linux-kernel - Re: [PATCH] arm: always update thread

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Date:   Tue, 27 Nov 2018 08:30:32 -0200
From:   Rafael David Tinoco <rafael.tinoco@...aro.org>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Rafael David Tinoco <rafael.tinoco@...aro.org>,
        Vladimir Murzin <vladimir.murzin@....com>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Timothy E Baldwin <T.E.Baldwin99@...bers.leeds.ac.uk>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm: always update thread_info->syscall

On 11/26/18 9:44 PM, Russell King - ARM Linux wrote:
> On Mon, Nov 26, 2018 at 11:41:11PM +0000, Russell King - ARM Linux wrote:
>> On Mon, Nov 26, 2018 at 11:33:03PM +0000, Russell King - ARM Linux wrote:
>>> On Mon, Nov 26, 2018 at 08:53:35PM -0200, Rafael David Tinoco wrote:
>>>> Right now, only way for task->thread_info->syscall to be updated is if
>>>> if _TIF_SYSCALL_WORK is set in current's task thread_info->flags
>>>> (similar to what has_syscall_work() checks for arm64).
>>>>
>>>> This means that "->syscall" will only be updated if we are tracing the
>>>> syscalls through ptrace, for example. This is NOT the same behavior as
>>>> arm64, when pt_regs->syscallno is updated in the beginning of svc0
>>>> handler for *every* syscall entry.
>>>
>>> So when was it decided that the syscall number will always be required
>>> (we need it to know how far back this has to be backported).
>>
>> PS, I rather object to the fact that the required behaviour seems to
>> change, arch maintainers aren't told about it until... some test is
>> created at some random point in the future which then fails.
>>
>> Surely there's a better way to communicate changes in requirements
>> than discovery-by-random-bug-report ?
> 
> Final comment for tonight - the commit introducing /proc/*/syscall says:
> 
>      This adds /proc/PID/syscall and /proc/PID/task/TID/syscall magic files.
>      These use task_current_syscall() to show the task's current system call
>      number and argument registers, stack pointer and PC.  For a task blocked
>      but not in a syscall, the file shows "-1" in place of the syscall number,
>      followed by only the SP and PC.  For a task that's not blocked, it shows
>      "running".
> 
> Please validate that a blocked task does indeed show -1 with your patch
> applied.

Will do. This is done in an upper level (collect_syscall <- 
task_current_syscall <- proc_pid_syscall):

	if (!try_get_task_stack(target)) {
		/* Task has no stack, so the task isn't in a syscall. */
		*sp = *pc = 0;
		*callno = -1;
		return 0;
	}

I think only missing part for arm was that one, but will confirm, after 
fixing usage of "r7" for obtaining "scno". Will send a v2 in this thread.

Thanks!

-- 
Rafael D. Tinoco
Linaro Kernel Validation