multitude of mld fixes, add entrypoint

lib/runtime.asm

@@ -0,0 +1,29 @@
+#
+# MIPS32r6 ASSEMBLY RUNTIME
+# - sets up the stack
+# - calls main
+# - exits
+#
+
+.extern main
+
+.stack
+.align 2
+
+.space 4096
+__mips_stack:
+
+.text
+.align 2
+
+_start:
+	# setup stack
+	la $sp, __mips_stack
+
+	# call main
+	jal main
+
+	# exit
+	move $a0, $v0
+	li $v0, 1
+	syscall

mld/Makefile

@@ -4,4 +4,6 @@ SRC=.
 BIN=../bin/mld
 OUT=mld
 
+CFLAGS += -DPREFIX=$(PREFIX)
+
 include ../makefile.mk

mld/link.c

@@ -16,8 +16,11 @@
 
 static int load_objects(struct linker *linker)
 {
+	int max_entries = linker->args->in_count + 1;
+	// 1 needed for the runtime
+
 	linker->objects = malloc(sizeof(struct object) *
-			  linker->args->in_count);
+			  max_entries);
 	linker->obj_len = 0;
 
 	if (linker->objects == NULL) {
@@ -46,6 +49,19 @@ static int load_objects(struct linker *linker)
 skip_obj:
 	}
 
+	if (linker->args->freestanding == false) {
+		#define _STR(x) _STR2(x)
+		#define _STR2(x) #x
+
+		char *path = _STR(PREFIX) "/lib/mips/runtime.o";
+		struct object *obj = &linker->objects[linker->obj_len++];
+		if (object_load(obj, path))
+			return M_ERROR;
+
+		#undef _STR
+		#undef _STR2
+	}
+
 	return M_SUCCESS;
 }
 
@@ -161,7 +177,7 @@ static int relocate_symbol(struct linker *linker, struct object *obj,
 	struct segment *sec = NULL;
 	for (size_t i = 0; i < obj->phdr_len; i++) {
 		Elf32_Phdr *temp = &obj->phdr[i];
-		if (shdr->sh_offset == temp->p_offset) {
+		if (PHDR_SHDR_MATCH(temp, shdr)) {
 			sec = &obj->segments[i];
 			break;
 		}
@@ -386,30 +402,22 @@ static int relocate_instruction_rela(struct linker *linker,
 		return M_ERROR;
 	}
 	Elf32_Sym *sym = &symtab->syms[idx];
-	const char *symname = symtab->strtab->data + B32(sym->st_name);
 
-
-	/// get the section header that the symbol is related to
-	Elf32_Shdr *shdr = NULL;
-	if (B16(sym->st_shndx) >= seg->obj->shdr_len) {
-		ERROR("shndx index [%d] out of bounds", B16(sym->st_shndx));
+	char const *sym_name = symtab->strtab->data + B32(sym->st_name);
+	if (B32(sym->st_name) >= symtab->strtab->len) {
+		ERROR("relocation symbol name out of bounds");
+		return M_ERROR;
+	}
+
+	// get the new sym for the new vaddr
+	Elf32_Sym *new_sym = NULL;
+	if (symtab_get(&linker->symtab, &new_sym, sym_name)) {
+		ERROR("relocation symbol not found");
 		return M_ERROR;
 	}
-	shdr = &seg->obj->shdr[B16(sym->st_shndx)];
 
 	/// get the segment that the symbol is in
-	struct segment_table_entry *ent;
-	const char *segname = seg->obj->shstrtab->data + B32(shdr->sh_name);
-	if (B32(shdr->sh_name) >= seg->obj->shstrtab->len) {
-		ERROR("relocation segment name out of bounds");
-		return M_ERROR;
-	}
-	if (segtab_get(&linker->segments, &ent, segname)) {
-		ERROR("could not locate segment for relocation");
-		return M_ERROR;
-	}
-
-	uint32_t sym_vaddr = B32(sym->st_value) + ent->vaddr;
+	uint32_t sym_vaddr = B32(new_sym->st_value);
 	uint32_t *ins_raw = (uint32_t *) &seg->bytes[off];
 
 	union mips_instruction_data ins;
@@ -462,7 +470,7 @@ static int relocate_instruction_rela(struct linker *linker,
 	*ins_raw = B32(ins.raw);
 
 	if (warn)
-		WARNING("truncating relocation for symbol '%s'", symname);
+		WARNING("truncating relocation for symbol '%s'", sym_name);
 
 	return M_SUCCESS;
 
@@ -647,6 +655,15 @@ static int link_executable(struct linker *linker)
 	ehdr->e_shnum = B16(linker->shdr_len);
 	ehdr->e_shstrndx = B16(linker->shstrtab_shidx);
 
+	Elf32_Sym *entry = NULL;
+	if (symtab_get(&linker->symtab, &entry, "_start")) {
+		ERROR("undefined symbol _start");
+		return M_ERROR;
+	}
+
+	// BE to BE, no endiness conversion needed
+	ehdr->e_entry = entry->st_value;
+
 	update_offsets(linker);
 
 	if (write_file(linker))

mld/link.h

@@ -26,6 +26,11 @@
 #define ADDR_CHK(lnk_f, seg_f, max) \
 	((lnk_f) > max - (seg_f) || (lnk_f) + (seg_f) > max)
 
+// checks if a phdr and shdr are matches
+#define PHDR_SHDR_MATCH(phdr, shdr) (              \
+	((phdr)->p_offset == (shdr)->sh_offset) && \
+	((phdr)->p_filesz == (shdr)->sh_size))     \
+
 // start addresses for each tyoe of segment
 #define TEXT_VADDR_MIN 0x00400000
 #define DATA_VADDR_MIN 0x10000000
@@ -119,6 +124,8 @@ struct segment {
 
 	// object im related to
 	struct object *obj;
+	// segment table entry im related to
+	struct segment_table_entry *ent;
 
 	// relocation table
 	struct relocation_table reltab;
@@ -252,6 +259,7 @@ struct linker_arguments {
 	char **in_files;
 	int in_count;
 	char *out_file;
+	bool freestanding;
 };
 
 /* link object files */

mld/main.c

@@ -10,6 +10,7 @@ void help(void) {
 	printf("options:\n");
 	printf("\t-h\t\tprints this help message\n");
 	printf("\t-o <output>\tselect a output file destination\n");
+	printf("\t-f\t\tmake this binary freestanding (no runtime)\n");
 }
 
 int main(int argc, char **argv) {
@@ -18,11 +19,12 @@ int main(int argc, char **argv) {
 		.in_files = NULL,
 		.in_count = 0,
 		.out_file = "a.out",
+		.freestanding = false,
 	};
 
 	int c;
 
-	while ((c = getopt(argc, argv, "ho:")) != 1) {
+	while ((c = getopt(argc, argv, "ho:f")) != 1) {
 		switch(c) {
 		case 'h':
 			help();
@@ -30,6 +32,9 @@ int main(int argc, char **argv) {
 		case 'o':
 			args.out_file = optarg;
 			break;
+		case 'f':
+			args.freestanding = true;
+			break;
 		case '?':
 			return M_ERROR;
 		default:

mld/seg.c

@@ -31,10 +31,9 @@ static int load_shdr(struct object *obj, struct segment *seg, size_t index)
 		Elf32_Shdr *hdr = &obj->shdr[i];
 
 		// find shdr that matches  the offset in phdr
-		if (seg->phdr->p_offset != hdr->sh_offset)
+		if (!PHDR_SHDR_MATCH(seg->phdr, hdr))
 			continue;
 
-
 		// get name
 		uint32_t name = B32(hdr->sh_name);
 		if (name >= obj->shstrtab->len) {
@@ -43,11 +42,6 @@ static int load_shdr(struct object *obj, struct segment *seg, size_t index)
 		}
 		seg->name = &obj->shstrtab->data[name];
 
-		if (seg->phdr->p_filesz != hdr->sh_size) {
-			ERROR("segment phdr and shdr file sizes to not match");
-			return M_ERROR;
-		}
-
 		// map bytes
 		uint32_t len = B32(hdr->sh_size);
 		uint32_t off = B32(hdr->sh_offset);
@@ -154,6 +148,7 @@ int segment_load(struct object *obj, struct segment *seg, size_t index)
 	seg->align = B32(seg->phdr->p_align);
 
 	seg->obj = obj;
+	seg->ent = NULL;
 
 	return M_SUCCESS;
 }

mld/segtab.c

@@ -136,6 +136,8 @@ int segtab_ent_push(struct segment_table_entry *ent, struct segment *seg)
 	}
 
 	ent->parts[ent->len++] = seg;
+	seg->ent = ent;
+
 	return M_SUCCESS;
 }