pstore/ram: Fix undefined usage of rounddown_pow_of_two(0)

record_size / console_size / ftrace_size can be 0 (this is how you disable
the feature), but rounddown_pow_of_two(0) is undefined. As suggested by
Kees Cook, use !is_power_of_2() as a condition to call
rounddown_pow_of_two and avoid its undefined behavior on the value 0. This
issue has been present since commit 1894a25 (ramoops: Move to
fs/pstore/ram.c).

Cc: stable@vger.kernel.org
Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Florian Fainelli <ffainelli@freebox.fr>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>

fs/pstore/ram.c

@@ -376,10 +376,14 @@ static int __devinit ramoops_probe(struct platform_device *pdev)
 		goto fail_out;
 	}
 
-	pdata->mem_size = rounddown_pow_of_two(pdata->mem_size);
-	pdata->record_size = rounddown_pow_of_two(pdata->record_size);
-	pdata->console_size = rounddown_pow_of_two(pdata->console_size);
-	pdata->ftrace_size = rounddown_pow_of_two(pdata->ftrace_size);
+	if (!is_power_of_2(pdata->mem_size))
+		pdata->mem_size = rounddown_pow_of_two(pdata->mem_size);
+	if (!is_power_of_2(pdata->record_size))
+		pdata->record_size = rounddown_pow_of_two(pdata->record_size);
+	if (!is_power_of_2(pdata->console_size))
+		pdata->console_size = rounddown_pow_of_two(pdata->console_size);
+	if (!is_power_of_2(pdata->ftrace_size))
+		pdata->ftrace_size = rounddown_pow_of_two(pdata->ftrace_size);
 
 	cxt->dump_read_cnt = 0;
 	cxt->size = pdata->mem_size;