On Wed, Jul 23, 2014 at 08:03:47AM +0100, AKASHI Takahiro wrote:
On 07/23/2014 05:15 AM, Kees Cook wrote:
On Tue, Jul 22, 2014 at 2:14 AM, AKASHI Takahiro takahiro.akashi@linaro.org wrote:
asmlinkage int syscall_trace_enter(struct pt_regs *regs) {
unsigned long saved_x0, saved_x8;saved_x0 = regs->regs[0];saved_x8 = regs->regs[8];if (test_thread_flag(TIF_SYSCALL_TRACE)) tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);regs->syscallno = regs->regs[8];if ((long)regs->syscallno == ~0UL) { /* skip this syscall */regs->regs[8] = saved_x8;if (regs->regs[0] == saved_x0) /* not changed by user */regs->regs[0] = -ENOSYS;I'm not sure this is right compared to other architectures. Generally when a tracer performs a syscall skip, it's up to them to also adjust the return value. They may want to be faking a syscall, and what if the value they want to return happens to be what x0 was going into the tracer? It would have no way to avoid this -ENOSYS case. I think things are fine without this test.
Yeah, I know this issue, but was not sure that setting a return value is mandatory. (x86 seems to return -ENOSYS by default if not explicitly specified.) Is "fake a system call" a more appropriate word than "skip"?
I will defer to Will.
I agree with Kees -- iirc, I only suggested restoring x8.
Will