objtool: Add elf_create_reloc() and elf_init_reloc()

elf_create_rela_section() is quite limited in that it requires the
caller to know how many relocations need to be allocated up front.

In preparation for the objtool klp diff subcommand, allow an arbitrary
number of relocations to be created and initialized on demand after
section creation.

Acked-by: Petr Mladek <pmladek@suse.com>
Tested-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

tools/objtool/elf.c

@@ -22,6 +22,8 @@
 #include <objtool/warn.h>
 
 #define ALIGN_UP(x, align_to) (((x) + ((align_to)-1)) & ~((align_to)-1))
+#define ALIGN_UP_POW2(x) (1U << ((8 * sizeof(x)) - __builtin_clz((x) - 1U)))
+#define MAX(a, b) ((a) > (b) ? (a) : (b))
 
 static inline u32 str_hash(const char *str)
 {
@@ -899,10 +901,9 @@ elf_create_prefix_symbol(struct elf *elf, struct symbol *orig, size_t size)
 				 offset, size);
 }
 
-static struct reloc *elf_init_reloc(struct elf *elf, struct section *rsec,
+struct reloc *elf_init_reloc(struct elf *elf, struct section *rsec,
-				    unsigned int reloc_idx,
+			     unsigned int reloc_idx, unsigned long offset,
-				    unsigned long offset, struct symbol *sym,
+			     struct symbol *sym, s64 addend, unsigned int type)
-				    s64 addend, unsigned int type)
 {
 	struct reloc *reloc, empty = { 0 };
 
@@ -1004,12 +1005,16 @@ static int read_relocs(struct elf *elf)
 
 		rsec->base->rsec = rsec;
 
-		nr_reloc = 0;
+		/* nr_alloc_relocs=0: libelf owns d_buf */
+		rsec->nr_alloc_relocs = 0;
+
 		rsec->relocs = calloc(sec_num_entries(rsec), sizeof(*reloc));
 		if (!rsec->relocs) {
 			ERROR_GLIBC("calloc");
 			return -1;
 		}
+
+		nr_reloc = 0;
 		for (i = 0; i < sec_num_entries(rsec); i++) {
 			reloc = &rsec->relocs[i];
 
@@ -1258,8 +1263,116 @@ struct section *elf_create_section(struct elf *elf, const char *name,
 	return sec;
 }
 
+static int elf_alloc_reloc(struct elf *elf, struct section *rsec)
+{
+	struct reloc *old_relocs, *old_relocs_end, *new_relocs;
+	unsigned int nr_relocs_old = sec_num_entries(rsec);
+	unsigned int nr_relocs_new = nr_relocs_old + 1;
+	unsigned long nr_alloc;
+	struct symbol *sym;
+
+	if (!rsec->data) {
+		rsec->data = elf_newdata(elf_getscn(elf->elf, rsec->idx));
+		if (!rsec->data) {
+			ERROR_ELF("elf_newdata");
+			return -1;
+		}
+
+		rsec->data->d_align = 1;
+		rsec->data->d_type = ELF_T_RELA;
+		rsec->data->d_buf = NULL;
+	}
+
+	rsec->data->d_size = nr_relocs_new * elf_rela_size(elf);
+	rsec->sh.sh_size   = rsec->data->d_size;
+
+	nr_alloc = MAX(64, ALIGN_UP_POW2(nr_relocs_new));
+	if (nr_alloc <= rsec->nr_alloc_relocs)
+		return 0;
+
+	if (rsec->data->d_buf && !rsec->nr_alloc_relocs) {
+		void *orig_buf = rsec->data->d_buf;
+
+		/*
+		 * The original d_buf is owned by libelf so it can't be
+		 * realloced.
+		 */
+		rsec->data->d_buf = malloc(nr_alloc * elf_rela_size(elf));
+		if (!rsec->data->d_buf) {
+			ERROR_GLIBC("malloc");
+			return -1;
+		}
+		memcpy(rsec->data->d_buf, orig_buf,
+		       nr_relocs_old * elf_rela_size(elf));
+	} else {
+		rsec->data->d_buf = realloc(rsec->data->d_buf,
+					    nr_alloc * elf_rela_size(elf));
+		if (!rsec->data->d_buf) {
+			ERROR_GLIBC("realloc");
+			return -1;
+		}
+	}
+
+	rsec->nr_alloc_relocs = nr_alloc;
+
+	old_relocs = rsec->relocs;
+	new_relocs = calloc(nr_alloc, sizeof(struct reloc));
+	if (!new_relocs) {
+		ERROR_GLIBC("calloc");
+		return -1;
+	}
+
+	if (!old_relocs)
+		goto done;
+
+	/*
+	 * The struct reloc's address has changed.  Update all the symbols and
+	 * relocs which reference it.
+	 */
+
+	old_relocs_end = &old_relocs[nr_relocs_old];
+	for_each_sym(elf, sym) {
+		struct reloc *reloc;
+
+		reloc = sym->relocs;
+		if (!reloc)
+			continue;
+
+		if (reloc >= old_relocs && reloc < old_relocs_end)
+			sym->relocs = &new_relocs[reloc - old_relocs];
+
+		while (1) {
+			struct reloc *next_reloc = sym_next_reloc(reloc);
+
+			if (!next_reloc)
+				break;
+
+			if (next_reloc >= old_relocs && next_reloc < old_relocs_end)
+				set_sym_next_reloc(reloc, &new_relocs[next_reloc - old_relocs]);
+
+			reloc = next_reloc;
+		}
+	}
+
+	memcpy(new_relocs, old_relocs, nr_relocs_old * sizeof(struct reloc));
+
+	for (int i = 0; i < nr_relocs_old; i++) {
+		struct reloc *old = &old_relocs[i];
+		struct reloc *new = &new_relocs[i];
+		u32 key = reloc_hash(old);
+
+		elf_hash_del(reloc, &old->hash, key);
+		elf_hash_add(reloc, &new->hash, key);
+	}
+
+	free(old_relocs);
+done:
+	rsec->relocs = new_relocs;
+	return 0;
+}
+
 struct section *elf_create_rela_section(struct elf *elf, struct section *sec,
-					unsigned int reloc_nr)
+					unsigned int nr_relocs)
 {
 	struct section *rsec;
 	char *rsec_name;
@@ -1272,34 +1385,63 @@ struct section *elf_create_rela_section(struct elf *elf, struct section *sec,
 	strcpy(rsec_name, ".rela");
 	strcat(rsec_name, sec->name);
 
-	rsec = elf_create_section(elf, rsec_name, reloc_nr * elf_rela_size(elf),
+	rsec = elf_create_section(elf, rsec_name, nr_relocs * elf_rela_size(elf),
 				  elf_rela_size(elf), SHT_RELA, elf_addr_size(elf),
 				  SHF_INFO_LINK);
 	free(rsec_name);
 	if (!rsec)
 		return NULL;
 
-	rsec->sh.sh_link = find_section_by_name(elf, ".symtab")->idx;
+	if (nr_relocs) {
-	rsec->sh.sh_info = sec->idx;
-
-	if (reloc_nr) {
 		rsec->data->d_type = ELF_T_RELA;
-		rsec->relocs = calloc(sec_num_entries(rsec), sizeof(struct reloc));
+
+		rsec->nr_alloc_relocs = nr_relocs;
+		rsec->relocs = calloc(nr_relocs, sizeof(struct reloc));
 		if (!rsec->relocs) {
 			ERROR_GLIBC("calloc");
 			return NULL;
 		}
 	}
 
+	rsec->sh.sh_link = find_section_by_name(elf, ".symtab")->idx;
+	rsec->sh.sh_info = sec->idx;
+
 	sec->rsec = rsec;
 	rsec->base = sec;
 
 	return rsec;
 }
 
+struct reloc *elf_create_reloc(struct elf *elf, struct section *sec,
+			       unsigned long offset,
+			       struct symbol *sym, s64 addend,
+			       unsigned int type)
+{
+	struct section *rsec = sec->rsec;
+
+	if (!rsec) {
+		rsec = elf_create_rela_section(elf, sec, 0);
+		if (!rsec)
+			return NULL;
+	}
+
+	if (find_reloc_by_dest(elf, sec, offset)) {
+		ERROR_FUNC(sec, offset, "duplicate reloc");
+		return NULL;
+	}
+
+	if (elf_alloc_reloc(elf, rsec))
+		return NULL;
+
+	mark_sec_changed(elf, rsec, true);
+
+	return elf_init_reloc(elf, rsec, sec_num_entries(rsec) - 1, offset, sym,
+			      addend, type);
+}
+
 struct section *elf_create_section_pair(struct elf *elf, const char *name,
 					size_t entsize, unsigned int nr,
-					unsigned int reloc_nr)
+					unsigned int nr_relocs)
 {
 	struct section *sec;
 
@@ -1308,7 +1450,7 @@ struct section *elf_create_section_pair(struct elf *elf, const char *name,
 	if (!sec)
 		return NULL;
 
-	if (!elf_create_rela_section(elf, sec, reloc_nr))
+	if (!elf_create_rela_section(elf, sec, nr_relocs))
 		return NULL;
 
 	return sec;

tools/objtool/include/objtool/elf.h

@@ -47,6 +47,7 @@ struct section {
 	int idx;
 	bool _changed, text, rodata, noinstr, init, truncate;
 	struct reloc *relocs;
+	unsigned long nr_alloc_relocs;
 };
 
 struct symbol {
@@ -140,6 +141,14 @@ void *elf_add_data(struct elf *elf, struct section *sec, const void *data,
 
 unsigned int elf_add_string(struct elf *elf, struct section *strtab, const char *str);
 
+struct reloc *elf_create_reloc(struct elf *elf, struct section *sec,
+			       unsigned long offset, struct symbol *sym,
+			       s64 addend, unsigned int type);
+
+struct reloc *elf_init_reloc(struct elf *elf, struct section *rsec,
+			     unsigned int reloc_idx, unsigned long offset,
+			     struct symbol *sym, s64 addend, unsigned int type);
+
 struct reloc *elf_init_reloc_text_sym(struct elf *elf, struct section *sec,
 				      unsigned long offset,
 				      unsigned int reloc_idx,