scripts/check_extable.sh

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