x86, ds: allow small debug store buffers

Check the buffer size more precisely to allow buffers for exactly
one element provided the base address is already properly aligned.

Add a debug store selftest.

Reported-by: Stephane Eranian <eranian@googlemail.com>
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
Cc: roland@redhat.com
Cc: eranian@googlemail.com
Cc: oleg@redhat.com
Cc: juan.villacis@intel.com
Cc: ak@linux.jf.intel.com
LKML-Reference: <20090403144606.139137000@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/ds.c

@@ -656,6 +656,7 @@ static int ds_request(struct ds_tracer *tracer, struct ds_trace *trace,
 {
 	struct ds_context *context;
 	int error;
+	size_t req_size;
 
 	error = -EOPNOTSUPP;
 	if (!ds_cfg.sizeof_rec[qual])
@@ -665,9 +666,13 @@ static int ds_request(struct ds_tracer *tracer, struct ds_trace *trace,
 	if (!base)
 		goto out;
 
-	/* We need space for alignment adjustments in ds_init_ds_trace(). */
+	req_size = ds_cfg.sizeof_rec[qual];
+	/* We might need space for alignment adjustments. */
+	if (!IS_ALIGNED((unsigned long)base, DS_ALIGNMENT))
+		req_size += DS_ALIGNMENT;
+
 	error = -EINVAL;
-	if (size < (DS_ALIGNMENT + ds_cfg.sizeof_rec[qual]))
+	if (size < req_size)
 		goto out;
 
 	if (th != (size_t)-1) {

arch/x86/kernel/ds_selftest.c

@@ -16,8 +16,8 @@
 #include <asm/ds.h>
 
 
-#define BUFFER_SIZE	521 /* Intentionally chose an odd size. */
-
+#define BUFFER_SIZE		521	/* Intentionally chose an odd size. */
+#define SMALL_BUFFER_SIZE	24	/* A single bts entry. */
 
 struct ds_selftest_bts_conf {
 	struct bts_tracer *tracer;
@@ -381,7 +381,7 @@ int ds_selftest_bts(void)
 	conf.suspend = ds_suspend_bts_noirq;
 	conf.resume = ds_resume_bts_noirq;
 	conf.tracer =
-		ds_request_bts_task(current, buffer, BUFFER_SIZE,
+		ds_request_bts_task(current, buffer, SMALL_BUFFER_SIZE,
 				   NULL, (size_t)-1, BTS_KERNEL);
 	local_irq_save(irq);
 	ds_selftest_bts_cpu(&conf);