Re: [PATCH] arm: always update thread_info->syscall - Russell King

From: Russell King - ARM Linux <linux@armlinux.org.uk>
To: Rafael David Tinoco <rafael.tinoco@linaro.org>
Cc: Vladimir Murzin <vladimir.murzin@arm.com>,
	Vincent Whitchurch <vincent.whitchurch@axis.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	linux-kernel@vger.kernel.org, stable@vger.kernel.org,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Timothy E Baldwin <T.E.Baldwin99@members.leeds.ac.uk>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arm: always update thread_info->syscall
Date: Mon, 26 Nov 2018 23:44:56 +0000	[thread overview]
Message-ID: <20181126234456.GB30658@n2100.armlinux.org.uk> (raw)
In-Reply-To: <20181126234111.GA30658@n2100.armlinux.org.uk>

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.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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