debug/elf: support relocations relative to sections with non-zero addresses

commit 72ec930fa70c20ce69b21bf32a7916c04c2e9c2f added basic support for
relocations, but assumed that the symbol value would be 0, likely because
.debug_info always has address == 0 in the ELF section headers.

CL 195679 added further support for relocations, but explicitly encoded
the original assumption that section addresses would be 0.

This change removes that assumption: all relocations will now be
properly computed based on the target symbol value even when that symbol
is a section with a non-zero address.

Typically, sections that are part of a LOAD program segment have
non-zero addresses. For example, .debug_ranges relocations could be
relative to .text, which usually has an address > 0.

Fixes #40879

Change-Id: Ib0a616bb8b05d6c96d179b03ca33a10946fc5d59
GitHub-Last-Rev: 4200de732641995f3a4958a13a5c78f65b7eae50
GitHub-Pull-Request: golang/go#41038
Reviewed-on: https://go-review.googlesource.com/c/go/+/250559
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

3 files changed, 28 insertions, 0 deletions

diff --git a/src/debug/elf/testdata/go-relocation-test-gcc930-ranges-no-rela-x86-64 b/src/debug/elf/testdata/go-relocation-test-gcc930-ranges-no-rela-x86-64
new file mode 100644
index 0000000000..c013f3e081
--- /dev/null
+++ b/src/debug/elf/testdata/go-relocation-test-gcc930-ranges-no-rela-x86-64
diff --git a/src/debug/elf/testdata/go-relocation-test-gcc930-ranges-with-rela-x86-64 b/src/debug/elf/testdata/go-relocation-test-gcc930-ranges-with-rela-x86-64
new file mode 100644
index 0000000000..51e03aa7b0
--- /dev/null
+++ b/src/debug/elf/testdata/go-relocation-test-gcc930-ranges-with-rela-x86-64
diff --git a/src/debug/elf/testdata/multiple-code-sections.c b/src/debug/elf/testdata/multiple-code-sections.c
new file mode 100644
index 0000000000..03b9d53ab9
--- /dev/null
+++ b/src/debug/elf/testdata/multiple-code-sections.c
@@ -0,0 +1,28 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Build with:
+// gcc -g multiple-code-sections.c -Wl,--emit-relocs -Wl,--discard-none -Wl,-zmax-page-size=1 -fno-asynchronous-unwind-tables -o go-relocation-test-gcc930-ranges-with-rela-x86-64
+// gcc -g multiple-code-sections.c -Wl,-zmax-page-size=1 -fno-asynchronous-unwind-tables -o go-relocation-test-gcc930-ranges-no-rela-x86-64
+// Strip with:
+// strip --only-keep-debug \
+//       --remove-section=.eh_frame \
+//       --remove-section=.eh_frame_hdr \
+//       --remove-section=.shstrtab \
+//       --remove-section=.strtab \
+//       --remove-section=.symtab \
+//       --remove-section=.note.gnu.build-id \
+//       --remove-section=.note.ABI-tag \
+//       --remove-section=.dynamic \
+//       --remove-section=.gnu.hash \
+//       --remove-section=.interp \
+//       --remove-section=.rodata
+__attribute__((section(".separate_section"))) // To get GCC to emit a DW_AT_ranges attribute for the CU.
+int func(void) {
+    return 0;
+}
+
+int main(int argc, char *argv[]) {
+    return 0;
+}