This patch series is only a refactoring of the existing implementation of PAuth
and returned-address signing. The existing behavior is preserved.
1. aarch64: store signing key and signing method in DWARF _Unwind_FrameState
_Unwind_FrameState already contains several CIE and FDE information (see the
attributes below the comment "The information we care about from the CIE/FDE"
in libgcc/unwind-dw2.h).
The patch aims at moving the information from DWARF CIE (signing key stored in
the augmentation string) and FDE (the used signing method) into
_Unwind_FrameState along the already-stored CIE and FDE information.
Note: those information have to be saved in frame_state_reg_info instead of
_Unwind_FrameState as they need to be savable by DW_CFA_remember_state and
restorable by DW_CFA_restore_state, that both rely on the attribute "prev".
Those new information in _Unwind_FrameState simplifies the look-up of the
signing key when the return address is demangled. It also allows future signing
methods to be easily added.
_Unwind_FrameState is not a part of the public API of libunwind, so the change
is backward compatible.
A new architecture-specific handler MD_ARCH_EXTENSION_FRAME_INIT allows to
reset values in the frame state and unwind context if needed by the
architecture extension before changing the frame state to the caller context.
A new architecture-specific handler MD_ARCH_EXTENSION_CIE_AUG_HANDLER isolates
the architecture-specific augmentation strings in AArch64 backend, and allows
others architectures to reuse augmentation strings that would have clashed with
AArch64 DWARF extensions.
aarch64_demangle_return_addr, DW_CFA_AARCH64_negate_ra_state and
DW_CFA_val_expression cases in libgcc/unwind-dw2-execute_cfa.h were documented
to clarify where the value of the RA state register is stored (FS and CONTEXT
respectively).
2. aarch64: skip copy of RA state register into target context
The RA state register is local to a frame, so it should not be copied to the
target frame during the context installation.
This patch adds a new backend handler that check whether a register needs to be
skipped or not before its installation.
3. libgcc: hide CIE and FDE data for DWARF architecture extensions behind a
handler.
This patch provides a new handler MD_ARCH_FRAME_STATE_T to hide an
architecture-specific structure containing CIE and FDE data related to DWARF
architecture extensions.
Hiding the architecture-specific attributes behind a handler has the following
benefits:
1. isolating those data from the generic ones in _Unwind_FrameState
2. avoiding casts to custom types.
3. preserving typing information when debugging with GDB, and so
facilitating their printing.
This approach required to add a new header md-unwind-def.h included at the top
of libgcc/unwind-dw2.h, and redirecting to the corresponding architecture
header via a symbolic link.
An obvious drawback is the increase in complexity with macros, and headers. It
also caused a split of architecture definitions between md-unwind-def.h (types
definitions used in unwind-dw2.h) and md-unwind.h (local types definitions and
handlers implementations).
The naming of md-unwind.h with .h extension is a bit misleading as the file is
only included in the middle of unwind-dw2.c. Changing this naming would require
modification of others backends, which I prefered to abstain from.
Overall the benefits are worth the added complexity from my perspective.
4. libgcc: update configure (regenerated by autoreconf)
Regenerate the build files.
Note: This patch should be squashed with the previous one before merging.
### Diff between revisions 1 & 2
1: code style formatting + remove resetting of the local frame register in the
context as recommended by Richard Sandiford.
2: add a new handler as recommended by Richard Sandiford to skip the local
frame registers before installation in the target context.
3: code style formatting.
4: no change.
## Testing
Those changes were testing by covering the 3 following cases:
- backtracing.
- exception handling in a C++ program.
- gcc/testsuite/gcc.target/aarch64/pr104689.c: pac-ret with unusual DWARF [1]
Regression tested on aarch64-unknown-linux-gnu, and no regression found.
[1]: https://gcc.gnu.org/pipermail/gcc-patches/2022-May/594414.html
Ok for master? I don't have commit access so I need someone to commit on my
behalf.
Regards,
Matthieu.
Matthieu Longo (4):
aarch64: store signing key and signing method in DWARF
_Unwind_FrameState
aarch64: skip copy of RA state register into target context
libgcc: hide CIE and FDE data for DWARF architecture extensions behind
a handler.
libgcc: update configure (regenerated by autoreconf)
libgcc/Makefile.in | 6 +-
libgcc/config.host | 13 +-
libgcc/config/aarch64/aarch64-unwind-def.h | 41 ++++++
libgcc/config/aarch64/aarch64-unwind.h | 152 +++++++++++++++++----
libgcc/config/no-unwind.h | 3 +-
libgcc/configure | 2 +
libgcc/configure.ac | 1 +
libgcc/unwind-dw2-execute_cfa.h | 26 ++--
libgcc/unwind-dw2.c | 24 +++-
libgcc/unwind-dw2.h | 19 ++-
10 files changed, 235 insertions(+), 52 deletions(-)
create mode 100644 libgcc/config/aarch64/aarch64-unwind-def.h
--
2.46.0
