From: Quan Zhou zhouquan@iscas.ac.cn
Due to the path that modifies a0 in syscall_enter_from_user_mode before the actual execution of syscall_handler [1], the kernel currently saves a0 to orig_a0 at the entry point of do_trap_ecall_u as an original copy of a0. Once the syscall is interrupted and later resumed, the restarted syscall will use orig_a0 to continue execution.
The above rules generally apply except for ptrace(PTRACE_SETREGSET,), where the kernel will ignore the tracer's setting of tracee/a0 and will restart with the tracee/orig_a0. For the current kernel implementation of ptrace, projects like CRIU/Proot will encounter issues where the a0 setting becomes ineffective when performing ptrace(PTRACE_SETREGSET,).
Here is a suggested solution, expose orig_a0 to userspace so that ptrace can choose whether to set orig_a0 based on the actual scenario. In fact, x86/orig_eax and loongArch/orig_a0 have adopted similar solutions.
[1] link: https://lore.kernel.org/lkml/20230403-crisping-animosity-04ed8a45c625@spud/T...
--- Changes from RFC->v1: - Rebased on Linux 6.10-rc5. - Updated the patch description. - Adjust MAX_REG_OFFSET to match the new bottom of pt_regs (Charlie). - Simplify selftest to verify if a0 can be set (Charlie). - Fix .gitignore error (Charlie).
--- RFC link: https://lore.kernel.org/all/cover.1718693532.git.zhouquan@iscas.ac.cn/
Quan Zhou (2): riscv: Expose orig_a0 in the user_regs_struct structure riscv: selftests: Add a ptrace test to verify syscall parameter modification
arch/riscv/include/asm/ptrace.h | 7 +- arch/riscv/include/uapi/asm/ptrace.h | 2 + tools/testing/selftests/riscv/Makefile | 2 +- tools/testing/selftests/riscv/abi/.gitignore | 1 + tools/testing/selftests/riscv/abi/Makefile | 12 ++ tools/testing/selftests/riscv/abi/ptrace.c | 124 +++++++++++++++++++ 6 files changed, 144 insertions(+), 4 deletions(-) create mode 100644 tools/testing/selftests/riscv/abi/.gitignore create mode 100644 tools/testing/selftests/riscv/abi/Makefile create mode 100644 tools/testing/selftests/riscv/abi/ptrace.c
base-commit: f2661062f16b2de5d7b6a5c42a9a5c96326b8454