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buildflags.md: Document _ld_as_needed, --as-needed

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From Fedora commit 92959df563.

Related: #2005080
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Florian Weimer 2021-11-15 15:21:22 +01:00
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@ -116,6 +116,21 @@ recognized, so it has to come after the hardening flags on the command
line (it has to be added at the end of `CFLAGS`, or specified after
the `CFLAGS` variable contents).
### Keeping dependencies on unused shared objects
By default, ELF shared objects which are listed on the linker command
line, but which have no referencing symbols in the preceding objects,
are not added to the output file during the final link.
In order to keep dependencies on shared objects even if none of
their symbols are used, include this in the RPM spec file:
%undefine _ld_as_needed
For example, this can be required if shared objects are used for their
side effects in ELF constructors, or for making them available to
dynamically loaded plugins.
### Strict symbol checks in the link editor (ld)
Optionally, the link editor will refuse to link shared objects which
@ -325,6 +340,10 @@ to the compiler driver `gcc`, and not directly to the link editor
dynamic linker is instructed to revoke write permissions after
dynamic linking. Full protection of relocation data requires the
`-z now` flag (see below).
* `--as-needed`: In the final link, only generate ELF dependencies
for shared objects that actually provide symbols required by the link.
Shared objects which are not needed to fulfill symbol dependencies
are essentially ignored due to this flag.
* `-z defs`: Refuse to link shared objects (DSOs) with undefined symbols
(optional, see above).