klp-post-link.c

  1// SPDX-License-Identifier: GPL-2.0-or-later
  2/*
  3 * Read the intermediate KLP reloc/symbol representations created by klp diff
  4 * and convert them to the proper format required by livepatch.  This needs to
  5 * run last to avoid linker wreckage.  Linkers don't tend to handle the "two
  6 * rela sections for a single base section" case very well, nor do they like
  7 * SHN_LIVEPATCH.
  8 *
  9 * This is the final tool in the livepatch module generation pipeline:
 10 *
 11 *   kernel builds -> objtool klp diff -> module link -> objtool klp post-link
 12 */
 13
 14#include <fcntl.h>
 15#include <gelf.h>
 16#include <objtool/objtool.h>
 17#include <objtool/warn.h>
 18#include <objtool/klp.h>
 19#include <objtool/util.h>
 20#include <linux/livepatch_external.h>
 21
 22static int fix_klp_relocs(struct elf *elf)
 23{
 24	struct section *symtab, *klp_relocs;
 25
 26	klp_relocs = find_section_by_name(elf, KLP_RELOCS_SEC);
 27	if (!klp_relocs)
 28		return 0;
 29
 30	symtab = find_section_by_name(elf, ".symtab");
 31	if (!symtab) {
 32		ERROR("missing .symtab");
 33		return -1;
 34	}
 35
 36	for (int i = 0; i < sec_size(klp_relocs) / sizeof(struct klp_reloc); i++) {
 37		struct klp_reloc *klp_reloc;
 38		unsigned long klp_reloc_off;
 39		struct section *sec, *tmp, *klp_rsec;
 40		unsigned long offset;
 41		struct reloc *reloc;
 42		char sym_modname[64];
 43		char rsec_name[SEC_NAME_LEN];
 44		u64 addend;
 45		struct symbol *sym, *klp_sym;
 46
 47		klp_reloc_off = i * sizeof(*klp_reloc);
 48		klp_reloc = klp_relocs->data->d_buf + klp_reloc_off;
 49
 50		/*
 51		 * Read __klp_relocs[i]:
 52		 */
 53
 54		/* klp_reloc.sec_offset */
 55		reloc = find_reloc_by_dest(elf, klp_relocs,
 56					   klp_reloc_off + offsetof(struct klp_reloc, offset));
 57		if (!reloc) {
 58			ERROR("malformed " KLP_RELOCS_SEC " section");
 59			return -1;
 60		}
 61
 62		sec = reloc->sym->sec;
 63		offset = reloc_addend(reloc);
 64
 65		/* klp_reloc.sym */
 66		reloc = find_reloc_by_dest(elf, klp_relocs,
 67					   klp_reloc_off + offsetof(struct klp_reloc, sym));
 68		if (!reloc) {
 69			ERROR("malformed " KLP_RELOCS_SEC " section");
 70			return -1;
 71		}
 72
 73		klp_sym = reloc->sym;
 74		addend = reloc_addend(reloc);
 75
 76		/* symbol format: .klp.sym.modname.sym_name,sympos */
 77		if (sscanf(klp_sym->name + strlen(KLP_SYM_PREFIX), "%55[^.]", sym_modname) != 1)
 78			ERROR("can't find modname in klp symbol '%s'", klp_sym->name);
 79
 80		/*
 81		 * Create the KLP rela:
 82		 */
 83
 84		/* section format: .klp.rela.sec_objname.section_name */
 85		if (snprintf_check(rsec_name, SEC_NAME_LEN,
 86				   KLP_RELOC_SEC_PREFIX "%s.%s",
 87				   sym_modname, sec->name))
 88			return -1;
 89
 90		klp_rsec = find_section_by_name(elf, rsec_name);
 91		if (!klp_rsec) {
 92			klp_rsec = elf_create_section(elf, rsec_name, 0,
 93						      elf_rela_size(elf),
 94						      SHT_RELA, elf_addr_size(elf),
 95						      SHF_ALLOC | SHF_INFO_LINK | SHF_RELA_LIVEPATCH);
 96			if (!klp_rsec)
 97				return -1;
 98
 99			klp_rsec->sh.sh_link = symtab->idx;
100			klp_rsec->sh.sh_info = sec->idx;
101			klp_rsec->base = sec;
102		}
103
104		tmp = sec->rsec;
105		sec->rsec = klp_rsec;
106		if (!elf_create_reloc(elf, sec, offset, klp_sym, addend, klp_reloc->type))
107			return -1;
108		sec->rsec = tmp;
109
110		/*
111		 * Fix up the corresponding KLP symbol:
112		 */
113
114		klp_sym->sym.st_shndx = SHN_LIVEPATCH;
115		if (!gelf_update_sym(symtab->data, klp_sym->idx, &klp_sym->sym)) {
116			ERROR_ELF("gelf_update_sym");
117			return -1;
118		}
119
120		/*
121		 * Disable the original non-KLP reloc by converting it to R_*_NONE:
122		 */
123
124		reloc = find_reloc_by_dest(elf, sec, offset);
125		sym = reloc->sym;
126		sym->sym.st_shndx = SHN_LIVEPATCH;
127		set_reloc_type(elf, reloc, 0);
128		if (!gelf_update_sym(symtab->data, sym->idx, &sym->sym)) {
129			ERROR_ELF("gelf_update_sym");
130			return -1;
131		}
132	}
133
134	return 0;
135}
136
137/*
138 * This runs on the livepatch module after all other linking has been done.  It
139 * converts the intermediate __klp_relocs section into proper KLP relocs to be
140 * processed by livepatch.  This needs to run last to avoid linker wreckage.
141 * Linkers don't tend to handle the "two rela sections for a single base
142 * section" case very well, nor do they appreciate SHN_LIVEPATCH.
143 */
144int cmd_klp_post_link(int argc, const char **argv)
145{
146	struct elf *elf;
147
148	argc--;
149	argv++;
150
151	if (argc != 1) {
152		fprintf(stderr, "%d\n", argc);
153		fprintf(stderr, "usage: objtool link <file.ko>\n");
154		return -1;
155	}
156
157	elf = elf_open_read(argv[0], O_RDWR);
158	if (!elf)
159		return -1;
160
161	if (fix_klp_relocs(elf))
162		return -1;
163
164	if (elf_write(elf))
165		return -1;
166
167	return elf_close(elf);
168}