scripts/check_extable.sh

   1 #! /bin/bash
   2 # SPDX-License-Identifier: GPL-2.0
   3 # (c) 2015, Quentin Casasnovas <quentin.casasnovas@oracle.com>
   4
   5 obj=$1
   6
   7 file ${obj} | grep -q ELF || (echo "${obj} is not an ELF file." 1>&2 ; exit 0)
   8
   9 # Bail out early if there isn't an __ex_table section in this object file.
  10 objdump -hj __ex_table ${obj} 2> /dev/null > /dev/null
  11 [ $? -ne 0 ] && exit 0
  12
  13 white_list=.text,.fixup
  14
  15 suspicious_relocs=$(objdump -rj __ex_table ${obj}  | tail -n +6 |
  16                         grep -v $(eval echo -e{${white_list}}) | awk '{print $3}')
  17
  18 # No suspicious relocs in __ex_table, jobs a good'un
  19 [ -z "${suspicious_relocs}" ] && exit 0
  20
  21
  22 # After this point, something is seriously wrong since we just found out we
  23 # have some relocations in __ex_table which point to sections which aren't
  24 # white listed.  If you're adding a new section in the Linux kernel, and
  25 # you're expecting this section to contain code which can fault (i.e. the
  26 # __ex_table relocation to your new section is expected), simply add your
  27 # new section to the white_list variable above.  If not, you're probably
  28 # doing something wrong and the rest of this code is just trying to print
  29 # you more information about it.
  30
  31 function find_section_offset_from_symbol()
  32 {
  33     eval $(objdump -t ${obj} | grep ${1} | sed 's/\([0-9a-f]\+\) .\{7\} \([^ \t]\+\).*/section="\2"; section_offset="0x\1" /')
  34
  35     # addr2line takes addresses in hexadecimal...
  36     section_offset=$(printf "0x%016x" $(( ${section_offset} + $2 )) )
  37 }
  38
  39 function find_symbol_and_offset_from_reloc()
  40 {
  41     # Extract symbol and offset from the objdump output
  42     eval $(echo $reloc | sed 's/\([^+]\+\)+\?\(0x[0-9a-f]\+\)\?/symbol="\1"; symbol_offset="\2"/')
  43
  44     # When the relocation points to the begining of a symbol or section, it
  45     # won't print the offset since it is zero.
  46     if [ -z "${symbol_offset}" ]; then
  47         symbol_offset=0x0
  48     fi
  49 }
  50
  51 function find_alt_replacement_target()
  52 {
  53     # The target of the .altinstr_replacement is the relocation just before
  54     # the .altinstr_replacement one.
  55     eval $(objdump -rj .altinstructions ${obj} | grep -B1 "${section}+${section_offset}" | head -n1 | awk '{print $3}' |
  56            sed 's/\([^+]\+\)+\(0x[0-9a-f]\+\)/alt_target_section="\1"; alt_target_offset="\2"/')
  57 }
  58
  59 function handle_alt_replacement_reloc()
  60 {
  61     # This will define alt_target_section and alt_target_section_offset
  62     find_alt_replacement_target ${section} ${section_offset}
  63
  64     echo "Error: found a reference to .altinstr_replacement in __ex_table:"
  65     addr2line -fip -j ${alt_target_section} -e ${obj} ${alt_target_offset} | awk '{print "\t" $0}'
  66
  67     error=true
  68 }
  69
  70 function is_executable_section()
  71 {
  72     objdump -hwj ${section} ${obj} | grep -q CODE
  73     return $?
  74 }
  75
  76 function handle_suspicious_generic_reloc()
  77 {
  78     if is_executable_section ${section}; then
  79         # We've got a relocation to a non white listed _executable_
  80         # section, print a warning so the developper adds the section to
  81         # the white list or fix his code.  We try to pretty-print the file
  82         # and line number where that relocation was added.
  83         echo "Warning: found a reference to section \"${section}\" in __ex_table:"
  84         addr2line -fip -j ${section} -e ${obj} ${section_offset} | awk '{print "\t" $0}'
  85     else
  86         # Something is definitively wrong here since we've got a relocation
  87         # to a non-executable section, there's no way this would ever be
  88         # running in the kernel.
  89         echo "Error: found a reference to non-executable section \"${section}\" in __ex_table at offset ${section_offset}"
  90         error=true
  91     fi
  92 }
  93
  94 function handle_suspicious_reloc()
  95 {
  96     case "${section}" in
  97         ".altinstr_replacement")
  98             handle_alt_replacement_reloc ${section} ${section_offset}
  99             ;;
 100         *)
 101             handle_suspicious_generic_reloc ${section} ${section_offset}
 102             ;;
 103     esac
 104 }
 105
 106 function diagnose()
 107 {
 108
 109     for reloc in ${suspicious_relocs}; do
 110         # Let's find out where the target of the relocation in __ex_table
 111         # is, this will define ${symbol} and ${symbol_offset}
 112         find_symbol_and_offset_from_reloc ${reloc}
 113
 114         # When there's a global symbol at the place of the relocation,
 115         # objdump will use it instead of giving us a section+offset, so
 116         # let's find out which section is this symbol in and the total
 117         # offset withing that section.
 118         find_section_offset_from_symbol ${symbol} ${symbol_offset}
 119
 120         # In this case objdump was presenting us with a reloc to a symbol
 121         # rather than a section. Now that we've got the actual section,
 122         # we can skip it if it's in the white_list.
 123         if [ -z "$( echo $section | grep -v $(eval echo -e{${white_list}}))" ]; then
 124             continue;
 125         fi
 126
 127         # Will either print a warning if the relocation happens to be in a
 128         # section we do not know but has executable bit set, or error out.
 129         handle_suspicious_reloc
 130     done
 131 }
 132
 133 function check_debug_info() {
 134     objdump -hj .debug_info ${obj} 2> /dev/null > /dev/null ||
 135         echo -e "${obj} does not contain debug information, the addr2line output will be limited.\n" \
 136              "Recompile ${obj} with CONFIG_DEBUG_INFO to get a more useful output."
 137 }
 138
 139 check_debug_info
 140
 141 diagnose
 142
 143 if [ "${error}" ]; then
 144     exit 1
 145 fi
 146
 147 exit 0