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Date: Thu, 28 Sep 2017 09:06:08 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Ananth N Mavinakayanahalli <ananth@linux.vnet.ibm.com>,
        Anton Blanchard <anton@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 3/3] kernel/uprobes: Fix check for active uprobe
Message-ID: <20170928070608.danalxb2obvhav4z@gmail.com>
References: <cover.1506061547.git.naveen.n.rao@linux.vnet.ibm.com>
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 <20170925161619.GA15325@redhat.com>
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* Oleg Nesterov <oleg@redhat.com> wrote:

> On 09/22, Naveen N. Rao wrote:
> >
> > --- a/kernel/events/uprobes.c
> > +++ b/kernel/events/uprobes.c
> > @@ -1751,6 +1751,19 @@ static struct uprobe *find_active_uprobe(unsigned long bp_vaddr, int *is_swbp)
> >  			uprobe = find_uprobe(inode, offset);
> >  		}
> >  
> > +		/* Ensure that the breakpoint was actually installed */
> > +		if (uprobe) {
> > +			/*
> > +			 * TODO: move copy_insn/etc into _register and remove
> > +			 * this hack.  After we hit the bp, _unregister +
> > +			 * _register can install the new and not-yet-analyzed
> > +			 * uprobe at the same address, restart.
> > +			 */
> > +			smp_rmb(); /* pairs with wmb() in prepare_uprobe() */
> > +			if (unlikely(!test_bit(UPROBE_COPY_INSN, &uprobe->flags)))
> > +				uprobe = NULL;
> > +		}
> 
> ACK ...
> 
> but the comment is no longer valid, it only mentions the race  unregister +
> register.
> 
> And note that "restart" is not true in that we are not going to simply restart,
> we will check is_trap_at_addr() and then either send SIGTRAP or restart.
> 
> This is correct because we do this check under mmap_sem so we can't race with
> install_breakpoint(), so is_trap_at_addr() == T can't be falsely true if
> UPROBE_COPY_INSN is not set.
> 
> And btw, perhaps you should do this check right after find_uprobe() in the
> if (valid_vma) block.

Ok guys - I'll wait for the next iteration that incorporates Oleg's suggestions.

Thanks,

	Ingo