tools/objtool/special.c

0001 // SPDX-License-Identifier: GPL-2.0-or-later
0002 /*
0003  * Copyright (C) 2015 Josh Poimboeuf <jpoimboe@redhat.com>
0004  */
0005
0006 /*
0007  * This file reads all the special sections which have alternate instructions
0008  * which can be patched in or redirected to at runtime.
0009  */
0010
0011 #include <stdlib.h>
0012 #include <string.h>
0013
0014 #include <arch/special.h>
0015 #include <objtool/builtin.h>
0016 #include <objtool/special.h>
0017 #include <objtool/warn.h>
0018 #include <objtool/endianness.h>
0019
0020 struct special_entry {
0021     const char *sec;
0022     bool group, jump_or_nop;
0023     unsigned char size, orig, new;
0024     unsigned char orig_len, new_len; /* group only */
0025     unsigned char feature; /* ALTERNATIVE macro CPU feature */
0026     unsigned char key; /* jump_label key */
0027 };
0028
0029 struct special_entry entries[] = {
0030     {
0031         .sec = ".altinstructions",
0032         .group = true,
0033         .size = ALT_ENTRY_SIZE,
0034         .orig = ALT_ORIG_OFFSET,
0035         .orig_len = ALT_ORIG_LEN_OFFSET,
0036         .new = ALT_NEW_OFFSET,
0037         .new_len = ALT_NEW_LEN_OFFSET,
0038         .feature = ALT_FEATURE_OFFSET,
0039     },
0040     {
0041         .sec = "__jump_table",
0042         .jump_or_nop = true,
0043         .size = JUMP_ENTRY_SIZE,
0044         .orig = JUMP_ORIG_OFFSET,
0045         .new = JUMP_NEW_OFFSET,
0046         .key = JUMP_KEY_OFFSET,
0047     },
0048     {
0049         .sec = "__ex_table",
0050         .size = EX_ENTRY_SIZE,
0051         .orig = EX_ORIG_OFFSET,
0052         .new = EX_NEW_OFFSET,
0053     },
0054     {},
0055 };
0056
0057 void __weak arch_handle_alternative(unsigned short feature, struct special_alt *alt)
0058 {
0059 }
0060
0061 static void reloc_to_sec_off(struct reloc *reloc, struct section **sec,
0062                  unsigned long *off)
0063 {
0064     *sec = reloc->sym->sec;
0065     *off = reloc->sym->offset + reloc->addend;
0066 }
0067
0068 static int get_alt_entry(struct elf *elf, struct special_entry *entry,
0069              struct section *sec, int idx,
0070              struct special_alt *alt)
0071 {
0072     struct reloc *orig_reloc, *new_reloc;
0073     unsigned long offset;
0074
0075     offset = idx * entry->size;
0076
0077     alt->group = entry->group;
0078     alt->jump_or_nop = entry->jump_or_nop;
0079
0080     if (alt->group) {
0081         alt->orig_len = *(unsigned char *)(sec->data->d_buf + offset +
0082                            entry->orig_len);
0083         alt->new_len = *(unsigned char *)(sec->data->d_buf + offset +
0084                           entry->new_len);
0085     }
0086
0087     if (entry->feature) {
0088         unsigned short feature;
0089
0090         feature = bswap_if_needed(*(unsigned short *)(sec->data->d_buf +
0091                                   offset +
0092                                   entry->feature));
0093         arch_handle_alternative(feature, alt);
0094     }
0095
0096     orig_reloc = find_reloc_by_dest(elf, sec, offset + entry->orig);
0097     if (!orig_reloc) {
0098         WARN_FUNC("can't find orig reloc", sec, offset + entry->orig);
0099         return -1;
0100     }
0101
0102     reloc_to_sec_off(orig_reloc, &alt->orig_sec, &alt->orig_off);
0103
0104     if (!entry->group || alt->new_len) {
0105         new_reloc = find_reloc_by_dest(elf, sec, offset + entry->new);
0106         if (!new_reloc) {
0107             WARN_FUNC("can't find new reloc",
0108                   sec, offset + entry->new);
0109             return -1;
0110         }
0111
0112         reloc_to_sec_off(new_reloc, &alt->new_sec, &alt->new_off);
0113
0114         /* _ASM_EXTABLE_EX hack */
0115         if (alt->new_off >= 0x7ffffff0)
0116             alt->new_off -= 0x7ffffff0;
0117     }
0118
0119     if (entry->key) {
0120         struct reloc *key_reloc;
0121
0122         key_reloc = find_reloc_by_dest(elf, sec, offset + entry->key);
0123         if (!key_reloc) {
0124             WARN_FUNC("can't find key reloc",
0125                   sec, offset + entry->key);
0126             return -1;
0127         }
0128         alt->key_addend = key_reloc->addend;
0129     }
0130
0131     return 0;
0132 }
0133
0134 /*
0135  * Read all the special sections and create a list of special_alt structs which
0136  * describe all the alternate instructions which can be patched in or
0137  * redirected to at runtime.
0138  */
0139 int special_get_alts(struct elf *elf, struct list_head *alts)
0140 {
0141     struct special_entry *entry;
0142     struct section *sec;
0143     unsigned int nr_entries;
0144     struct special_alt *alt;
0145     int idx, ret;
0146
0147     INIT_LIST_HEAD(alts);
0148
0149     for (entry = entries; entry->sec; entry++) {
0150         sec = find_section_by_name(elf, entry->sec);
0151         if (!sec)
0152             continue;
0153
0154         if (sec->sh.sh_size % entry->size != 0) {
0155             WARN("%s size not a multiple of %d",
0156                  sec->name, entry->size);
0157             return -1;
0158         }
0159
0160         nr_entries = sec->sh.sh_size / entry->size;
0161
0162         for (idx = 0; idx < nr_entries; idx++) {
0163             alt = malloc(sizeof(*alt));
0164             if (!alt) {
0165                 WARN("malloc failed");
0166                 return -1;
0167             }
0168             memset(alt, 0, sizeof(*alt));
0169
0170             ret = get_alt_entry(elf, entry, sec, idx, alt);
0171             if (ret > 0)
0172                 continue;
0173             if (ret < 0)
0174                 return ret;
0175
0176             list_add_tail(&alt->list, alts);
0177         }
0178     }
0179
0180     return 0;
0181 }