linux-kernel - Re: [PATCH] signal/x86: Delay calling signals in atomic

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Date:   Mon, 04 Apr 2022 09:29:10 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Oleg Nesterov <oleg@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ben Segall <bsegall@...gle.com>,
        Borislav Petkov <bp@...en8.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Mel Gorman <mgorman@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH] signal/x86: Delay calling signals in atomic

Sebastian Andrzej Siewior <bigeasy@...utronix.de> writes:

> On 2022-03-30 13:10:05 [-0500], Eric W. Biederman wrote:
>> But it looks like if we are coming from userspace then we use the same
>> stack as any other time we would come from userspace.  AKA a stack
>> that allows the kernel to sleep.
>> 
>> So I don't see what the problem is that is trying to be fixed.
>
> It is not only the stack. In atomic context / disabled interrupts it is
> not possible to acquire a spinlock_t (sighand_struct::siglock) which is
> done later.

Looking at do_int3_user the interrupts must be enabled.
>
>> I know that code has been changed over the years, perhaps this is
>> something that was fixed upstream and the real time tree didn't realize
>> there was no longer a need to fix anything?
>> 
>> Or am I missing something subtle when reading the idtentry assembly?
>
> It certainly is true that the code changed over the years. The per-CPU
> stack is one problem, the siglock in atomic context is the other one.
> Thank you for the input. Let me digest the informations I have here and
> get back.

Certainly.  I case it helps this is the relevant bit of code:

static void do_int3_user(struct pt_regs *regs)
{
	if (do_int3(regs))
		return;

	cond_local_irq_enable(regs);
	do_trap(X86_TRAP_BP, SIGTRAP, "int3", regs, 0, 0, NULL);
	cond_local_irq_disable(regs);
}

The signal delivery where siglock is take happens inside of do_trap.  If
we are coming from kernel mode only do_int3 is called.

Coming from user_mode we switch to the task stack and
enable interrupts.

Unless I am misreading the code the cond_local_irq_{enable/disable} can
correctly be replaced local_irq_{enable/disable} as coming from user
mode interrupts are always enabled.

Unless I am misreading cond_local_irq_enable.  If for some reason
cond_local_irq_enable doesn't enable interrupts when come from user
mode fixing that appears to be the fix that is needed.


Eric