[PATCH 0/4 v3] arm64/ptrace: allow to get all registers on syscall traps

Andrei Vagin
2021-03-22T22:54:01Z

Here are two known problems with registers when a tracee is stopped in
syscall traps.

The first problem is about the x7 register that is used to indicate
whether or not the stop has been signalled from syscall entry or syscall
exit. This means that:

- Any writes by the tracer to this register during the stop are
ignored/discarded.

- The actual value of the register is not available during the stop,
so the tracer cannot save it and restore it later.

For applications like the user-mode Linux or gVisor, it is critical to
have access to the full set of registers in any moment. For example,
they need to change values of all registers to emulate rt_sigreturn or
execve and they need to have the full set of registers to build a signal
frame.

The second problem is that orig_x0 isn't exposed to user-space. orig_x0
is recorded at the start of the syscall entry and then it is used for
resetting the argument back to its original value during syscall
restarts.

This series extends the user_pt_regs structure with orig_x0 and orig_x7.
This doesn't break the backward compatibility. There are two interfaces
how user-space can receive user_pt_regs from the kernel. The first one
is PTRACE_{GET,SET}REGSET. In this case, the user-space provides a
buffer and its size and the kernel fills only the part that fits the
size. The second interface is a core dump file where registers are
written in a separate note and the user-space can parse only the part
that it knows.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Martin <Dave.Martin@arm.com>
Cc: Keno Fischer <keno@juliacomputing.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Will Deacon <will@kernel.org>

Andrei Vagin (4):
arm64: expose orig_x0 in the user_pt_regs structure
arm64/ptrace: introduce orig_x7 in the user_pt_regs structure
selftest/arm64/ptrace: add a test for orig_x0
selftest/arm64/ptrace: add a test for orig_x7

v2: use the ptrace option instead of adding a new regset.
v3: append orig_x0 and orig_x7 to the user_pt_regs.

arch/arm64/include/asm/ptrace.h | 5 +
arch/arm64/kernel/ptrace.c | 130 +++++++++++-----
include/uapi/linux/elf.h | 1 +
tools/testing/selftests/arm64/Makefile | 2 +-
tools/testing/selftests/arm64/ptrace/Makefile | 6 +
.../arm64/ptrace/ptrace_syscall_regs_test.c | 142 ++++++++++++++++++
6 files changed, 246 insertions(+), 40 deletions(-)
create mode 100644 tools/testing/selftests/arm64/ptrace/Makefile
create mode 100644 tools/testing/selftests/arm64/ptrace/ptrace_syscall_regs_test.c

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2.29.2