linux-kernel - Re: [PATCH v2 2.6.38-rc8-tip 11/20] 11: uprobes: slot allocation for uprobes

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Message-ID: <20110315131020.36477a1c@bike.lwn.net>
Date:	Tue, 15 Mar 2011 13:10:20 -0600
From:	Jonathan Corbet <corbet@....net>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux-mm <linux-mm@...ck.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	SystemTap <systemtap@...rces.redhat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2 2.6.38-rc8-tip 11/20] 11: uprobes: slot allocation
 for uprobes

Just a couple of minor notes while I was looking at this code...

> +static struct uprobes_xol_area *xol_alloc_area(void)
> +{
> +	struct uprobes_xol_area *area = NULL;
> +
> +	area = kzalloc(sizeof(*area), GFP_USER);
> +	if (unlikely(!area))
> +		return NULL;
> +
> +	area->bitmap = kzalloc(BITS_TO_LONGS(UINSNS_PER_PAGE) * sizeof(long),
> +								GFP_USER);

Why GFP_USER?  That causes extra allocation limits to be enforced.  Given
that in part 14 you have:

+/* Prepare to single-step probed instruction out of line. */
+static int pre_ssout(struct uprobe *uprobe, struct pt_regs *regs,
+				unsigned long vaddr)
+{
+	xol_get_insn_slot(uprobe, vaddr);
+	BUG_ON(!current->utask->xol_vaddr);

It seems to me that you really don't want those allocations to fail.

back to xol_alloc_area():

> +	if (!area->bitmap)
> +		goto fail;
> +
> +	spin_lock_init(&area->slot_lock);
> +	if (!xol_add_vma(area) && !current->mm->uprobes_xol_area) {
> +		task_lock(current);
> +		if (!current->mm->uprobes_xol_area) {
> +			current->mm->uprobes_xol_area = area;
> +			task_unlock(current);
> +			return area;
> +		}
> +		task_unlock(current);
> +	}
> +
> +fail:
> +	if (area) {
> +		if (area->bitmap)
> +			kfree(area->bitmap);
> +		kfree(area);
> +	}

You've already checked area against NULL, and kfree() can handle null
pointers, so both of those tests are unneeded.

> +	return current->mm->uprobes_xol_area;
> +}

jon
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