From ef9133af166ca531ee9818d01fdeb58e56eaef37 Mon Sep 17 00:00:00 2001
From: Michael Forney <mforney@mforney.org>
Date: Sun, 10 Oct 2021 13:10:57 -0700
Subject: Use Elf64_Rela for relocations.

The x86_64 psABI[0] says that

> The AMD64 LP64 ABI architecture uses only Elf64_Rela relocation
> entries with explicit addends. The r_addend member serves as the
> relocation addend.

[0] https://raw.githubusercontent.com/wiki/hjl-tools/x86-psABI/x86-64-psABI-draft.pdf
---
 main.c   | 21 ++++++++++-----------
 minias.h |  1 +
 2 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/main.c b/main.c
index 147aede..227a3d2 100644
--- a/main.c
+++ b/main.c
@@ -136,16 +136,16 @@ static void initsections(void) {
   text->hdr.sh_addralign = 4;
 
   textrel = newsection();
-  textrel->hdr.sh_type = SHT_REL;
+  textrel->hdr.sh_type = SHT_RELA;
   textrel->hdr.sh_info = text->idx;
   textrel->hdr.sh_link = symtab->idx;
-  textrel->hdr.sh_entsize = sizeof(Elf64_Rel);
+  textrel->hdr.sh_entsize = sizeof(Elf64_Rela);
 
   datarel = newsection();
-  datarel->hdr.sh_type = SHT_REL;
+  datarel->hdr.sh_type = SHT_RELA;
   datarel->hdr.sh_info = data->idx;
   datarel->hdr.sh_link = symtab->idx;
-  datarel->hdr.sh_entsize = sizeof(Elf64_Rel);
+  datarel->hdr.sh_entsize = sizeof(Elf64_Rela);
 }
 
 Relocation *newreloc() {
@@ -415,6 +415,8 @@ static void assemblereloc(const char *l, int64_t c, int nbytes, int type) {
     reloc->section = cursection;
     reloc->sym = sym;
     reloc->offset = cursection->hdr.sh_size;
+    reloc->addend = c;
+    c = 0;
   }
   assembleconstant(c, nbytes);
 }
@@ -1035,9 +1037,7 @@ static void resolvereloc(Relocation *reloc) {
   switch (reloc->type) {
   case R_X86_64_32: {
     rdata = &reloc->section->data[reloc->offset];
-    addend = (int32_t)rdata[0] | (int32_t)(rdata[1] << 8) |
-             (int32_t)(rdata[2] << 16) | (int32_t)(rdata[3] << 24);
-    value = sym->offset - reloc->offset + addend;
+    value = sym->offset - reloc->offset + reloc->addend;
     rdata[0] = ((uint32_t)value & 0xff);
     rdata[1] = ((uint32_t)value & 0xff00) >> 8;
     rdata[2] = ((uint32_t)value & 0xff0000) >> 16;
@@ -1046,9 +1046,7 @@ static void resolvereloc(Relocation *reloc) {
   }
   case R_X86_64_PC32: {
     rdata = &reloc->section->data[reloc->offset];
-    addend = (int32_t)rdata[0] | (int32_t)(rdata[1] << 8) |
-             (int32_t)(rdata[2] << 16) | (int32_t)(rdata[3] << 24);
-    value = sym->offset - reloc->offset + addend;
+    value = sym->offset - reloc->offset + reloc->addend;
     rdata[0] = ((uint32_t)value & 0xff);
     rdata[1] = ((uint32_t)value & 0xff00) >> 8;
     rdata[2] = ((uint32_t)value & 0xff0000) >> 16;
@@ -1063,7 +1061,7 @@ static void resolvereloc(Relocation *reloc) {
 static void appendreloc(Relocation *reloc) {
   Symbol *sym;
   Section *relsection;
-  Elf64_Rel elfrel;
+  Elf64_Rela elfrel;
 
   memset(&elfrel, 0, sizeof(elfrel));
 
@@ -1080,6 +1078,7 @@ static void appendreloc(Relocation *reloc) {
   case R_X86_64_32:
     elfrel.r_info = ELF64_R_INFO(sym->idx, reloc->type);
     elfrel.r_offset = reloc->offset;
+    elfrel.r_addend = reloc->addend;
     break;
   default:
     unreachable();
diff --git a/minias.h b/minias.h
index abf1f06..750f481 100644
--- a/minias.h
+++ b/minias.h
@@ -35,6 +35,7 @@ typedef struct {
   Symbol *sym;
   int type;
   int64_t offset;
+  int64_t addend;
 } Relocation;
 
 typedef enum {
-- 
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