linux-kernel - Re: [PATCH v3 3/3] x86/ptrace, x86/signal: Remove TS_I386_REGS

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Message-ID: <CALCETrUn_=1Jwj7VcHUpo9r1tp+k3fyx7ngaTrSMF5nM3OFyoQ@mail.gmail.com>
Date:	Thu, 23 Jun 2016 14:53:18 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Pedro Alves <palves@...hat.com>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v3 3/3] x86/ptrace, x86/signal: Remove TS_I386_REGS_POKED

On Thu, Jun 23, 2016 at 2:26 PM, Oleg Nesterov <oleg@...hat.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