Re: [livepatch-build-tools part2 4/6] livepatch-build: detect special section group sizes

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Tue, Apr 16, 2019 at 12:07:14PM +0000, Pawel Wieczorkiewicz wrote:
> Hard-coding the special section group sizes is unreliable. Instead,
> determine them dynamically by finding the related struct definitions
> in the DWARF metadata.
> 
> This is a livepatch backport of kpatch upstream commit [1]:
> kpatch-build: detect special section group sizes 170449847136a48b19fc
> 
> Xen only deals with alt_instr, bug_frame and exception_table_entry
> structures, so sizes of these structers are obtained from xen-syms.
> 
> This change is needed since with recent Xen the alt_instr structure
> has changed size from 12 to 14 bytes.

Oh this is so much better than the "solution" we coded.

Thank you!

Ross, will commit to repo unless you have concerns..
> 
> [1] https://github.com/jpoimboe/kpatch/commit/170449847136a48b19fcceb19c1d4d257d386b56
> 
> Signed-off-by: Pawel Wieczorkiewicz <wipawel@amazon.de>
> Reviewed-by: Bjoern Doebel <doebel@amazon.de>
> Reviewed-by: Martin Mazein <amazein@amazon.de>
> ---
>  create-diff-object.c | 60 ++++++++++++++++++++++++++++++++++++++++++++--------
>  livepatch-build      | 23 ++++++++++++++++++++
>  2 files changed, 74 insertions(+), 9 deletions(-)
> 
> diff --git a/create-diff-object.c b/create-diff-object.c
> index 1e6e617..b0b4dcb 100644
> --- a/create-diff-object.c
> +++ b/create-diff-object.c
> @@ -958,12 +958,54 @@ static void kpatch_mark_constant_labels_same(struct kpatch_elf *kelf)
>  	}
>  }
>  
> -static int bug_frames_0_group_size(struct kpatch_elf *kelf, int offset) { return 8; }
> -static int bug_frames_1_group_size(struct kpatch_elf *kelf, int offset) { return 8; }
> -static int bug_frames_2_group_size(struct kpatch_elf *kelf, int offset) { return 8; }
> -static int bug_frames_3_group_size(struct kpatch_elf *kelf, int offset) { return 16; }
> -static int ex_table_group_size(struct kpatch_elf *kelf, int offset) { return 8; }
> -static int altinstructions_group_size(struct kpatch_elf *kelf, int offset) { return 12; }
> +static int bug_frames_group_size(struct kpatch_elf *kelf, int offset)
> +{
> +    static int size = 0;
> +    char *str;
> +    if (!size) {
> +        str = getenv("BUG_STRUCT_SIZE");
> +        size = str ? atoi(str) : 8;
> +    }
> +
> +    return size;
> +}
> +
> +static int bug_frames_3_group_size(struct kpatch_elf *kelf, int offset)
> +{
> +    static int size = 0;
> +    char *str;
> +    if (!size) {
> +        str = getenv("BUG_STRUCT_SIZE");
> +        size = str ? atoi(str) : 16;
> +    }
> +
> +    return size;
> +}
> +
> +static int ex_table_group_size(struct kpatch_elf *kelf, int offset)
> +{
> +    static int size = 0;
> +    char *str;
> +    if (!size) {
> +        str = getenv("EX_STRUCT_SIZE");
> +        size = str ? atoi(str) : 8;
> +    }
> +
> +    return size;
> +}
> +
> +static int altinstructions_group_size(struct kpatch_elf *kelf, int offset)
> +{
> +    static int size = 0;
> +    char *str;
> +    if (!size) {
> +        str = getenv("ALT_STRUCT_SIZE");
> +        size = str ? atoi(str) : 12;
> +    }
> +
> +    printf("altinstr_size=%d\n", size);
> +    return size;
> +}
>  
>  /*
>   * The rela groups in the .fixup section vary in size.  The beginning of each
> @@ -1016,15 +1058,15 @@ static int fixup_group_size(struct kpatch_elf *kelf, int offset)
>  static struct special_section special_sections[] = {
>  	{
>  		.name		= ".bug_frames.0",
> -		.group_size	= bug_frames_0_group_size,
> +		.group_size	= bug_frames_group_size,
>  	},
>  	{
>  		.name		= ".bug_frames.1",
> -		.group_size	= bug_frames_1_group_size,
> +		.group_size	= bug_frames_group_size,
>  	},
>  	{
>  		.name		= ".bug_frames.2",
> -		.group_size	= bug_frames_2_group_size,
> +		.group_size	= bug_frames_group_size,
>  	},
>  	{
>  		.name		= ".bug_frames.3",
> diff --git a/livepatch-build b/livepatch-build
> index c057fa1..a6cae12 100755
> --- a/livepatch-build
> +++ b/livepatch-build
> @@ -284,6 +284,29 @@ echo "Output directory: ${OUTPUT}"
>  echo "================================================"
>  echo
>  
> +if [ -f "$XENSYMS" ]; then
> +    echo "Reading special section data"
> +    SPECIAL_VARS=$(readelf -wi "$XENSYMS" |
> +               gawk --non-decimal-data '
> +               BEGIN { a = b = e = 0 }
> +               a == 0 && /DW_AT_name.* alt_instr/ {a = 1; next}
> +               b == 0 && /DW_AT_name.* bug_frame/ {b = 1; next}
> +               e == 0 && /DW_AT_name.* exception_table_entry/ {e = 1; next}
> +               a == 1 {printf("export ALT_STRUCT_SIZE=%d\n", $4); a = 2}
> +               b == 1 {printf("export BUG_STRUCT_SIZE=%d\n", $4); b = 2}
> +               e == 1 {printf("export EX_STRUCT_SIZE=%d\n", $4); e = 2}
> +               a == 2 && b == 2 && e == 2 {exit}')
> +    [[ -n $SPECIAL_VARS ]] && eval "$SPECIAL_VARS"
> +    if [[ -z $ALT_STRUCT_SIZE ]] || [[ -z $BUG_STRUCT_SIZE ]] || [[ -z $EX_STRUCT_SIZE ]]; then
> +        die "can't find special struct size"
> +    fi
> +    for i in $ALT_STRUCT_SIZE $BUG_STRUCT_SIZE $EX_STRUCT_SIZE; do
> +        if [[ ! $i -gt 0 ]] || [[ ! $i -le 16 ]]; then
> +            die "invalid special struct size $i"
> +        fi
> +    done
> +fi
> +
>  if [ "${SKIP}" != "build" ]; then
>      [ -e "${OUTPUT}" ] && die "Output directory exists"
>      grep -q 'CONFIG_LIVEPATCH=y' "${CONFIGFILE}" || die "CONFIG_LIVEPATCH must be enabled"
> -- 
> 2.16.5
> 
> 
> 
> 
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