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Re: [Xen-devel] [livepatch-build-tools part3 v2 3/3] create-diff-object: Strip all undefined entires of known size

On 8/8/19 1:51 PM, Pawel Wieczorkiewicz wrote:
The patched ELF object file contains all sections and symbols as
resulted from the compilation. However, certain symbols may not be
copied over to the resulting object file, due to being unchanged or
not included for other reasons.
In such situation the resulting object file has the entire sections
copied along (with all their entries unchanged), while some of the
corresponding symbols are not copied along at all.
This leads to having incorrect undefined (STN_UNDEF) entries in the
final hotpatch ELF file.

The newly added function livepatch_strip_undefined_elements() detects
and removes all undefined RELA entries as well as their corresponding
PROGBITS section entries.
Since the sections may contain elements of unknown size (sh.sh_entsize
== 0), perform the strip only on sections with well defined entry

After replacing the stripped rela list, it is assumed that the next
invocation of the kpatch_rebuild_rela_section_data() will adjust all
section header parameters according to the current state.

The code in this patch seems to be very similar (i.e. somewhat copy-and-pasted) to kpatch_regenerate_special_section() which prunes the rela list and rebuilds the corresponding text section according to the predicate should_keep_rela_group(). The intent of the function also seems to be the same (only keep elements that are needed).

In what situations does the existing function not do the right thing?

Can should_keep_rela_group() be updated instead?

Ross Lagerwall

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