Re: [PATCH 1/3] arm64: ptrace: Add is_syscall_success to handle compat

On Thu, Apr 22, 2021 at 05:07:53PM +0100, Will Deacon wrote: > On Wed, Apr 21, 2021 at 06:10:05PM +0100, Mark Rutland wrote: > > On Mon, Apr 19, 2021 at 01:19:33PM +0100, Will Deacon wrote: > > > On Fri, Apr 16, 2021 at 02:34:41PM +0100, Mark Rutland wrote: > > > > I think this is a problem we created for ourselves back in commit: > > > > > > > > 15956689a0e60aa0 ("arm64: compat: Ensure upper 32 bits of x0 are zero on syscall return) > > > > > > > > AFAICT, the perf regs samples are the only place this matters, since for > > > > ptrace the compat regs are implicitly truncated to compat_ulong_t, and > > > > audit expects the non-truncated return value. Other architectures don't > > > > truncate here, so I think we're setting ourselves up for a game of > > > > whack-a-mole to truncate and extend wherever we need to. > > > > > > > > Given that, I suspect it'd be better to do something like the below. > > > > > > > > Will, thoughts? > > > > > > I think perf is one example, but this is also visible to userspace via the > > > native ptrace interface and I distinctly remember needing this for some > > > versions of arm64 strace to work correctly when tracing compat tasks. > > > > FWIW, you've convinced me on your approach (more on that below), but > > when I went digging here this didn't seem to be exposed via ptrace -- > > for any task tracing a compat task, the GPRs are exposed via > > compat_gpr_{get,set}(), which always truncate to compat_ulong_t, giving > > the lower 32 bits. See task_user_regset_view() for where we get the > > regset. > > > > Am I missing something, or are you thinking of another issue you fixed > > at the same time? > > I think it may depend on whether strace pokes at the GPRs or instead issues > a PTRACE_GET_SYSCALL_INFO request but I've forgotten the details, > unfortunately. I do remember seeing an issue though, and it was only last > year. Ah; I hadn't spotted PTRACE_GET_SYSCALL_INFO, thanks for the pointer. I see that gets at the regs via syscall_get_arguments(), which doesn't truncate them. That makes me wonder whether x86 and others do the right thing here...