lib/buildid.c

   1 // SPDX-License-Identifier: GPL-2.0
   2
   3 #include <linux/buildid.h>
   4 #include <linux/cache.h>
   5 #include <linux/elf.h>
   6 #include <linux/kernel.h>
   7 #include <linux/pagemap.h>
   8
   9 #define BUILD_ID 3
  10
  11 #define MAX_PHDR_CNT 256
  12
  13 struct freader {
  14         void *buf;
  15         u32 buf_sz;
  16         int err;
  17         union {
  18                 struct {
  19                         struct file *file;
  20                         struct folio *folio;
  21                         void *addr;
  22                         loff_t folio_off;
  23                         bool may_fault;
  24                 };
  25                 struct {
  26                         const char *data;
  27                         u64 data_sz;
  28                 };
  29         };
  30 };
  31
  32 static void freader_init_from_file(struct freader *r, void *buf, u32 buf_sz,
  33                                    struct file *file, bool may_fault)
  34 {
  35         memset(r, 0, sizeof(*r));
  36         r->buf = buf;
  37         r->buf_sz = buf_sz;
  38         r->file = file;
  39         r->may_fault = may_fault;
  40 }
  41
  42 static void freader_init_from_mem(struct freader *r, const char *data, u64 data_sz)
  43 {
  44         memset(r, 0, sizeof(*r));
  45         r->data = data;
  46         r->data_sz = data_sz;
  47 }
  48
  49 static void freader_put_folio(struct freader *r)
  50 {
  51         if (!r->folio)
  52                 return;
  53         kunmap_local(r->addr);
  54         folio_put(r->folio);
  55         r->folio = NULL;
  56 }
  57
  58 static int freader_get_folio(struct freader *r, loff_t file_off)
  59 {
  60         /* check if we can just reuse current folio */
  61         if (r->folio && file_off >= r->folio_off &&
  62             file_off < r->folio_off + folio_size(r->folio))
  63                 return 0;
  64
  65         freader_put_folio(r);
  66
  67         r->folio = filemap_get_folio(r->file->f_mapping, file_off >> PAGE_SHIFT);
  68
  69         /* if sleeping is allowed, wait for the page, if necessary */
  70         if (r->may_fault && (IS_ERR(r->folio) || !folio_test_uptodate(r->folio))) {
  71                 filemap_invalidate_lock_shared(r->file->f_mapping);
  72                 r->folio = read_cache_folio(r->file->f_mapping, file_off >> PAGE_SHIFT,
  73                                             NULL, r->file);
  74                 filemap_invalidate_unlock_shared(r->file->f_mapping);
  75         }
  76
  77         if (IS_ERR(r->folio) || !folio_test_uptodate(r->folio)) {
  78                 if (!IS_ERR(r->folio))
  79                         folio_put(r->folio);
  80                 r->folio = NULL;
  81                 return -EFAULT;
  82         }
  83
  84         r->folio_off = folio_pos(r->folio);
  85         r->addr = kmap_local_folio(r->folio, 0);
  86
  87         return 0;
  88 }
  89
  90 static const void *freader_fetch(struct freader *r, loff_t file_off, size_t sz)
  91 {
  92         size_t folio_sz;
  93
  94         /* provided internal temporary buffer should be sized correctly */
  95         if (WARN_ON(r->buf && sz > r->buf_sz)) {
  96                 r->err = -E2BIG;
  97                 return NULL;
  98         }
  99
 100         if (unlikely(file_off + sz < file_off)) {
 101                 r->err = -EOVERFLOW;
 102                 return NULL;
 103         }
 104
 105         /* working with memory buffer is much more straightforward */
 106         if (!r->buf) {
 107                 if (file_off + sz > r->data_sz) {
 108                         r->err = -ERANGE;
 109                         return NULL;
 110                 }
 111                 return r->data + file_off;
 112         }
 113
 114         /* fetch or reuse folio for given file offset */
 115         r->err = freader_get_folio(r, file_off);
 116         if (r->err)
 117                 return NULL;
 118
 119         /* if requested data is crossing folio boundaries, we have to copy
 120          * everything into our local buffer to keep a simple linear memory
 121          * access interface
 122          */
 123         folio_sz = folio_size(r->folio);
 124         if (file_off + sz > r->folio_off + folio_sz) {
 125                 int part_sz = r->folio_off + folio_sz - file_off;
 126
 127                 /* copy the part that resides in the current folio */
 128                 memcpy(r->buf, r->addr + (file_off - r->folio_off), part_sz);
 129
 130                 /* fetch next folio */
 131                 r->err = freader_get_folio(r, r->folio_off + folio_sz);
 132                 if (r->err)
 133                         return NULL;
 134
 135                 /* copy the rest of requested data */
 136                 memcpy(r->buf + part_sz, r->addr, sz - part_sz);
 137
 138                 return r->buf;
 139         }
 140
 141         /* if data fits in a single folio, just return direct pointer */
 142         return r->addr + (file_off - r->folio_off);
 143 }
 144
 145 static void freader_cleanup(struct freader *r)
 146 {
 147         if (!r->buf)
 148                 return; /* non-file-backed mode */
 149
 150         freader_put_folio(r);
 151 }
 152
 153 /*
 154  * Parse build id from the note segment. This logic can be shared between
 155  * 32-bit and 64-bit system, because Elf32_Nhdr and Elf64_Nhdr are
 156  * identical.
 157  */
 158 static int parse_build_id(struct freader *r, unsigned char *build_id, __u32 *size,
 159                           loff_t note_off, Elf32_Word note_size)
 160 {
 161         const char note_name[] = "GNU";
 162         const size_t note_name_sz = sizeof(note_name);
 163         u32 build_id_off, new_off, note_end, name_sz, desc_sz;
 164         const Elf32_Nhdr *nhdr;
 165         const char *data;
 166
 167         if (check_add_overflow(note_off, note_size, &note_end))
 168                 return -EINVAL;
 169
 170         while (note_end - note_off > sizeof(Elf32_Nhdr) + note_name_sz) {
 171                 nhdr = freader_fetch(r, note_off, sizeof(Elf32_Nhdr) + note_name_sz);
 172                 if (!nhdr)
 173                         return r->err;
 174
 175                 name_sz = READ_ONCE(nhdr->n_namesz);
 176                 desc_sz = READ_ONCE(nhdr->n_descsz);
 177
 178                 new_off = note_off + sizeof(Elf32_Nhdr);
 179                 if (check_add_overflow(new_off, ALIGN(name_sz, 4), &new_off) ||
 180                     check_add_overflow(new_off, ALIGN(desc_sz, 4), &new_off) ||
 181                     new_off > note_end)
 182                         break;
 183
 184                 if (nhdr->n_type == BUILD_ID &&
 185                     name_sz == note_name_sz &&
 186                     memcmp(nhdr + 1, note_name, note_name_sz) == 0 &&
 187                     desc_sz > 0 && desc_sz <= BUILD_ID_SIZE_MAX) {
 188                         build_id_off = note_off + sizeof(Elf32_Nhdr) + ALIGN(note_name_sz, 4);
 189
 190                         /* freader_fetch() will invalidate nhdr pointer */
 191                         data = freader_fetch(r, build_id_off, desc_sz);
 192                         if (!data)
 193                                 return r->err;
 194
 195                         memcpy(build_id, data, desc_sz);
 196                         memset(build_id + desc_sz, 0, BUILD_ID_SIZE_MAX - desc_sz);
 197                         if (size)
 198                                 *size = desc_sz;
 199                         return 0;
 200                 }
 201
 202                 note_off = new_off;
 203         }
 204
 205         return -EINVAL;
 206 }
 207
 208 /* Parse build ID from 32-bit ELF */
 209 static int get_build_id_32(struct freader *r, unsigned char *build_id, __u32 *size)
 210 {
 211         const Elf32_Ehdr *ehdr;
 212         const Elf32_Phdr *phdr;
 213         __u32 phnum, phoff, i;
 214
 215         ehdr = freader_fetch(r, 0, sizeof(Elf32_Ehdr));
 216         if (!ehdr)
 217                 return r->err;
 218
 219         /* subsequent freader_fetch() calls invalidate pointers, so remember locally */
 220         phnum = READ_ONCE(ehdr->e_phnum);
 221         phoff = READ_ONCE(ehdr->e_phoff);
 222
 223         /* set upper bound on amount of segments (phdrs) we iterate */
 224         if (phnum > MAX_PHDR_CNT)
 225                 phnum = MAX_PHDR_CNT;
 226
 227         /* check that phoff is not large enough to cause an overflow */
 228         if (phoff + phnum * sizeof(Elf32_Phdr) < phoff)
 229                 return -EINVAL;
 230
 231         for (i = 0; i < phnum; ++i) {
 232                 phdr = freader_fetch(r, phoff + i * sizeof(Elf32_Phdr), sizeof(Elf32_Phdr));
 233                 if (!phdr)
 234                         return r->err;
 235
 236                 if (phdr->p_type == PT_NOTE &&
 237                     !parse_build_id(r, build_id, size, READ_ONCE(phdr->p_offset),
 238                                     READ_ONCE(phdr->p_filesz)))
 239                         return 0;
 240         }
 241         return -EINVAL;
 242 }
 243
 244 /* Parse build ID from 64-bit ELF */
 245 static int get_build_id_64(struct freader *r, unsigned char *build_id, __u32 *size)
 246 {
 247         const Elf64_Ehdr *ehdr;
 248         const Elf64_Phdr *phdr;
 249         __u32 phnum, i;
 250         __u64 phoff;
 251
 252         ehdr = freader_fetch(r, 0, sizeof(Elf64_Ehdr));
 253         if (!ehdr)
 254                 return r->err;
 255
 256         /* subsequent freader_fetch() calls invalidate pointers, so remember locally */
 257         phnum = READ_ONCE(ehdr->e_phnum);
 258         phoff = READ_ONCE(ehdr->e_phoff);
 259
 260         /* set upper bound on amount of segments (phdrs) we iterate */
 261         if (phnum > MAX_PHDR_CNT)
 262                 phnum = MAX_PHDR_CNT;
 263
 264         /* check that phoff is not large enough to cause an overflow */
 265         if (phoff + phnum * sizeof(Elf64_Phdr) < phoff)
 266                 return -EINVAL;
 267
 268         for (i = 0; i < phnum; ++i) {
 269                 phdr = freader_fetch(r, phoff + i * sizeof(Elf64_Phdr), sizeof(Elf64_Phdr));
 270                 if (!phdr)
 271                         return r->err;
 272
 273                 if (phdr->p_type == PT_NOTE &&
 274                     !parse_build_id(r, build_id, size, READ_ONCE(phdr->p_offset),
 275                                     READ_ONCE(phdr->p_filesz)))
 276                         return 0;
 277         }
 278
 279         return -EINVAL;
 280 }
 281
 282 /* enough for Elf64_Ehdr, Elf64_Phdr, and all the smaller requests */
 283 #define MAX_FREADER_BUF_SZ 64
 284
 285 static int __build_id_parse(struct vm_area_struct *vma, unsigned char *build_id,
 286                             __u32 *size, bool may_fault)
 287 {
 288         const Elf32_Ehdr *ehdr;
 289         struct freader r;
 290         char buf[MAX_FREADER_BUF_SZ];
 291         int ret;
 292
 293         /* only works for page backed storage  */
 294         if (!vma->vm_file)
 295                 return -EINVAL;
 296
 297         freader_init_from_file(&r, buf, sizeof(buf), vma->vm_file, may_fault);
 298
 299         /* fetch first 18 bytes of ELF header for checks */
 300         ehdr = freader_fetch(&r, 0, offsetofend(Elf32_Ehdr, e_type));
 301         if (!ehdr) {
 302                 ret = r.err;
 303                 goto out;
 304         }
 305
 306         ret = -EINVAL;
 307
 308         /* compare magic x7f "ELF" */
 309         if (memcmp(ehdr->e_ident, ELFMAG, SELFMAG) != 0)
 310                 goto out;
 311
 312         /* only support executable file and shared object file */
 313         if (ehdr->e_type != ET_EXEC && ehdr->e_type != ET_DYN)
 314                 goto out;
 315
 316         if (ehdr->e_ident[EI_CLASS] == ELFCLASS32)
 317                 ret = get_build_id_32(&r, build_id, size);
 318         else if (ehdr->e_ident[EI_CLASS] == ELFCLASS64)
 319                 ret = get_build_id_64(&r, build_id, size);
 320 out:
 321         freader_cleanup(&r);
 322         return ret;
 323 }
 324
 325 /*
 326  * Parse build ID of ELF file mapped to vma
 327  * @vma:      vma object
 328  * @build_id: buffer to store build id, at least BUILD_ID_SIZE long
 329  * @size:     returns actual build id size in case of success
 330  *
 331  * Assumes no page fault can be taken, so if relevant portions of ELF file are
 332  * not already paged in, fetching of build ID fails.
 333  *
 334  * Return: 0 on success; negative error, otherwise
 335  */
 336 int build_id_parse_nofault(struct vm_area_struct *vma, unsigned char *build_id, __u32 *size)
 337 {
 338         return __build_id_parse(vma, build_id, size, false /* !may_fault */);
 339 }
 340
 341 /*
 342  * Parse build ID of ELF file mapped to VMA
 343  * @vma:      vma object
 344  * @build_id: buffer to store build id, at least BUILD_ID_SIZE long
 345  * @size:     returns actual build id size in case of success
 346  *
 347  * Assumes faultable context and can cause page faults to bring in file data
 348  * into page cache.
 349  *
 350  * Return: 0 on success; negative error, otherwise
 351  */
 352 int build_id_parse(struct vm_area_struct *vma, unsigned char *build_id, __u32 *size)
 353 {
 354         return __build_id_parse(vma, build_id, size, true /* may_fault */);
 355 }
 356
 357 /**
 358  * build_id_parse_buf - Get build ID from a buffer
 359  * @buf:      ELF note section(s) to parse
 360  * @buf_size: Size of @buf in bytes
 361  * @build_id: Build ID parsed from @buf, at least BUILD_ID_SIZE_MAX long
 362  *
 363  * Return: 0 on success, -EINVAL otherwise
 364  */
 365 int build_id_parse_buf(const void *buf, unsigned char *build_id, u32 buf_size)
 366 {
 367         struct freader r;
 368         int err;
 369
 370         freader_init_from_mem(&r, buf, buf_size);
 371
 372         err = parse_build_id(&r, build_id, NULL, 0, buf_size);
 373
 374         freader_cleanup(&r);
 375         return err;
 376 }
 377
 378 #if IS_ENABLED(CONFIG_STACKTRACE_BUILD_ID) || IS_ENABLED(CONFIG_VMCORE_INFO)
 379 unsigned char vmlinux_build_id[BUILD_ID_SIZE_MAX] __ro_after_init;
 380
 381 /**
 382  * init_vmlinux_build_id - Compute and stash the running kernel's build ID
 383  */
 384 void __init init_vmlinux_build_id(void)
 385 {
 386         extern const void __start_notes;
 387         extern const void __stop_notes;
 388         unsigned int size = &__stop_notes - &__start_notes;
 389
 390         build_id_parse_buf(&__start_notes, vmlinux_build_id, size);
 391 }
 392 #endif