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From: Frederic Weisbecker on 25 Mar 2010 22:00
The trace event buffer used by perf to record raw sample events
is typed as an array of char and may then not be aligned to 8
by alloc_percpu().

But we need it to be aligned to 8 in sparc64 because we cast
this buffer into a random structure type built by the TRACE_EVENT()
macro to store the traces. So if a random 64 bits field is accessed
inside, it may be not under an expected good alignment.

Use an array of long instead to force the appropriate alignment, and
perform a compile time check to ensure the size in byte of the buffer
is a multiple of sizeof(long) so that its actual size doesn't get
shrinked under us.

This fixes unaligned accesses reported while using perf lock
in sparc 64.

Suggested-by: David Miller <davem(a)davemloft.net>
Suggested-by: Tejun Heo <htejun(a)gmail.com>
Signed-off-by: Frederic Weisbecker <fweisbec(a)gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra(a)chello.nl>
Cc: Arnaldo Carvalho de Melo <acme(a)redhat.com>
Cc: Paul Mackerras <paulus(a)samba.org>
Cc: Ingo Molnar <mingo(a)elte.hu>
Cc: David Miller <davem(a)davemloft.net>
Cc: Tejun Heo <htejun(a)gmail.com>
Cc: Steven Rostedt <rostedt(a)goodmis.org>
---
kernel/trace/trace_event_perf.c | 11 +++++++++--
1 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c
index 8e9edcd..30314f5 100644
--- a/kernel/trace/trace_event_perf.c
+++ b/kernel/trace/trace_event_perf.c
@@ -15,7 +15,12 @@ EXPORT_PER_CPU_SYMBOL_GPL(perf_trace_regs);
static char *perf_trace_buf;
static char *perf_trace_buf_nmi;

-typedef typeof(char [PERF_MAX_TRACE_SIZE]) perf_trace_t ;
+/*
+ * Force it to be aligned to unsigned long to avoid misaligned accesses
+ * suprises
+ */
+typedef typeof(unsigned long [PERF_MAX_TRACE_SIZE / sizeof(unsigned long)])
+ perf_trace_t;

/* Count the events in use (per event id, not per instance) */
static int total_ref_count;
@@ -128,6 +133,8 @@ __kprobes void *perf_trace_buf_prepare(int size, unsigned short type,
char *trace_buf, *raw_data;
int pc, cpu;

+ BUILD_BUG_ON(PERF_MAX_TRACE_SIZE % sizeof(unsigned long));
+
pc = preempt_count();

/* Protect the per cpu buffer, begin the rcu read side */
@@ -150,7 +157,7 @@ __kprobes void *perf_trace_buf_prepare(int size, unsigned short type,
raw_data = per_cpu_ptr(trace_buf, cpu);

/* zero the dead bytes from align to not leak stack to user */
- *(u64 *)(&raw_data[size - sizeof(u64)]) = 0ULL;
+ memset(&raw_data[size - sizeof(u64)], 0, sizeof(u64));

entry = (struct trace_entry *)raw_data;
tracing_generic_entry_update(entry, *irq_flags, pc);
--
1.6.2.3

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