Re: [Xen-devel] [livepatch-build-tools part2 6/6] create-diff-object: Do not include all .rodata sections

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

On 4/16/19 1:07 PM, Pawel Wieczorkiewicz wrote:
> Starting with the "2af6f1aa6233 Fix patch creation with GCC 6.1+" commit
> all .rodata sections are included by default (regardless of whether they
> are needed or not).
> During stacked hotpatch builds it leads to unnecessary duplication of
> the .rodata sections as each and every consecutive hotpatch contains all
> the .rodata section of previous hotpatches.
This commit message is a bit confusing.

1) All of this only applies to .rodata.str* sections. Other .rodata 
sections are handled separately.
2) The above commit _did not_ introduce the problem. Previous versions 
of GCC did not split .rodata.str sections by function which meant that 
the entire section was always included. The commit fixed patch creation 
and _maintained_ the existing behaviour for GCC 6.1+ by including all 
the .rodata.str* sections. This patch now includes only what is needed 
(but it should be noted that this would likely not be useful on older 
versions of GCC since they don't split .rodata.str properly).
> To prevent this situation, mark the .rodata section for inclusion only
> if they are referenced by any of the current hotpatch symbols (or a
> corresponding RELA section).
>
> Extend patchability verification to detect all non-standard, non-rela,
> non-debug and non-special sections that are not referenced by any of
> the symbols or RELA sections.
>
> Signed-off-by: Pawel Wieczorkiewicz <wipawel@xxxxxxxxx>
> Reviewed-by: Andra-Irina Paraschiv <andraprs@xxxxxxxxxx>
> Reviewed-by: Bjoern Doebel <doebel@xxxxxxxxx>
> Reviewed-by: Norbert Manthey <nmanthey@xxxxxxxxx>
> ---
>   create-diff-object.c | 27 ++++++++++++++++++++++++++-
>   1 file changed, 26 insertions(+), 1 deletion(-)
>
> diff --git a/create-diff-object.c b/create-diff-object.c
> index f6060cd..f7eb421 100644
> --- a/create-diff-object.c
> +++ b/create-diff-object.c
> @@ -1341,7 +1341,7 @@ static void kpatch_include_standard_elements(struct 
> kpatch_elf *kelf)
>   
>   	list_for_each_entry(sec, &kelf->sections, list) {
>                 /* include these sections even if they haven't changed */
> -               if (is_standard_section(sec) || 
> should_include_str_section(sec->name)) {
> +               if (is_standard_section(sec)) {
>                         sec->include = 1;
>                         if (sec->secsym)
>                                 sec->secsym->include = 1;
> @@ -1352,6 +1352,19 @@ static void kpatch_include_standard_elements(struct 
> kpatch_elf *kelf)
>         list_entry(kelf->symbols.next, struct symbol, list)->include = 1;
>   }
>   
> +static void kpatch_include_standard_string_elements(struct kpatch_elf *kelf)
> +{
> +       struct section *sec;
> +
> +       list_for_each_entry(sec, &kelf->sections, list) {
> +               if (should_include_str_section(sec->name) && 
> is_referenced_section(sec, kelf)) {
> +                       sec->include = 1;
> +                       if (sec->secsym)
> +                               sec->secsym->include = 1;
> +               }
> +       }
> +}
I think it would be simpler to just amend the previous function rather 
than introducing a new one. E.g.
... || (should_include_str_section(sec->name) && 
is_referenced_section(sec, kelf))
Regards,
--
Ross Lagerwall

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