/boot/build32.lds
@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2016 Oracle and/or its affiliates. All rights reserved.
+ *      Daniel Kiper <daniel.kiper@xxxxxxxxxx>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+ENTRY(_start)
+
+SECTIONS
+{
+  /* Merge code and data into one section. */
+  .text : {
+        *(.text)
+        *(.text.*)
+        *(.rodata)
+  }
+
+  /DISCARD/ : {
+        /*
+         * .got.plt section is used only by dynamic linker
+         * and our output is not supposed to be loaded by
+         * dynamic linker. Additionally, it just contains
+         * .PLT0 which is referenced from nowhere. So, we
+         * can safely drop .got.plt here.
+         *
+         * Ha! This should be really discarded here. However,
+         * .got.plt section contains _GLOBAL_OFFSET_TABLE_
+         * symbol too and it is used as a reference for relative
+         * addressing (and only for that thing). Hence, ld
+         * complains if we remove that section because it
+         * cannot find _GLOBAL_OFFSET_TABLE_. So, drop .got.plt
+         * section during conversion to plain binary format.
+         * Please check build32.mk for more details.
+         */
+        /* *(.got.plt) */
+  }
+}
diff --git a/xen/arch/x86/boot/build32.mk b/xen/arch/x86/boot/build32.mk
index 4a7d388..eb02b4b 100644
--- a/xen/arch/x86/boot/build32.mk
+++ b/xen/arch/x86/boot/build32.mk
@@ -12,20 +12,24 @@ CFLAGS := $(filter-out -flto,$(CFLAGS))
        (od -v -t x $< | tr -s ' ' | awk 'NR > 1 {print s} {s=$$0}' | \
        sed 's/ /,0x/g' | sed 's/,0x$$//' | sed 's/^[0-9]*,/ .long /') >$@
 
+#
+# Drop .got.plt during conversion to plain binary format.
+# Please check build32.lds for more details.
+#
 %.bin: %.lnk
-       $(OBJCOPY) -O binary $< $@
+       $(OBJCOPY) -O binary -R .got.plt $< $@
 
 %.lnk: %.o
        $(OBJDUMP) -h $< | sed -n '/[0-9]/{s,00*,0,g;p;}' |\
                while read idx name sz rest; do \
                        case "$$name" in \
-                       .data|.data.*|.rodata|.rodata.*|.bss|.bss.*) \
+                       .data|.data.*|.rodata.*|.bss|.bss.*) \
                                test $$sz != 0 || continue; \
                                echo "Error: non-empty $$name: 0x$$sz" >&2; \
                                exit $$(expr $$idx + 1);; \
                        esac; \
                done
-       $(LD) $(LDFLAGS_DIRECT) -N -Ttext 0 -o $@ $<
+       $(LD) $(LDFLAGS_DIRECT) -N -T build32.lds -o $@ $<
 
 %.o: %.c
        $(CC) $(CFLAGS) -c -fpic $< -o $@
-- 
1.7.10.4


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