[ELF] Align the end of PT_GNU_RELRO associated PT_LOAD to a common-page-size boundary (#66042)
Close #57618: currently we align the end of PT_GNU_RELRO to a
common-page-size
boundary, but do not align the end of the associated PT_LOAD. This is
benign
when runtime_page_size >= common-page-size.
However, when runtime_page_size < common-page-size, it is possible that
`alignUp(end(PT_LOAD), page_size) < alignDown(end(PT_GNU_RELRO),
page_size)`.
In this case, rtld's mprotect call for PT_GNU_RELRO will apply to
unmapped
regions and lead to an error, e.g.
```
error while loading shared libraries: cannot apply additional memory protection after relocation: Cannot allocate memory
```
To fix the issue, add a padding section .relro_padding like mold, which
is contained in the PT_GNU_RELRO segment and the associated PT_LOAD
segment. The section also prevents strip from corrupting PT_LOAD program
headers.
.relro_padding has the largest `sortRank` among RELRO sections.
Therefore, it is naturally placed at the end of `PT_GNU_RELRO` segment
in the absence of `PHDRS`/`SECTIONS` commands.
In the presence of `SECTIONS` commands, we place .relro_padding
immediately before a symbol assignment using DATA_SEGMENT_RELRO_END (see
also https://reviews.llvm.org/D124656), if present.
DATA_SEGMENT_RELRO_END is changed to align to max-page-size instead of
common-page-size.
Some edge cases worth mentioning:
* ppc64-toc-addis-nop.s: when PHDRS is present, do not append
.relro_padding
* avoid-empty-program-headers.s: when the only RELRO section is .tbss,
it is not part of PT_LOAD segment, therefore we do not append
.relro_padding.
---
Close #65002: GNU ld from 2.39 onwards aligns the end of PT_GNU_RELRO to
a
max-page-size boundary (https://sourceware.org/PR28824) so that the last
page is
protected even if runtime_page_size > common-page-size.
In my opinion, losing protection for the last page when the runtime page
size is
larger than common-page-size is not really an issue. Double mapping a
page of up
to max-common-page for the protection could cause undesired VM waste.
Internally
we had users complaining about 2MiB max-page-size applying to shared
objects.
Therefore, the end of .relro_padding is padded to a common-page-size
boundary. Users who are really anxious can set common-page-size to match
their runtime page size.
---
17 tests need updating as there are lots of change detectors.
diff --git a/lld/docs/ELF/linker_script.rst b/lld/docs/ELF/linker_script.rst
index bc20375..fbd96ab 100644
--- a/lld/docs/ELF/linker_script.rst
+++ b/lld/docs/ELF/linker_script.rst
@@ -172,3 +172,18 @@
still applies (possibly after orphan section placement). It is recommended to
leave the brace empty (i.e. ``section : {}``) for the insert command, because
its description will be ignored anyway.
+
+Built-in functions
+~~~~~~~~~~~~~~~~~~
+
+``DATA_SEGMENT_RELRO_END(offset, exp)`` defines the end of the ``PT_GNU_RELRO``
+segment when ``-z relro`` (default) is in effect. Sections between
+``DATA_SEGMENT_ALIGN`` and ``DATA_SEGMENT_RELRO_END`` are considered RELRO.
+
+The typical use case is ``. = DATA_SEGMENT_RELRO_END(0, .);`` followed by
+writable but non-RELRO sections. LLD ignores ``offset`` and ``exp`` and aligns
+the current location to a max-page-size boundary, ensuring that the next
+``PT_LOAD`` segment will not overlap with the ``PT_GNU_RELRO`` segment.
+
+LLD will insert ``.relro_padding`` immediately before the symbol assignment
+using ``DATA_SEGMENT_RELRO_END``.
