linux-kernel - [RFC PATCH 0/2] riscv: Expose orig_a0 to userspace for ptrace to set the actual a0

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Date: Wed, 19 Jun 2024 10:00:37 +0800
From: zhouquan@...as.ac.cn
To: linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org
Cc: oleg@...hat.com,
	paul.walmsley@...ive.com,
	palmer@...belt.com,
	aou@...s.berkeley.edu,
	andy.chiu@...ive.com,
	shuah@...nel.org,
	charlie@...osinc.com,
	zhouquan@...as.ac.cn
Subject: [RFC PATCH 0/2] riscv: Expose orig_a0 to userspace for ptrace to set the actual a0

From: Quan Zhou <zhouquan@...as.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's original a0 value. For the current
kernel implementation of ptrace, projects like CRIU/Proot will
encounter issues where the a0 setting becomes ineffective when
performing ptrace(PTRACE_{SET/GET}REGSET,).

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/

Quan Zhou (2):
  riscv: Expose orig_a0 in the user_regs_struct structure
  riscv: selftests: Add a ptrace test to check a0 of restarted syscall

 arch/riscv/include/asm/ptrace.h               |   4 +-
 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 ++
 .../riscv/abi/ptrace_restart_syscall.c        | 148 ++++++++++++++++++
 6 files changed, 166 insertions(+), 3 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_restart_syscall.c

base-commit: 6ba59ff4227927d3a8530fc2973b80e94b54d58f
-- 
2.34.1