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Sami Tolvanen <samitolvanen@google.com> says: This series adds KCFI support for RISC-V. KCFI is a fine-grained forward-edge control-flow integrity scheme supported in Clang >=16, which ensures indirect calls in instrumented code can only branch to functions whose type matches the function pointer type, thus making code reuse attacks more difficult. Patch 1 implements a pt_regs based syscall wrapper to address function pointer type mismatches in syscall handling. Patches 2 and 3 annotate indirectly called assembly functions with CFI types. Patch 4 implements error handling for indirect call checks. Patch 5 disables CFI for arch/riscv/purgatory. Patch 6 finally allows CONFIG_CFI_CLANG to be enabled for RISC-V. Note that Clang 16 has a generic architecture-agnostic KCFI implementation, which does work with the kernel, but doesn't produce a stable code sequence for indirect call checks, which means potential failures just trap and won't result in informative error messages. Clang 17 includes a RISC-V specific back-end implementation for KCFI, which emits a predictable code sequence for the checks and a .kcfi_traps section with locations of the traps, which patch 5 uses to produce more useful errors. The type mismatch fixes and annotations in the first three patches also become necessary in future if the kernel decides to support fine-grained CFI implemented using the hardware landing pad feature proposed in the in-progress Zicfisslp extension. Once the specification is ratified and hardware support emerges, implementing runtime patching support that replaces KCFI instrumentation with Zicfisslp landing pads might also be feasible (similarly to KCFI to FineIBT patching on x86_64), allowing distributions to ship a unified kernel binary for all devices. * b4-shazam-merge: riscv: Allow CONFIG_CFI_CLANG to be selected riscv/purgatory: Disable CFI riscv: Add CFI error handling riscv: Add ftrace_stub_graph riscv: Add types to indirectly called assembly functions riscv: Implement syscall wrappers Link: https://lore.kernel.org/r/20230710183544.999540-8-samitolvanen@google.com Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.