oProfile: oops when profile_pc() returns ~0LU

Instruction pointer returned by profile_pc() can be a random value.  This
break the assumption than we can safely set struct op_sample.eip field to a
magic value to signal to the per-cpu buffer reader side special event like
task switch ending up in a segfault in get_task_mm() when profile_pc()
return ~0UL.  Fixed by sanitizing the sampled eip and reject/log invalid
eip.

Problem reported by Sami Farin, patch tested by him.

Signed-off-by: Philippe Elie <phil.el@wanadoo.fr>
Tested-by: Sami Farin <safari-kernel@safari.iki.fi>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/oprofile/cpu_buffer.c

@@ -64,6 +64,8 @@ int alloc_cpu_buffers(void)
 		b->head_pos = 0;
 		b->sample_received = 0;
 		b->sample_lost_overflow = 0;
+		b->backtrace_aborted = 0;
+		b->sample_invalid_eip = 0;
 		b->cpu = i;
 		INIT_DELAYED_WORK(&b->work, wq_sync_buffer);
 	}
@@ -175,6 +177,11 @@ static int log_sample(struct oprofile_cpu_buffer * cpu_buf, unsigned long pc,
 
 	cpu_buf->sample_received++;
 
+	if (pc == ESCAPE_CODE) {
+		cpu_buf->sample_invalid_eip++;
+		return 0;
+	}
+
 	if (nr_available_slots(cpu_buf) < 3) {
 		cpu_buf->sample_lost_overflow++;
 		return 0;

drivers/oprofile/cpu_buffer.h

@@ -42,6 +42,7 @@ struct oprofile_cpu_buffer {
 	unsigned long sample_received;
 	unsigned long sample_lost_overflow;
 	unsigned long backtrace_aborted;
+	unsigned long sample_invalid_eip;
 	int cpu;
 	struct delayed_work work;
 } ____cacheline_aligned;

drivers/oprofile/oprofile_stats.c

@@ -26,6 +26,8 @@ void oprofile_reset_stats(void)
 		cpu_buf = &cpu_buffer[i]; 
 		cpu_buf->sample_received = 0;
 		cpu_buf->sample_lost_overflow = 0;
+		cpu_buf->backtrace_aborted = 0;
+		cpu_buf->sample_invalid_eip = 0;
 	}
  
 	atomic_set(&oprofile_stats.sample_lost_no_mm, 0);
@@ -61,6 +63,8 @@ void oprofile_create_stats_files(struct super_block * sb, struct dentry * root)
 			&cpu_buf->sample_lost_overflow);
 		oprofilefs_create_ro_ulong(sb, cpudir, "backtrace_aborted",
 			&cpu_buf->backtrace_aborted);
+		oprofilefs_create_ro_ulong(sb, cpudir, "sample_invalid_eip",
+			&cpu_buf->sample_invalid_eip);
 	}
  
 	oprofilefs_create_ro_atomic(sb, dir, "sample_lost_no_mm",