[for-next][PATCH 1/5] tracing: Show real address for trace event arguments

Steven Rostedt
2021-02-15T19:57:19Z

To help debugging kernel, show real address for trace event arguments
in tracefs/trace{,pipe} instead of hashed pointer value.

Since ftrace human-readable format uses vsprintf(), all %p are
translated to hash values instead of pointer address.

However, when debugging the kernel, raw address value gives a
hint when comparing with the memory mapping in the kernel.
(Those are sometimes used with crash log, which is not hashed too)
So converting %p with %px when calling trace_seq_printf().

Moreover, this is not improving the security because the tracefs
can be used only by root user and the raw address values are readable
from tracefs/percpu/cpu*/trace_pipe_raw file.

Link: https://lkml.kernel.org/r/160277370703.29307.5134475491761971203.stgit@devnote2

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
include/linux/trace_events.h | 4 ++
include/trace/trace_events.h | 2 +-
kernel/trace/trace.c | 71 +++++++++++++++++++++++++++++++++++-
kernel/trace/trace.h | 2 +
kernel/trace/trace_output.c | 12 +++++-
5 files changed, 88 insertions(+), 3 deletions(-)

diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
index 5d1eeac4bfbe..7077fec653bb 100644
--- a/include/linux/trace_events.h
+++ b/include/linux/trace_events.h
@@ -55,6 +55,8 @@ struct trace_event;

int trace_raw_output_prep(struct trace_iterator *iter,
struct trace_event *event);
+extern __printf(2, 3)
+void trace_event_printf(struct trace_iterator *iter, const char *fmt, ...);

/*
* The trace entry - the most basic unit of tracing. This is what
@@ -87,6 +89,8 @@ struct trace_iterator {
unsigned long iter_flags;
void *temp; /* temp holder */
unsigned int temp_size;
+ char *fmt; /* modified format holder */
+ unsigned int fmt_size;

/* trace_seq for __print_flags() and __print_symbolic() etc. */
struct trace_seq tmp_seq;
diff --git a/include/trace/trace_events.h b/include/trace/trace_events.h
index 7785961d82ba..e6a8b3febc49 100644
--- a/include/trace/trace_events.h
+++ b/include/trace/trace_events.h
@@ -364,7 +364,7 @@ trace_raw_output_##call(struct trace_iterator *iter, int flags, \
if (ret != TRACE_TYPE_HANDLED) \
return ret; \
\
- trace_seq_printf(s, print); \
+ trace_event_printf(iter, print); \
\
return trace_handle_return(s); \
} \
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index b79bcacdd6f9..39f8a537196e 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3530,6 +3530,62 @@ __find_next_entry(struct trace_iterator *iter, int *ent_cpu,
return next;
}

+#define STATIC_FMT_BUF_SIZE 128
+static char static_fmt_buf[STATIC_FMT_BUF_SIZE];
+
+static char *trace_iter_expand_format(struct trace_iterator *iter)
+{
+ char *tmp;
+
+ if (iter->fmt == static_fmt_buf)
+ return NULL;
+
+ tmp = krealloc(iter->fmt, iter->fmt_size + STATIC_FMT_BUF_SIZE,
+ GFP_KERNEL);
+ if (tmp) {
+ iter->fmt_size += STATIC_FMT_BUF_SIZE;
+ iter->fmt = tmp;
+ }
+
+ return tmp;
+}
+
+const char *trace_event_format(struct trace_iterator *iter, const char *fmt)
+{
+ const char *p, *new_fmt;
+ char *q;
+
+ if (WARN_ON_ONCE(!fmt))
+ return fmt;
+
+ p = fmt;
+ new_fmt = q = iter->fmt;
+ while (*p) {
+ if (unlikely(q - new_fmt + 3 > iter->fmt_size)) {
+ if (!trace_iter_expand_format(iter))
+ return fmt;
+
+ q += iter->fmt - new_fmt;
+ new_fmt = iter->fmt;
+ }
+
+ *q++ = *p++;
+
+ /* Replace %p with %px */
+ if (p[-1] == '%') {
+ if (p[0] == '%') {
+ *q++ = *p++;
+ } else if (p[0] == 'p' && !isalnum(p[1])) {
+ *q++ = *p++;
+ *q++ = 'x';
+ }
+ }
+ }
+ *q = '\0';
+
+ return new_fmt;
+}
+
#define STATIC_TEMP_BUF_SIZE 128
static char static_temp_buf[STATIC_TEMP_BUF_SIZE] __aligned(4);

@@ -4322,6 +4378,16 @@ __tracing_open(struct inode *inode, struct file *file, bool snapshot)
if (iter->temp)
iter->temp_size = 128;

+ /*
+ * trace_event_printf() may need to modify given format
+ * string to replace %p with %px so that it shows real address
+ * instead of hash value. However, that is only for the event
+ * tracing, other tracer may not need. Defer the allocation
+ * until it is needed.
+ */
+ iter->fmt = NULL;
+ iter->fmt_size = 0;
+
/*
* We make a copy of the current tracer to avoid concurrent
* changes on it while we are reading.
@@ -4473,6 +4539,7 @@ static int tracing_release(struct inode *inode, struct file *file)

mutex_destroy(&iter->mutex);
free_cpumask_var(iter->started);
+ kfree(iter->fmt);
kfree(iter->temp);
kfree(iter->trace);
kfree(iter->buffer_iter);
@@ -9331,9 +9398,11 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)

/* Simulate the iterator */
trace_init_global_iter(&iter);
- /* Can not use kmalloc for iter.temp */
+ /* Can not use kmalloc for iter.temp and iter.fmt */
iter.temp = static_temp_buf;
iter.temp_size = STATIC_TEMP_BUF_SIZE;
+ iter.fmt = static_fmt_buf;
+ iter.fmt_size = STATIC_FMT_BUF_SIZE;

for_each_tracing_cpu(cpu) {
atomic_inc(&per_cpu_ptr(iter.array_buffer->data, cpu)->disabled);
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index a9e13bd5a41b..6c3ea6f95e68 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -581,6 +581,8 @@ struct trace_entry *trace_find_next_entry(struct trace_iterator *iter,
void trace_buffer_unlock_commit_nostack(struct trace_buffer *buffer,
struct ring_buffer_event *event);

+const char *trace_event_format(struct trace_iterator *iter, const char *fmt);
+
int trace_empty(struct trace_iterator *iter);

void *trace_find_next_entry_inc(struct trace_iterator *iter);
diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
index 92b1575ae0ca..61255bad7e01 100644
--- a/kernel/trace/trace_output.c
+++ b/kernel/trace/trace_output.c
@@ -312,13 +312,23 @@ int trace_raw_output_prep(struct trace_iterator *iter,
}
EXPORT_SYMBOL(trace_raw_output_prep);

+void trace_event_printf(struct trace_iterator *iter, const char *fmt, ...)
+{
+ va_list ap;
+
+ va_start(ap, fmt);
+ trace_seq_vprintf(&iter->seq, trace_event_format(iter, fmt), ap);
+ va_end(ap);
+}
+EXPORT_SYMBOL(trace_event_printf);
+
static int trace_output_raw(struct trace_iterator *iter, char *name,
char *fmt, va_list ap)
{
struct trace_seq *s = &iter->seq;

trace_seq_printf(s, "%s: ", name);
- trace_seq_vprintf(s, fmt, ap);
+ trace_seq_vprintf(s, trace_event_format(iter, fmt), ap);

return trace_handle_return(s);
}
--
2.30.0