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

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

On 4/16/19 1:07 PM, 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.
structers -> structures

> This change is needed since with recent Xen the alt_instr structure
> has changed size from 12 to 14 bytes.
>
> [1] 
> https://github.com/jpoimboe/kpatch/commit/170449847136a48b19fcceb19c1d4d257d386b56
>
> Signed-off-by: Pawel Wieczorkiewicz <wipawel@xxxxxxxxx>
> Reviewed-by: Bjoern Doebel <doebel@xxxxxxxxx>
> Reviewed-by: Martin Mazein <amazein@xxxxxxxxx>
> ---
>   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;
Why did you remove the hard error here from the original kpatch commit? 
I think a hard error is preferable to guessing.
> +    }
> +
> +    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 $i"
> +        fi
> +    done
> +fi
> +
If this hunk is moved after the call to build_full(), then it can be run 
directly on the xen-syms that has just been built which would allow 
dropping `if [ -f "$XENSYMS" ]...` and guaranteeing that *_STRUCT_SIZE 
are always set.
--
Ross Lagerwall

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