[ELF] adjustOutputSections: don't copy SHF_EXECINSTR when an output does not contain input sections (#70911)
For an output section with no input section, GNU ld eliminates the
output section when there are only symbol assignments (e.g.
`.foo : { symbol = 42; }`) but not for `.foo : { . += 42; }`
(`SHF_ALLOC|SHF_WRITE`).
We choose to retain such an output section with a symbol assignment
(unless unreferenced `PROVIDE`). We copy the previous section flag (see
https://reviews.llvm.org/D37736) to hopefully make the current PT_LOAD
segment extend to the current output section:
* decrease the number of PT_LOAD segments
* If a new PT_LOAD segment is introduced without a page-size
alignment as a separator, there may be a run-time crash.
However, this `flags` copying behavior is not suitable for
`.foo : { . += 42; }` when `flags` contains `SHF_EXECINSTR`. The
executable bit is surprising
(https://discourse.llvm.org/t/lld-output-section-flag-assignment-behavior/74359).
I think we should drop SHF_EXECINSTR when copying `flags`. The risk is a
code section followed by `.foo : { symbol = 42; }` will be broken, which
I believe is unrelated as such uses are almost always related to data
sections.
For data-command-only output sections (e.g. `.foo : { QUAD(42) }`), we
keep allowing copyable SHF_WRITE.
Some tests are updated to drop the SHF_EXECINSTR flag. GNU ld doesn't
set SHF_EXECINSTR as well, though it sets SHF_WRITE for some tests while
we don't.diff --git a/lld/docs/ELF/linker_script.rst b/lld/docs/ELF/linker_script.rst
index fbd96ab..3606ef4 100644
--- a/lld/docs/ELF/linker_script.rst
+++ b/lld/docs/ELF/linker_script.rst
@@ -97,6 +97,16 @@
GNU ld from Binutils 2.35 onwards will reduce sh_addralign so that
sh_addr=0 (modulo sh_addralign).
+When an output section has no input section, GNU ld will eliminate it if it
+only contains symbol assignments (e.g. ``.foo { symbol = 42; }``). LLD will
+retain such sections unless all the symbol assignments are unreferenced
+``PROVIDED``.
+
+When an output section has no input section but advances the location counter,
+GNU ld sets the ``SHF_WRITE`` flag. LLD sets the SHF_WRITE flag only if the
+preceding output section with non-empty input sections also has the SHF_WRITE
+flag.
+
Output section type
-------------------
