elf/dl-map-segments.h

   1 /* Map in a shared object's segments.  Generic version.
   2    Copyright (C) 1995-2022 Free Software Foundation, Inc.
   3    Copyright The GNU Toolchain Authors.
   4    This file is part of the GNU C Library.
   5
   6    The GNU C Library is free software; you can redistribute it and/or
   7    modify it under the terms of the GNU Lesser General Public
   8    License as published by the Free Software Foundation; either
   9    version 2.1 of the License, or (at your option) any later version.
  10
  11    The GNU C Library is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14    Lesser General Public License for more details.
  15
  16    You should have received a copy of the GNU Lesser General Public
  17    License along with the GNU C Library; if not, see
  18    <https://www.gnu.org/licenses/>.  */
  19
  20 #include <dl-load.h>
  21
  22 /* Map a segment and align it properly.  */
  23
  24 static __always_inline ElfW(Addr)
  25 _dl_map_segment (const struct loadcmd *c, ElfW(Addr) mappref,
  26                  const size_t maplength, int fd)
  27 {
  28   if (__glibc_likely (c->mapalign <= GLRO(dl_pagesize)))
  29     return (ElfW(Addr)) __mmap ((void *) mappref, maplength, c->prot,
  30                                 MAP_COPY|MAP_FILE, fd, c->mapoff);
  31
  32   /* If the segment alignment > the page size, allocate enough space to
  33      ensure that the segment can be properly aligned.  */
  34   ElfW(Addr) maplen = (maplength >= c->mapalign
  35                        ? (maplength + c->mapalign)
  36                        : (2 * c->mapalign));
  37   ElfW(Addr) map_start = (ElfW(Addr)) __mmap ((void *) mappref, maplen,
  38                                               PROT_NONE,
  39                                               MAP_ANONYMOUS|MAP_PRIVATE,
  40                                               -1, 0);
  41   if (__glibc_unlikely ((void *) map_start == MAP_FAILED))
  42     return map_start;
  43
  44   ElfW(Addr) map_start_aligned = ALIGN_UP (map_start, c->mapalign);
  45   map_start_aligned = (ElfW(Addr)) __mmap ((void *) map_start_aligned,
  46                                            maplength, c->prot,
  47                                            MAP_COPY|MAP_FILE|MAP_FIXED,
  48                                            fd, c->mapoff);
  49   if (__glibc_unlikely ((void *) map_start_aligned == MAP_FAILED))
  50     __munmap ((void *) map_start, maplen);
  51   else
  52     {
  53       /* Unmap the unused regions.  */
  54       ElfW(Addr) delta = map_start_aligned - map_start;
  55       if (delta)
  56         __munmap ((void *) map_start, delta);
  57       ElfW(Addr) map_end = map_start_aligned + maplength;
  58       map_end = ALIGN_UP (map_end, GLRO(dl_pagesize));
  59       delta = map_start + maplen - map_end;
  60       if (delta)
  61         __munmap ((void *) map_end, delta);
  62     }
  63
  64   return map_start_aligned;
  65 }
  66
  67 /* This implementation assumes (as does the corresponding implementation
  68    of _dl_unmap_segments, in dl-unmap-segments.h) that shared objects
  69    are always laid out with all segments contiguous (or with gaps
  70    between them small enough that it's preferable to reserve all whole
  71    pages inside the gaps with PROT_NONE mappings rather than permitting
  72    other use of those parts of the address space).  */
  73
  74 static __always_inline const char *
  75 _dl_map_segments (struct link_map *l, int fd,
  76                   const ElfW(Ehdr) *header, int type,
  77                   const struct loadcmd loadcmds[], size_t nloadcmds,
  78                   const size_t maplength, bool has_holes,
  79                   struct link_map *loader)
  80 {
  81   const struct loadcmd *c = loadcmds;
  82
  83   if (__glibc_likely (type == ET_DYN))
  84     {
  85       /* This is a position-independent shared object.  We can let the
  86          kernel map it anywhere it likes, but we must have space for all
  87          the segments in their specified positions relative to the first.
  88          So we map the first segment without MAP_FIXED, but with its
  89          extent increased to cover all the segments.  Then we remove
  90          access from excess portion, and there is known sufficient space
  91          there to remap from the later segments.
  92
  93          As a refinement, sometimes we have an address that we would
  94          prefer to map such objects at; but this is only a preference,
  95          the OS can do whatever it likes. */
  96       ElfW(Addr) mappref
  97         = (ELF_PREFERRED_ADDRESS (loader, maplength, c->mapstart)
  98            - MAP_BASE_ADDR (l));
  99
 100       /* Remember which part of the address space this object uses.  */
 101       l->l_map_start = _dl_map_segment (c, mappref, maplength, fd);
 102       if (__glibc_unlikely ((void *) l->l_map_start == MAP_FAILED))
 103         return DL_MAP_SEGMENTS_ERROR_MAP_SEGMENT;
 104
 105       l->l_map_end = l->l_map_start + maplength;
 106       l->l_addr = l->l_map_start - c->mapstart;
 107
 108       if (has_holes)
 109         {
 110           /* Change protection on the excess portion to disallow all access;
 111              the portions we do not remap later will be inaccessible as if
 112              unallocated.  Then jump into the normal segment-mapping loop to
 113              handle the portion of the segment past the end of the file
 114              mapping.  */
 115           if (__glibc_unlikely (loadcmds[nloadcmds - 1].mapstart <
 116                                 c->mapend))
 117             return N_("ELF load command address/offset not page-aligned");
 118           if (__glibc_unlikely
 119               (__mprotect ((caddr_t) (l->l_addr + c->mapend),
 120                            loadcmds[nloadcmds - 1].mapstart - c->mapend,
 121                            PROT_NONE) < 0))
 122             return DL_MAP_SEGMENTS_ERROR_MPROTECT;
 123         }
 124
 125       l->l_contiguous = 1;
 126
 127       goto postmap;
 128     }
 129
 130   /* Remember which part of the address space this object uses.  */
 131   l->l_map_start = c->mapstart + l->l_addr;
 132   l->l_map_end = l->l_map_start + maplength;
 133   l->l_contiguous = !has_holes;
 134
 135   while (c < &loadcmds[nloadcmds])
 136     {
 137       if (c->mapend > c->mapstart
 138           /* Map the segment contents from the file.  */
 139           && (__mmap ((void *) (l->l_addr + c->mapstart),
 140                       c->mapend - c->mapstart, c->prot,
 141                       MAP_FIXED|MAP_COPY|MAP_FILE,
 142                       fd, c->mapoff)
 143               == MAP_FAILED))
 144         return DL_MAP_SEGMENTS_ERROR_MAP_SEGMENT;
 145
 146     postmap:
 147       _dl_postprocess_loadcmd (l, header, c);
 148
 149       if (c->allocend > c->dataend)
 150         {
 151           /* Extra zero pages should appear at the end of this segment,
 152              after the data mapped from the file.   */
 153           ElfW(Addr) zero, zeroend, zeropage;
 154
 155           zero = l->l_addr + c->dataend;
 156           zeroend = l->l_addr + c->allocend;
 157           zeropage = ((zero + GLRO(dl_pagesize) - 1)
 158                       & ~(GLRO(dl_pagesize) - 1));
 159
 160           if (zeroend < zeropage)
 161             /* All the extra data is in the last page of the segment.
 162                We can just zero it.  */
 163             zeropage = zeroend;
 164
 165           if (zeropage > zero)
 166             {
 167               /* Zero the final part of the last page of the segment.  */
 168               if (__glibc_unlikely ((c->prot & PROT_WRITE) == 0))
 169                 {
 170                   /* Dag nab it.  */
 171                   if (__mprotect ((caddr_t) (zero
 172                                              & ~(GLRO(dl_pagesize) - 1)),
 173                                   GLRO(dl_pagesize), c->prot|PROT_WRITE) < 0)
 174                     return DL_MAP_SEGMENTS_ERROR_MPROTECT;
 175                 }
 176               memset ((void *) zero, '\0', zeropage - zero);
 177               if (__glibc_unlikely ((c->prot & PROT_WRITE) == 0))
 178                 __mprotect ((caddr_t) (zero & ~(GLRO(dl_pagesize) - 1)),
 179                             GLRO(dl_pagesize), c->prot);
 180             }
 181
 182           if (zeroend > zeropage)
 183             {
 184               /* Map the remaining zero pages in from the zero fill FD.  */
 185               caddr_t mapat;
 186               mapat = __mmap ((caddr_t) zeropage, zeroend - zeropage,
 187                               c->prot, MAP_ANON|MAP_PRIVATE|MAP_FIXED,
 188                               -1, 0);
 189               if (__glibc_unlikely (mapat == MAP_FAILED))
 190                 return DL_MAP_SEGMENTS_ERROR_MAP_ZERO_FILL;
 191             }
 192         }
 193
 194       ++c;
 195     }
 196
 197   /* Notify ELF_PREFERRED_ADDRESS that we have to load this one
 198      fixed.  */
 199   ELF_FIXED_ADDRESS (loader, c->mapstart);
 200
 201   return NULL;
 202 }