linux-kernel - [PATCH 5.10 030/130] objtool: Add elf_create_undef

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Date:   Tue, 12 Jul 2022 20:37:56 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Borislav Petkov <bp@...e.de>, Ingo Molnar <mingo@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Ben Hutchings <ben@...adent.org.uk>
Subject: [PATCH 5.10 030/130] objtool: Add elf_create_undef_symbol()

From: Peter Zijlstra <peterz@...radead.org>

commit 2f2f7e47f0525cbaad5dd9675fd9d8aa8da12046 upstream.

Allow objtool to create undefined symbols; this allows creating
relocations to symbols not currently in the symbol table.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Borislav Petkov <bp@...e.de>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Reviewed-by: Miroslav Benes <mbenes@...e.cz>
Link: https://lkml.kernel.org/r/20210326151300.064743095@infradead.org
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 tools/objtool/elf.c |   60 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 tools/objtool/elf.h |    1 
 2 files changed, 61 insertions(+)

--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -766,6 +766,66 @@ static int elf_add_string(struct elf *el
 	return len;
 }
 
+struct symbol *elf_create_undef_symbol(struct elf *elf, const char *name)
+{
+	struct section *symtab;
+	struct symbol *sym;
+	Elf_Data *data;
+	Elf_Scn *s;
+
+	sym = malloc(sizeof(*sym));
+	if (!sym) {
+		perror("malloc");
+		return NULL;
+	}
+	memset(sym, 0, sizeof(*sym));
+
+	sym->name = strdup(name);
+
+	sym->sym.st_name = elf_add_string(elf, NULL, sym->name);
+	if (sym->sym.st_name == -1)
+		return NULL;
+
+	sym->sym.st_info = GELF_ST_INFO(STB_GLOBAL, STT_NOTYPE);
+	// st_other 0
+	// st_shndx 0
+	// st_value 0
+	// st_size 0
+
+	symtab = find_section_by_name(elf, ".symtab");
+	if (!symtab) {
+		WARN("can't find .symtab");
+		return NULL;
+	}
+
+	s = elf_getscn(elf->elf, symtab->idx);
+	if (!s) {
+		WARN_ELF("elf_getscn");
+		return NULL;
+	}
+
+	data = elf_newdata(s);
+	if (!data) {
+		WARN_ELF("elf_newdata");
+		return NULL;
+	}
+
+	data->d_buf = &sym->sym;
+	data->d_size = sizeof(sym->sym);
+	data->d_align = 1;
+
+	sym->idx = symtab->len / sizeof(sym->sym);
+
+	symtab->len += data->d_size;
+	symtab->changed = true;
+
+	sym->sec = find_section_by_index(elf, 0);
+
+	elf_add_symbol(elf, sym);
+
+	return sym;
+}
+
 struct section *elf_create_section(struct elf *elf, const char *name,
 				   unsigned int sh_flags, size_t entsize, int nr)
 {
--- a/tools/objtool/elf.h
+++ b/tools/objtool/elf.h
@@ -133,6 +133,7 @@ int elf_write_insn(struct elf *elf, stru
 		   unsigned long offset, unsigned int len,
 		   const char *insn);
 int elf_write_reloc(struct elf *elf, struct reloc *reloc);
+struct symbol *elf_create_undef_symbol(struct elf *elf, const char *name);
 int elf_write(struct elf *elf);
 void elf_close(struct elf *elf);