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Build flags: Improve description of -z defs remedies

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Florian Weimer 2018-01-23 11:42:25 +01:00
parent 078af19261
commit c873752b33
1 changed files with 30 additions and 12 deletions
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42
buildflags.md
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@ -44,7 +44,10 @@ It is possible to set RPM macros to change some aspects of the
compiler flags. Changing these flags should be used as a last
recourse if other workarunds are not available.
### Disable hardened builds
### Hardened builds
By default, the build flags enable fully hardened builds. To change
this, include this in the RPM spec file:
%undefine _hardened_build
@ -52,20 +55,38 @@ This turns off certain hardening features, as described in detail
below. The main difference is that executables will be
position-dependent (no full ASLR) and use lazy binding.
### Disable annotated builds/watermarking
### Annotated builds/watermarking
By default, the build flags cause a special output section to be
included in ELF files which describes certain aspects of the build.
To change this, include this in the RPM spec file:
%undefine _annotated_build
This turns off watermarking, making it impossible to do full hardening
coverage analysis for any binaries produced.
### Disable strict symbol checks in the link editor (ld)
### Strict symbol checks in the link editor (ld)
By default, the link editor will refuse to link shared objects which
contain undefined symbols. In some cases (such as when a DSO is
loaded as a plugin and is expected to bind to symbols in the main
executable), undefined symbols are expected. In this case, you can
add
contain undefined symbols. Such symbols lack symbol versioning
information and can be bound to the wrong (compatibility) symbol
version at run time, and not the actual (default) symbol version which
would have been used if the symbol definition had been available at
static link time. Furthermore, at run time, the dynamic linker will
not have complete dependency information (in the form of DT_NEEDED
entries), which can lead to errors (crashes) if IFUNC resolvers are
executed before the shared object containing them is fully relocated.
With the default flags, link failures will occur if the linker command
line does not list all shared objects which are needed. In this case,
you need to add the missing DSOs (with linker arguments such as
`-lm`). As a result, the link editor will also generated the
necessary DT_NEEDED entries.
In some cases (such as when a DSO is loaded as a plugin and is
expected to bind to symbols in the main executable), undefined symbols
are expected. In this case, you can add
%undefine _strict_symbol_defs_build
@ -227,11 +248,8 @@ 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).
* `-z defs`: Refuse to link shared objects (DSOs) with undefined symbols.
Such symbols lack symbol versioning information and can be bound to
the wrong (compatibility) symbol version at run time, and not the
actual (default) symbol version which would have been used if the
symbol definition had been available and static link time.
* `-z defs`: Refuse to link shared objects (DSOs) with undefined symbols
(see above).
For hardened builds, the
`-specs=/usr/lib/rpm/redhat/redhat-hardened-ld` flag is added to the