Message-ID: <20150318090925.GK19131@chrystal.uk.oracle.com>
Date:	Wed, 18 Mar 2015 10:09:25 +0100
From:	Quentin Casasnovas <quentin.casasnovas@...cle.com>
To:	Quentin Casasnovas <quentin.casasnovas@...cle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Michal Marek <mmarek@...e.cz>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 6/7] scripts: add check_extable.sh script.

Adding Rusty and Michal to CC.

On Tue, Mar 17, 2015 at 01:40:01PM +0100, Quentin Casasnovas wrote:
> This shell script can be used to sanity check the __ex_table section on an
> object file, making sure the relocations in there are pointing to valid
> executable sections.  If it finds some suspicious relocations, it'll use
> addr2line to try and dump where this is coming from.
> 
> This works best with CONFIG_DEBUG_INFO.
> 
> Signed-off-by: Quentin Casasnovas <quentin.casasnovas@...cle.com>
> ---
>  scripts/check_extable.sh | 146 +++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 146 insertions(+)
>  create mode 100755 scripts/check_extable.sh
> 
> diff --git a/scripts/check_extable.sh b/scripts/check_extable.sh
> new file mode 100755
> index 0000000..0fb6b1c
> --- /dev/null
> +++ b/scripts/check_extable.sh
> @@ -0,0 +1,146 @@
> +#! /bin/bash
> +# (c) 2015, Quentin Casasnovas <quentin.casasnovas@...cle.com>
> +
> +obj=$1
> +
> +file ${obj} | grep -q ELF || (echo "${obj} is not and ELF file." 1>&2 ; exit 0)
> +
> +# Bail out early if there isn't an __ex_table section in this object file.
> +objdump -hj __ex_table ${obj} 2> /dev/null > /dev/null
> +[ $? -ne 0 ] && exit 0
> +
> +white_list=.text,.fixup
> +
> +suspicious_relocs=$(objdump -rj __ex_table ${obj}  | tail -n +6 |
> +			grep -v $(eval echo -e{${white_list}}) | awk '{print $3}')
> +
> +# No suspicious relocs in __ex_table, jobs a good'un
> +[ -z "${suspicious_relocs}" ] && exit 0
> +
> +
> +# After this point, something is seriously wrong since we just found out we
> +# have some relocations in __ex_table which point to sections which aren't
> +# white listed.  If you're adding a new section in the Linux kernel, and
> +# you're expecting this section to contain code which can fault (i.e. the
> +# __ex_table relocation to your new section is expected), simply add your
> +# new section to the white_list variable above.  If not, you're probably
> +# doing something wrong and the rest of this code is just trying to print
> +# you more information about it.
> +
> +function find_section_offset_from_symbol()
> +{
> +    eval $(objdump -t ${obj} | grep ${1} | sed 's/\([0-9a-f]\+\) .\{7\} \([^ \t]\+\).*/section="\2"; section_offset="0x\1" /')
> +
> +    # addr2line takes addresses in hexadecimal...
> +    section_offset=$(printf "0x%016x" $(( ${section_offset} + $2 )) )
> +}
> +
> +function find_symbol_and_offset_from_reloc()
> +{
> +    # Extract symbol and offset from the objdump output
> +    eval $(echo $reloc | sed 's/\([^+]\+\)+\?\(0x[0-9a-f]\+\)\?/symbol="\1"; symbol_offset="\2"/')
> +
> +    # When the relocation points to the begining of a symbol or section, it
> +    # won't print the offset since it is zero.
> +    if [ -z "${symbol_offset}" ]; then
> +	symbol_offset=0x0
> +    fi
> +}
> +
> +function find_alt_replacement_target()
> +{
> +    # The target of the .altinstr_replacement is the relocation just before
> +    # the .altinstr_replacement one.
> +    eval $(objdump -rj .altinstructions ${obj} | grep -B1 "${section}+${section_offset}" | head -n1 | awk '{print $3}' |
> +	   sed 's/\([^+]\+\)+\(0x[0-9a-f]\+\)/alt_target_section="\1"; alt_target_offset="\2"/')
> +}
> +
> +function handle_alt_replacement_reloc()
> +{
> +    # This will define alt_target_section and alt_target_section_offset
> +    find_alt_replacement_target ${section} ${section_offset}
> +
> +    echo "Error: found a reference to .altinstr_replacement in __ex_table:"
> +    addr2line -fip -j ${alt_target_section} -e ${obj} ${alt_target_offset} | awk '{print "\t" $0}'
> +
> +    error=true
> +}
> +
> +function is_executable_section()
> +{
> +    objdump -hwj ${section} ${obj} | grep -q CODE
> +    return $?
> +}
> +
> +function handle_suspicious_generic_reloc()
> +{
> +    if is_executable_section ${section}; then
> +	# We've got a relocation to a non white listed _executable_
> +	# section, print a warning so the developper adds the section to
> +	# the white list or fix his code.  We try to pretty-print the file
> +	# and line number where that relocation was added.
> +	echo "Warning: found a reference to section \"${section}\" in __ex_table:"
> +	addr2line -fip -j ${section} -e ${obj} ${section_offset} | awk '{print "\t" $0}'
> +    else
> +	# Something is definitively wrong here since we've got a relocation
> +	# to a non-executable section, there's no way this would ever be
> +	# running in the kernel.
> +	echo "Error: found a reference to non-executable section \"${section}\" in __ex_table at offset ${section_offset}"
> +	error=true
> +    fi
> +}
> +
> +function handle_suspicious_reloc()
> +{
> +    case "${section}" in
> +	".altinstr_replacement")
> +	    handle_alt_replacement_reloc ${section} ${section_offset}
> +	    ;;
> +	*)
> +	    handle_suspicious_generic_reloc ${section} ${section_offset}
> +	    ;;
> +    esac
> +}
> +
> +function diagnose()
> +{
> +
> +    for reloc in ${suspicious_relocs}; do
> +	# Let's find out where the target of the relocation in __ex_table
> +	# is, this will define ${symbol} and ${symbol_offset}
> +	find_symbol_and_offset_from_reloc ${reloc}
> +
> +	# When there's a global symbol at the place of the relocation,
> +	# objdump will use it instead of giving us a section+offset, so
> +	# let's find out which section is this symbol in and the total
> +	# offset withing that section.
> +	find_section_offset_from_symbol ${symbol} ${symbol_offset}
> +
> +	# In this case objdump was presenting us with a reloc to a symbol
> +	# rather than a section. Now that we've got the actual section,
> +	# we can skip it if it's in the white_list.
> +	if [ -z "$( echo $section | grep -v $(eval echo -e{${white_list}}))" ]; then
> +	    continue;
> +	fi
> +
> +	# Will either print a warning if the relocation happens to be in a
> +	# section we do not know but has executable bit set, or error out.
> +	handle_suspicious_reloc
> +    done
> +}
> +
> +function check_debug_info() {
> +    objdump -hj .debug_info ${obj} 2> /dev/null > /dev/null ||
> +	echo -e "${obj} does not contain debug information, the addr2line output will be limited.\n" \
> +	     "Recompile ${obj} with CONFIG_DEBUG_INFO to get a more useful output."
> +}
> +
> +check_debug_info
> +
> +diagnose
> +
> +if [ "${error}" ]; then
> +    exit 1
> +fi
> +
> +exit 0
> -- 
> 2.0.5
> 
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