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Date: Mon, 25 Sep 2017 18:16:19 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>,
        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: <20170925161619.GA15325@redhat.com>
References: <cover.1506061547.git.naveen.n.rao@linux.vnet.ibm.com>
 <6718c4ebec1d71524bb81c907180bad90fc8d3c3.1506061547.git.naveen.n.rao@linux.vnet.ibm.com>
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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but the comment is no longer valid, it only mentions the race  unregister +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.

Oleg.