Our tracehook logic for syscall entry/exit raises a SIGTRAP back to the tracer following a ptrace request such as PTRACE_SYSCALL. As part of this procedure, we clobber the reported value of one of the tracee's general purpose registers (x7 for native tasks, r12 for compat) to indicate whether the stop occurred on syscall entry or exit. This is a slightly unfortunate ABI, as it prevents the tracer from accessing the real register value and is at odds with other similar stops such as seccomp traps.

Since we're stuck with this ABI, expand the comment in our tracehook logic to acknowledge the issue and descibe the behaviour in more detail.

Cc: stable@vger.kernel.org Cc: Mark Rutland mark.rutland@arm.com Cc: Luis Machado luis.machado@linaro.org Reported-by: Keno Fischer keno@juliacomputing.com Signed-off-by: Will Deacon will@kernel.org --- arch/arm64/kernel/ptrace.c | 16 ++++++++++++++-- 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c index 68b7f34a08f5..d71795dc3dbd 100644 --- a/arch/arm64/kernel/ptrace.c +++ b/arch/arm64/kernel/ptrace.c @@ -1811,8 +1811,20 @@ static void tracehook_report_syscall(struct pt_regs *regs, unsigned long saved_reg;

/* - * A scratch register (ip(r12) on AArch32, x7 on AArch64) is - * used to denote syscall entry/exit: + * We have some ABI weirdness here in the way that we handle syscall + * exit stops because we indicate whether or not the stop has been + * signalled from syscall entry or syscall exit by clobbering a general + * purpose register (ip/r12 for AArch32, x7 for AArch64) in the tracee + * and restoring its old value after the stop. 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. + * + * - Syscall stops behave differently to seccomp and pseudo-step traps + * (the latter do not nobble any registers). */ regno = (is_compat_task() ? 12 : 7); saved_reg = regs->regs[regno];