Re: [PATCH v3 3/3] x86/ptrace, x86/signal: Remove TS_I386_REGS_POKED

Andy Lutomirski
2016-06-23T21:53:40Z

On Thu, Jun 23, 2016 at 2:26 PM, Oleg Nesterov <oleg@redhat.com> wrote:
> Again, I think the patch is fine, just a question
>
> On 06/20, Andy Lutomirski wrote:
>>
>> System call restart has some oddities wrt ptrace:
>>
>> 1. For whatever reason, the kernel delivers signals and triggers
>> ptrace before handling syscall restart. This means that
>> -ERESTART_RESTARTBLOCK, etc is visible to userspace. We could
>> plausibly get away with changing that, but it seems quite risky.
>
> How we can change this?
>
> The kernel simply can't know how it should react to (say) -ERESTARTSYS
> until debugger acks/nacks/changes the signal reported by tracee.

Hmm, good point. I don't know whether our current behavior is fully
correct or whether we could change it.

>
>> + /*
>> + * A 32-bit ptracer has the following expectations:
>> + *
>> + * - Storing -1 (i.e. 0xffffffff) to orig_eax will prevent
>> + * syscall restart handling.
>> + *
>> + * - Restoring regs saved on exit from an interrupted
>> + * restartable syscall will trigger syscall restart. Such
>> + * regs will have non-negative orig_eax and negative eax.
>> + *
>> + * The kernel's syscall restart code treats regs->orig_ax and
>> + * regs->ax as 64-bit signed quantities. 32-bit user code
>> + * doesn't care about the high bits. Keep it simple and just
>> + * sign-extend both values.
>> + */
>> + R32_SIGNED(orig_eax, orig_ax);
>> + R32_SIGNED(eax, ax);
>
> OK. but do we really need R32_SIGNED(orig_eax) ? syscall_get_nr()
> returns "int", not "long".

Fair enough, maybe we don't. I'll drop that part and just keep
R32_SIGNED(eax, ax).

--Andy