tools/objtool/arch/x86/special.c

   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2 #include <string.h>
   3
   4 #include "../../special.h"
   5 #include "../../builtin.h"
   6
   7 #define X86_FEATURE_POPCNT (4 * 32 + 23)
   8 #define X86_FEATURE_SMAP   (9 * 32 + 20)
   9
  10 void arch_handle_alternative(unsigned short feature, struct special_alt *alt)
  11 {
  12         switch (feature) {
  13         case X86_FEATURE_SMAP:
  14                 /*
  15                  * If UACCESS validation is enabled; force that alternative;
  16                  * otherwise force it the other way.
  17                  *
  18                  * What we want to avoid is having both the original and the
  19                  * alternative code flow at the same time, in that case we can
  20                  * find paths that see the STAC but take the NOP instead of
  21                  * CLAC and the other way around.
  22                  */
  23                 if (uaccess)
  24                         alt->skip_orig = true;
  25                 else
  26                         alt->skip_alt = true;
  27                 break;
  28         case X86_FEATURE_POPCNT:
  29                 /*
  30                  * It has been requested that we don't validate the !POPCNT
  31                  * feature path which is a "very very small percentage of
  32                  * machines".
  33                  */
  34                 alt->skip_orig = true;
  35                 break;
  36         default:
  37                 break;
  38         }
  39 }
  40
  41 bool arch_support_alt_relocation(struct special_alt *special_alt,
  42                                  struct instruction *insn,
  43                                  struct reloc *reloc)
  44 {
  45         /*
  46          * The x86 alternatives code adjusts the offsets only when it
  47          * encounters a branch instruction at the very beginning of the
  48          * replacement group.
  49          */
  50         return insn->offset == special_alt->new_off &&
  51                (insn->type == INSN_CALL || is_static_jump(insn));
  52 }
  53
  54 /*
  55  * There are 3 basic jump table patterns:
  56  *
  57  * 1. jmpq *[rodata addr](,%reg,8)
  58  *
  59  *    This is the most common case by far.  It jumps to an address in a simple
  60  *    jump table which is stored in .rodata.
  61  *
  62  * 2. jmpq *[rodata addr](%rip)
  63  *
  64  *    This is caused by a rare GCC quirk, currently only seen in three driver
  65  *    functions in the kernel, only with certain obscure non-distro configs.
  66  *
  67  *    As part of an optimization, GCC makes a copy of an existing switch jump
  68  *    table, modifies it, and then hard-codes the jump (albeit with an indirect
  69  *    jump) to use a single entry in the table.  The rest of the jump table and
  70  *    some of its jump targets remain as dead code.
  71  *
  72  *    In such a case we can just crudely ignore all unreachable instruction
  73  *    warnings for the entire object file.  Ideally we would just ignore them
  74  *    for the function, but that would require redesigning the code quite a
  75  *    bit.  And honestly that's just not worth doing: unreachable instruction
  76  *    warnings are of questionable value anyway, and this is such a rare issue.
  77  *
  78  * 3. mov [rodata addr],%reg1
  79  *    ... some instructions ...
  80  *    jmpq *(%reg1,%reg2,8)
  81  *
  82  *    This is a fairly uncommon pattern which is new for GCC 6.  As of this
  83  *    writing, there are 11 occurrences of it in the allmodconfig kernel.
  84  *
  85  *    As of GCC 7 there are quite a few more of these and the 'in between' code
  86  *    is significant. Esp. with KASAN enabled some of the code between the mov
  87  *    and jmpq uses .rodata itself, which can confuse things.
  88  *
  89  *    TODO: Once we have DWARF CFI and smarter instruction decoding logic,
  90  *    ensure the same register is used in the mov and jump instructions.
  91  *
  92  *    NOTE: RETPOLINE made it harder still to decode dynamic jumps.
  93  */
  94 struct reloc *arch_find_switch_table(struct objtool_file *file,
  95                                     struct instruction *insn)
  96 {
  97         struct reloc  *text_reloc, *rodata_reloc;
  98         struct section *table_sec;
  99         unsigned long table_offset;
 100
 101         /* look for a relocation which references .rodata */
 102         text_reloc = find_reloc_by_dest_range(file->elf, insn->sec,
 103                                               insn->offset, insn->len);
 104         if (!text_reloc || text_reloc->sym->type != STT_SECTION ||
 105             !text_reloc->sym->sec->rodata)
 106                 return NULL;
 107
 108         table_offset = text_reloc->addend;
 109         table_sec = text_reloc->sym->sec;
 110
 111         if (text_reloc->type == R_X86_64_PC32)
 112                 table_offset += 4;
 113
 114         /*
 115          * Make sure the .rodata address isn't associated with a
 116          * symbol.  GCC jump tables are anonymous data.
 117          *
 118          * Also support C jump tables which are in the same format as
 119          * switch jump tables.  For objtool to recognize them, they
 120          * need to be placed in the C_JUMP_TABLE_SECTION section.  They
 121          * have symbols associated with them.
 122          */
 123         if (find_symbol_containing(table_sec, table_offset) &&
 124             strcmp(table_sec->name, C_JUMP_TABLE_SECTION))
 125                 return NULL;
 126
 127         /*
 128          * Each table entry has a rela associated with it.  The rela
 129          * should reference text in the same function as the original
 130          * instruction.
 131          */
 132         rodata_reloc = find_reloc_by_dest(file->elf, table_sec, table_offset);
 133         if (!rodata_reloc)
 134                 return NULL;
 135
 136         /*
 137          * Use of RIP-relative switch jumps is quite rare, and
 138          * indicates a rare GCC quirk/bug which can leave dead
 139          * code behind.
 140          */
 141         if (text_reloc->type == R_X86_64_PC32)
 142                 file->ignore_unreachables = true;
 143
 144         return rodata_reloc;
 145 }