objtool/klp: Add post-link subcommand to finalize livepatch modules

Livepatch needs some ELF magic which linkers don't like:

- Two relocation sections (.rela*, .klp.rela*) for the same text
section.

- Use of SHN_LIVEPATCH to mark livepatch symbols.

Unfortunately linkers tend to mangle such things. To work around that,
klp diff generates a linker-compliant intermediate binary which encodes
the relevant KLP section/reloc/symbol metadata.

After module linking, the .ko then needs to be converted to an actual
livepatch module. Introduce a new klp post-link subcommand to do so.

Acked-by: Petr Mladek <[e-mail]>
Tested-by: Joe Lawrence <[e-mail]>
Signed-off-by: Josh Poimboeuf <[e-mail]>

tools/objtool/Build

···
 objtool-y += objtool.o
 
 objtool-$(BUILD_ORC) += orc_gen.o orc_dump.o
-objtool-$(BUILD_KLP) += builtin-klp.o klp-diff.o
+objtool-$(BUILD_KLP) += builtin-klp.o klp-diff.o klp-post-link.o
 
 objtool-y += libstring.o
 objtool-y += libctype.o

tools/objtool/builtin-klp.c

···
 
 static struct subcmd subcmds[] = {
 	{ "diff",		"Generate binary diff of two object files",		cmd_klp_diff, },
+	{ "post-link",		"Finalize klp symbols/relocs after module linking",	cmd_klp_post_link, },
 };
 
 static void cmd_klp_usage(void)

tools/objtool/include/objtool/klp.h

···
 #ifndef _OBJTOOL_KLP_H
 #define _OBJTOOL_KLP_H
 
+#define SHF_RELA_LIVEPATCH	0x00100000
+#define SHN_LIVEPATCH		0xff20
+
 /*
  * __klp_objects and __klp_funcs are created by klp diff and used by the patch
  * module init code to build the klp_patch, klp_object and klp_func structs
···
 };
 
 int cmd_klp_diff(int argc, const char **argv);
+int cmd_klp_post_link(int argc, const char **argv);
 
 #endif /* _OBJTOOL_KLP_H */

tools/objtool/klp-post-link.c

···
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Read the intermediate KLP reloc/symbol representations created by klp diff
+ * and convert them to the proper format required by livepatch.  This needs to
+ * run last to avoid linker wreckage.  Linkers don't tend to handle the "two
+ * rela sections for a single base section" case very well, nor do they like
+ * SHN_LIVEPATCH.
+ *
+ * This is the final tool in the livepatch module generation pipeline:
+ *
+ *   kernel builds -> objtool klp diff -> module link -> objtool klp post-link
+ */
+
+#include <fcntl.h>
+#include <gelf.h>
+#include <objtool/objtool.h>
+#include <objtool/warn.h>
+#include <objtool/klp.h>
+#include <objtool/util.h>
+#include <linux/livepatch_external.h>
+
+static int fix_klp_relocs(struct elf *elf)
+{
+	struct section *symtab, *klp_relocs;
+
+	klp_relocs = find_section_by_name(elf, KLP_RELOCS_SEC);
+	if (!klp_relocs)
+		return 0;
+
+	symtab = find_section_by_name(elf, ".symtab");
+	if (!symtab) {
+		ERROR("missing .symtab");
+		return -1;
+	}
+
+	for (int i = 0; i < sec_size(klp_relocs) / sizeof(struct klp_reloc); i++) {
+		struct klp_reloc *klp_reloc;
+		unsigned long klp_reloc_off;
+		struct section *sec, *tmp, *klp_rsec;
+		unsigned long offset;
+		struct reloc *reloc;
+		char sym_modname[64];
+		char rsec_name[SEC_NAME_LEN];
+		u64 addend;
+		struct symbol *sym, *klp_sym;
+
+		klp_reloc_off = i * sizeof(*klp_reloc);
+		klp_reloc = klp_relocs->data->d_buf + klp_reloc_off;
+
+		/*
+		 * Read __klp_relocs[i]:
+		 */
+
+		/* klp_reloc.sec_offset */
+		reloc = find_reloc_by_dest(elf, klp_relocs,
+					   klp_reloc_off + offsetof(struct klp_reloc, offset));
+		if (!reloc) {
+			ERROR("malformed " KLP_RELOCS_SEC " section");
+			return -1;
+		}
+
+		sec = reloc->sym->sec;
+		offset = reloc_addend(reloc);
+
+		/* klp_reloc.sym */
+		reloc = find_reloc_by_dest(elf, klp_relocs,
+					   klp_reloc_off + offsetof(struct klp_reloc, sym));
+		if (!reloc) {
+			ERROR("malformed " KLP_RELOCS_SEC " section");
+			return -1;
+		}
+
+		klp_sym = reloc->sym;
+		addend = reloc_addend(reloc);
+
+		/* symbol format: .klp.sym.modname.sym_name,sympos */
+		if (sscanf(klp_sym->name + strlen(KLP_SYM_PREFIX), "%55[^.]", sym_modname) != 1)
+			ERROR("can't find modname in klp symbol '%s'", klp_sym->name);
+
+		/*
+		 * Create the KLP rela:
+		 */
+
+		/* section format: .klp.rela.sec_objname.section_name */
+		if (snprintf_check(rsec_name, SEC_NAME_LEN,
+				   KLP_RELOC_SEC_PREFIX "%s.%s",
+				   sym_modname, sec->name))
+			return -1;
+
+		klp_rsec = find_section_by_name(elf, rsec_name);
+		if (!klp_rsec) {
+			klp_rsec = elf_create_section(elf, rsec_name, 0,
+						      elf_rela_size(elf),
+						      SHT_RELA, elf_addr_size(elf),
+						      SHF_ALLOC | SHF_INFO_LINK | SHF_RELA_LIVEPATCH);
+			if (!klp_rsec)
+				return -1;
+
+			klp_rsec->sh.sh_link = symtab->idx;
+			klp_rsec->sh.sh_info = sec->idx;
+			klp_rsec->base = sec;
+		}
+
+		tmp = sec->rsec;
+		sec->rsec = klp_rsec;
+		if (!elf_create_reloc(elf, sec, offset, klp_sym, addend, klp_reloc->type))
+			return -1;
+		sec->rsec = tmp;
+
+		/*
+		 * Fix up the corresponding KLP symbol:
+		 */
+
+		klp_sym->sym.st_shndx = SHN_LIVEPATCH;
+		if (!gelf_update_sym(symtab->data, klp_sym->idx, &klp_sym->sym)) {
+			ERROR_ELF("gelf_update_sym");
+			return -1;
+		}
+
+		/*
+		 * Disable the original non-KLP reloc by converting it to R_*_NONE:
+		 */
+
+		reloc = find_reloc_by_dest(elf, sec, offset);
+		sym = reloc->sym;
+		sym->sym.st_shndx = SHN_LIVEPATCH;
+		set_reloc_type(elf, reloc, 0);
+		if (!gelf_update_sym(symtab->data, sym->idx, &sym->sym)) {
+			ERROR_ELF("gelf_update_sym");
+			return -1;
+		}
+	}
+
+	return 0;
+}
+
+/*
+ * This runs on the livepatch module after all other linking has been done.  It
+ * converts the intermediate __klp_relocs section into proper KLP relocs to be
+ * processed by livepatch.  This needs to run last to avoid linker wreckage.
+ * Linkers don't tend to handle the "two rela sections for a single base
+ * section" case very well, nor do they appreciate SHN_LIVEPATCH.
+ */
+int cmd_klp_post_link(int argc, const char **argv)
+{
+	struct elf *elf;
+
+	argc--;
+	argv++;
+
+	if (argc != 1) {
+		fprintf(stderr, "%d\n", argc);
+		fprintf(stderr, "usage: objtool link <file.ko>\n");
+		return -1;
+	}
+
+	elf = elf_open_read(argv[0], O_RDWR);
+	if (!elf)
+		return -1;
+
+	if (fix_klp_relocs(elf))
+		return -1;
+
+	if (elf_write(elf))
+		return -1;
+
+	return elf_close(elf);
+}