qemu/trace/simple.c
Markus Armbruster db25d56c01 trace/simple: Fix unauthorized enable
st_set_trace_file() accidentally enables tracing.  It's called
unconditionally during startup, which is why QEMU built with the
simple trace backend always writes a trace file "trace-$PID".

This has been broken for quite a while.  I didn't track down the exact
commit.

Fix st_set_trace_file() to restore the state.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20200527065613.25322-1-armbru@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2020-06-24 11:21:00 +01:00

425 lines
12 KiB
C

/*
* Simple trace backend
*
* Copyright IBM, Corp. 2010
*
* This work is licensed under the terms of the GNU GPL, version 2. See
* the COPYING file in the top-level directory.
*
*/
#include "qemu/osdep.h"
#ifndef _WIN32
#include <pthread.h>
#endif
#include "qemu/timer.h"
#include "trace/control.h"
#include "trace/simple.h"
#include "qemu/error-report.h"
#include "qemu/qemu-print.h"
/** Trace file header event ID, picked to avoid conflict with real event IDs */
#define HEADER_EVENT_ID (~(uint64_t)0)
/** Trace file magic number */
#define HEADER_MAGIC 0xf2b177cb0aa429b4ULL
/** Trace file version number, bump if format changes */
#define HEADER_VERSION 4
/** Records were dropped event ID */
#define DROPPED_EVENT_ID (~(uint64_t)0 - 1)
/** Trace record is valid */
#define TRACE_RECORD_VALID ((uint64_t)1 << 63)
/*
* Trace records are written out by a dedicated thread. The thread waits for
* records to become available, writes them out, and then waits again.
*/
static GMutex trace_lock;
static GCond trace_available_cond;
static GCond trace_empty_cond;
static bool trace_available;
static bool trace_writeout_enabled;
enum {
TRACE_BUF_LEN = 4096 * 64,
TRACE_BUF_FLUSH_THRESHOLD = TRACE_BUF_LEN / 4,
};
uint8_t trace_buf[TRACE_BUF_LEN];
static volatile gint trace_idx;
static unsigned int writeout_idx;
static volatile gint dropped_events;
static uint32_t trace_pid;
static FILE *trace_fp;
static char *trace_file_name;
#define TRACE_RECORD_TYPE_MAPPING 0
#define TRACE_RECORD_TYPE_EVENT 1
/* * Trace buffer entry */
typedef struct {
uint64_t event; /* event ID value */
uint64_t timestamp_ns;
uint32_t length; /* in bytes */
uint32_t pid;
uint64_t arguments[];
} TraceRecord;
typedef struct {
uint64_t header_event_id; /* HEADER_EVENT_ID */
uint64_t header_magic; /* HEADER_MAGIC */
uint64_t header_version; /* HEADER_VERSION */
} TraceLogHeader;
static void read_from_buffer(unsigned int idx, void *dataptr, size_t size);
static unsigned int write_to_buffer(unsigned int idx, void *dataptr, size_t size);
static void clear_buffer_range(unsigned int idx, size_t len)
{
uint32_t num = 0;
while (num < len) {
if (idx >= TRACE_BUF_LEN) {
idx = idx % TRACE_BUF_LEN;
}
trace_buf[idx++] = 0;
num++;
}
}
/**
* Read a trace record from the trace buffer
*
* @idx Trace buffer index
* @record Trace record to fill
*
* Returns false if the record is not valid.
*/
static bool get_trace_record(unsigned int idx, TraceRecord **recordptr)
{
uint64_t event_flag = 0;
TraceRecord record;
/* read the event flag to see if its a valid record */
read_from_buffer(idx, &record, sizeof(event_flag));
if (!(record.event & TRACE_RECORD_VALID)) {
return false;
}
smp_rmb(); /* read memory barrier before accessing record */
/* read the record header to know record length */
read_from_buffer(idx, &record, sizeof(TraceRecord));
*recordptr = malloc(record.length); /* don't use g_malloc, can deadlock when traced */
/* make a copy of record to avoid being overwritten */
read_from_buffer(idx, *recordptr, record.length);
smp_rmb(); /* memory barrier before clearing valid flag */
(*recordptr)->event &= ~TRACE_RECORD_VALID;
/* clear the trace buffer range for consumed record otherwise any byte
* with its MSB set may be considered as a valid event id when the writer
* thread crosses this range of buffer again.
*/
clear_buffer_range(idx, record.length);
return true;
}
/**
* Kick writeout thread
*
* @wait Whether to wait for writeout thread to complete
*/
static void flush_trace_file(bool wait)
{
g_mutex_lock(&trace_lock);
trace_available = true;
g_cond_signal(&trace_available_cond);
if (wait) {
g_cond_wait(&trace_empty_cond, &trace_lock);
}
g_mutex_unlock(&trace_lock);
}
static void wait_for_trace_records_available(void)
{
g_mutex_lock(&trace_lock);
while (!(trace_available && trace_writeout_enabled)) {
g_cond_signal(&trace_empty_cond);
g_cond_wait(&trace_available_cond, &trace_lock);
}
trace_available = false;
g_mutex_unlock(&trace_lock);
}
static gpointer writeout_thread(gpointer opaque)
{
TraceRecord *recordptr;
union {
TraceRecord rec;
uint8_t bytes[sizeof(TraceRecord) + sizeof(uint64_t)];
} dropped;
unsigned int idx = 0;
int dropped_count;
size_t unused __attribute__ ((unused));
uint64_t type = TRACE_RECORD_TYPE_EVENT;
for (;;) {
wait_for_trace_records_available();
if (g_atomic_int_get(&dropped_events)) {
dropped.rec.event = DROPPED_EVENT_ID;
dropped.rec.timestamp_ns = get_clock();
dropped.rec.length = sizeof(TraceRecord) + sizeof(uint64_t);
dropped.rec.pid = trace_pid;
do {
dropped_count = g_atomic_int_get(&dropped_events);
} while (!g_atomic_int_compare_and_exchange(&dropped_events,
dropped_count, 0));
dropped.rec.arguments[0] = dropped_count;
unused = fwrite(&type, sizeof(type), 1, trace_fp);
unused = fwrite(&dropped.rec, dropped.rec.length, 1, trace_fp);
}
while (get_trace_record(idx, &recordptr)) {
unused = fwrite(&type, sizeof(type), 1, trace_fp);
unused = fwrite(recordptr, recordptr->length, 1, trace_fp);
writeout_idx += recordptr->length;
free(recordptr); /* don't use g_free, can deadlock when traced */
idx = writeout_idx % TRACE_BUF_LEN;
}
fflush(trace_fp);
}
return NULL;
}
void trace_record_write_u64(TraceBufferRecord *rec, uint64_t val)
{
rec->rec_off = write_to_buffer(rec->rec_off, &val, sizeof(uint64_t));
}
void trace_record_write_str(TraceBufferRecord *rec, const char *s, uint32_t slen)
{
/* Write string length first */
rec->rec_off = write_to_buffer(rec->rec_off, &slen, sizeof(slen));
/* Write actual string now */
rec->rec_off = write_to_buffer(rec->rec_off, (void*)s, slen);
}
int trace_record_start(TraceBufferRecord *rec, uint32_t event, size_t datasize)
{
unsigned int idx, rec_off, old_idx, new_idx;
uint32_t rec_len = sizeof(TraceRecord) + datasize;
uint64_t event_u64 = event;
uint64_t timestamp_ns = get_clock();
do {
old_idx = g_atomic_int_get(&trace_idx);
smp_rmb();
new_idx = old_idx + rec_len;
if (new_idx - writeout_idx > TRACE_BUF_LEN) {
/* Trace Buffer Full, Event dropped ! */
g_atomic_int_inc(&dropped_events);
return -ENOSPC;
}
} while (!g_atomic_int_compare_and_exchange(&trace_idx, old_idx, new_idx));
idx = old_idx % TRACE_BUF_LEN;
rec_off = idx;
rec_off = write_to_buffer(rec_off, &event_u64, sizeof(event_u64));
rec_off = write_to_buffer(rec_off, &timestamp_ns, sizeof(timestamp_ns));
rec_off = write_to_buffer(rec_off, &rec_len, sizeof(rec_len));
rec_off = write_to_buffer(rec_off, &trace_pid, sizeof(trace_pid));
rec->tbuf_idx = idx;
rec->rec_off = (idx + sizeof(TraceRecord)) % TRACE_BUF_LEN;
return 0;
}
static void read_from_buffer(unsigned int idx, void *dataptr, size_t size)
{
uint8_t *data_ptr = dataptr;
uint32_t x = 0;
while (x < size) {
if (idx >= TRACE_BUF_LEN) {
idx = idx % TRACE_BUF_LEN;
}
data_ptr[x++] = trace_buf[idx++];
}
}
static unsigned int write_to_buffer(unsigned int idx, void *dataptr, size_t size)
{
uint8_t *data_ptr = dataptr;
uint32_t x = 0;
while (x < size) {
if (idx >= TRACE_BUF_LEN) {
idx = idx % TRACE_BUF_LEN;
}
trace_buf[idx++] = data_ptr[x++];
}
return idx; /* most callers wants to know where to write next */
}
void trace_record_finish(TraceBufferRecord *rec)
{
TraceRecord record;
read_from_buffer(rec->tbuf_idx, &record, sizeof(TraceRecord));
smp_wmb(); /* write barrier before marking as valid */
record.event |= TRACE_RECORD_VALID;
write_to_buffer(rec->tbuf_idx, &record, sizeof(TraceRecord));
if (((unsigned int)g_atomic_int_get(&trace_idx) - writeout_idx)
> TRACE_BUF_FLUSH_THRESHOLD) {
flush_trace_file(false);
}
}
static int st_write_event_mapping(void)
{
uint64_t type = TRACE_RECORD_TYPE_MAPPING;
TraceEventIter iter;
TraceEvent *ev;
trace_event_iter_init(&iter, NULL);
while ((ev = trace_event_iter_next(&iter)) != NULL) {
uint64_t id = trace_event_get_id(ev);
const char *name = trace_event_get_name(ev);
uint32_t len = strlen(name);
if (fwrite(&type, sizeof(type), 1, trace_fp) != 1 ||
fwrite(&id, sizeof(id), 1, trace_fp) != 1 ||
fwrite(&len, sizeof(len), 1, trace_fp) != 1 ||
fwrite(name, len, 1, trace_fp) != 1) {
return -1;
}
}
return 0;
}
/**
* Enable / disable tracing, return whether it was enabled.
*
* @enable: enable if %true, else disable.
*/
bool st_set_trace_file_enabled(bool enable)
{
bool was_enabled = trace_fp;
if (enable == !!trace_fp) {
return was_enabled; /* no change */
}
/* Halt trace writeout */
flush_trace_file(true);
trace_writeout_enabled = false;
flush_trace_file(true);
if (enable) {
static const TraceLogHeader header = {
.header_event_id = HEADER_EVENT_ID,
.header_magic = HEADER_MAGIC,
/* Older log readers will check for version at next location */
.header_version = HEADER_VERSION,
};
trace_fp = fopen(trace_file_name, "wb");
if (!trace_fp) {
return was_enabled;
}
if (fwrite(&header, sizeof header, 1, trace_fp) != 1 ||
st_write_event_mapping() < 0) {
fclose(trace_fp);
trace_fp = NULL;
return was_enabled;
}
/* Resume trace writeout */
trace_writeout_enabled = true;
flush_trace_file(false);
} else {
fclose(trace_fp);
trace_fp = NULL;
}
return was_enabled;
}
/**
* Set the name of a trace file
*
* @file The trace file name or NULL for the default name-<pid> set at
* config time
*/
void st_set_trace_file(const char *file)
{
bool saved_enable = st_set_trace_file_enabled(false);
g_free(trace_file_name);
if (!file) {
/* Type cast needed for Windows where getpid() returns an int. */
trace_file_name = g_strdup_printf(CONFIG_TRACE_FILE, (pid_t)getpid());
} else {
trace_file_name = g_strdup_printf("%s", file);
}
st_set_trace_file_enabled(saved_enable);
}
void st_print_trace_file_status(void)
{
qemu_printf("Trace file \"%s\" %s.\n",
trace_file_name, trace_fp ? "on" : "off");
}
void st_flush_trace_buffer(void)
{
flush_trace_file(true);
}
/* Helper function to create a thread with signals blocked. Use glib's
* portable threads since QEMU abstractions cannot be used due to reentrancy in
* the tracer. Also note the signal masking on POSIX hosts so that the thread
* does not steal signals when the rest of the program wants them blocked.
*/
static GThread *trace_thread_create(GThreadFunc fn)
{
GThread *thread;
#ifndef _WIN32
sigset_t set, oldset;
sigfillset(&set);
pthread_sigmask(SIG_SETMASK, &set, &oldset);
#endif
thread = g_thread_new("trace-thread", fn, NULL);
#ifndef _WIN32
pthread_sigmask(SIG_SETMASK, &oldset, NULL);
#endif
return thread;
}
bool st_init(void)
{
GThread *thread;
trace_pid = getpid();
thread = trace_thread_create(writeout_thread);
if (!thread) {
warn_report("unable to initialize simple trace backend");
return false;
}
atexit(st_flush_trace_buffer);
return true;
}