kernel/trace/trace_seq.c - kernel/common - Git at Google

 /*
  * trace_seq.c
  *
  * Copyright (C) 2008-2014 Red Hat Inc, Steven Rostedt <srostedt@redhat.com>
  *
  * The trace_seq is a handy tool that allows you to pass a descriptor around
  * to a buffer that other functions can write to. It is similar to the
  * seq_file functionality but has some differences.
  *
  * To use it, the trace_seq must be initialized with trace_seq_init().
  * This will set up the counters within the descriptor. You can call
  * trace_seq_init() more than once to reset the trace_seq to start
  * from scratch.
  *
  * The buffer size is currently PAGE_SIZE, although it may become dynamic
  * in the future.
  *
  * A write to the buffer will either succed or fail. That is, unlike
  * sprintf() there will not be a partial write (well it may write into
  * the buffer but it wont update the pointers). This allows users to
  * try to write something into the trace_seq buffer and if it fails
  * they can flush it and try again.
  *
  */
 #include <linux/uaccess.h>
 #include <linux/seq_file.h>
 #include <linux/trace_seq.h>

 /* How much buffer is left on the trace_seq? */
 #define TRACE_SEQ_BUF_LEFT(s) ((PAGE_SIZE - 1) - (s)->len)

 /* How much buffer is written? */
 #define TRACE_SEQ_BUF_USED(s) min((s)->len, (unsigned int)(PAGE_SIZE - 1))

 /**
  * trace_print_seq - move the contents of trace_seq into a seq_file
  * @m: the seq_file descriptor that is the destination
  * @s: the trace_seq descriptor that is the source.
  *
  * Returns 0 on success and non zero on error. If it succeeds to
  * write to the seq_file it will reset the trace_seq, otherwise
  * it does not modify the trace_seq to let the caller try again.
  */
 int trace_print_seq(struct seq_file *m, struct trace_seq *s)
 {
 	unsigned int len = TRACE_SEQ_BUF_USED(s);
 	int ret;

 	ret = seq_write(m, s->buffer, len);

 	/*
 	 * Only reset this buffer if we successfully wrote to the
 	 * seq_file buffer. This lets the caller try again or
 	 * do something else with the contents.
 	 */
 	if (!ret)
 		trace_seq_init(s);

 	return ret;
 }

 /**
  * trace_seq_printf - sequence printing of trace information
  * @s: trace sequence descriptor
  * @fmt: printf format string
  *
  * The tracer may use either sequence operations or its own
  * copy to user routines. To simplify formating of a trace
  * trace_seq_printf() is used to store strings into a special
  * buffer (@s). Then the output may be either used by
  * the sequencer or pulled into another buffer.
  *
  * Returns 1 if we successfully written all the contents to
  *   the buffer.
   * Returns 0 if we the length to write is bigger than the
  *   reserved buffer space. In this case, nothing gets written.
  */
 int trace_seq_printf(struct trace_seq *s, const char *fmt, ...)
 {
 	unsigned int len = TRACE_SEQ_BUF_LEFT(s);
 	va_list ap;
 	int ret;

 	if (s->full || !len)
 		return 0;

 	va_start(ap, fmt);
 	ret = vsnprintf(s->buffer + s->len, len, fmt, ap);
 	va_end(ap);

 	/* If we can't write it all, don't bother writing anything */
 	if (ret >= len) {
 		s->full = 1;
 		return 0;
 	}

 	s->len += ret;

 	return 1;
 }
 EXPORT_SYMBOL_GPL(trace_seq_printf);

 /**
  * trace_seq_bitmask - write a bitmask array in its ASCII representation
  * @s:		trace sequence descriptor
  * @maskp:	points to an array of unsigned longs that represent a bitmask
  * @nmaskbits:	The number of bits that are valid in @maskp
  *
  * Writes a ASCII representation of a bitmask string into @s.
  *
  * Returns 1 if we successfully written all the contents to
  *   the buffer.
  * Returns 0 if we the length to write is bigger than the
  *   reserved buffer space. In this case, nothing gets written.
  */
 int trace_seq_bitmask(struct trace_seq *s, const unsigned long *maskp,
 		      int nmaskbits)
 {
 	unsigned int len = TRACE_SEQ_BUF_LEFT(s);
 	int ret;

 	if (s->full || !len)
 		return 0;

 	ret = bitmap_scnprintf(s->buffer, len, maskp, nmaskbits);
 	s->len += ret;

 	return 1;
 }
 EXPORT_SYMBOL_GPL(trace_seq_bitmask);

 /**
  * trace_seq_vprintf - sequence printing of trace information
  * @s: trace sequence descriptor
  * @fmt: printf format string
  *
  * The tracer may use either sequence operations or its own
  * copy to user routines. To simplify formating of a trace
  * trace_seq_printf is used to store strings into a special
  * buffer (@s). Then the output may be either used by
  * the sequencer or pulled into another buffer.
  *
  * Returns how much it wrote to the buffer.
  */
 int trace_seq_vprintf(struct trace_seq *s, const char *fmt, va_list args)
 {
 	unsigned int len = TRACE_SEQ_BUF_LEFT(s);
 	int ret;

 	if (s->full || !len)
 		return 0;

 	ret = vsnprintf(s->buffer + s->len, len, fmt, args);

 	/* If we can't write it all, don't bother writing anything */
 	if (ret >= len) {
 		s->full = 1;
 		return 0;
 	}

 	s->len += ret;

 	return len;
 }
 EXPORT_SYMBOL_GPL(trace_seq_vprintf);

 /**
  * trace_seq_bprintf - Write the printf string from binary arguments
  * @s: trace sequence descriptor
  * @fmt: The format string for the @binary arguments
  * @binary: The binary arguments for @fmt.
  *
  * When recording in a fast path, a printf may be recorded with just
  * saving the format and the arguments as they were passed to the
  * function, instead of wasting cycles converting the arguments into
  * ASCII characters. Instead, the arguments are saved in a 32 bit
  * word array that is defined by the format string constraints.
  *
  * This function will take the format and the binary array and finish
  * the conversion into the ASCII string within the buffer.
  *
  * Returns how much it wrote to the buffer.
  */
 int trace_seq_bprintf(struct trace_seq *s, const char *fmt, const u32 *binary)
 {
 	unsigned int len = TRACE_SEQ_BUF_LEFT(s);
 	int ret;

 	if (s->full || !len)
 		return 0;

 	ret = bstr_printf(s->buffer + s->len, len, fmt, binary);

 	/* If we can't write it all, don't bother writing anything */
 	if (ret >= len) {
 		s->full = 1;
 		return 0;
 	}

 	s->len += ret;

 	return len;
 }
 EXPORT_SYMBOL_GPL(trace_seq_bprintf);

 /**
  * trace_seq_puts - trace sequence printing of simple string
  * @s: trace sequence descriptor
  * @str: simple string to record
  *
  * The tracer may use either the sequence operations or its own
  * copy to user routines. This function records a simple string
  * into a special buffer (@s) for later retrieval by a sequencer
  * or other mechanism.
  *
  * Returns how much it wrote to the buffer.
  */
 int trace_seq_puts(struct trace_seq *s, const char *str)
 {
 	unsigned int len = strlen(str);

 	if (s->full)
 		return 0;

 	if (len > TRACE_SEQ_BUF_LEFT(s)) {
 		s->full = 1;
 		return 0;
 	}

 	memcpy(s->buffer + s->len, str, len);
 	s->len += len;

 	return len;
 }
 EXPORT_SYMBOL_GPL(trace_seq_puts);

 /**
  * trace_seq_putc - trace sequence printing of simple character
  * @s: trace sequence descriptor
  * @c: simple character to record
  *
  * The tracer may use either the sequence operations or its own
  * copy to user routines. This function records a simple charater
  * into a special buffer (@s) for later retrieval by a sequencer
  * or other mechanism.
  *
  * Returns how much it wrote to the buffer.
  */
 int trace_seq_putc(struct trace_seq *s, unsigned char c)
 {
 	if (s->full)
 		return 0;

 	if (TRACE_SEQ_BUF_LEFT(s) < 1) {
 		s->full = 1;
 		return 0;
 	}

 	s->buffer[s->len++] = c;

 	return 1;
 }
 EXPORT_SYMBOL_GPL(trace_seq_putc);

 /**
  * trace_seq_putmem - write raw data into the trace_seq buffer
  * @s: trace sequence descriptor
  * @mem: The raw memory to copy into the buffer
  * @len: The length of the raw memory to copy (in bytes)
  *
  * There may be cases where raw memory needs to be written into the
  * buffer and a strcpy() would not work. Using this function allows
  * for such cases.
  *
  * Returns how much it wrote to the buffer.
  */
 int trace_seq_putmem(struct trace_seq *s, const void *mem, unsigned int len)
 {
 	if (s->full)
 		return 0;

 	if (len > TRACE_SEQ_BUF_LEFT(s)) {
 		s->full = 1;
 		return 0;
 	}

 	memcpy(s->buffer + s->len, mem, len);
 	s->len += len;

 	return len;
 }
 EXPORT_SYMBOL_GPL(trace_seq_putmem);

 #define MAX_MEMHEX_BYTES	8U
 #define HEX_CHARS		(MAX_MEMHEX_BYTES*2 + 1)

 /**
  * trace_seq_putmem_hex - write raw memory into the buffer in ASCII hex
  * @s: trace sequence descriptor
  * @mem: The raw memory to write its hex ASCII representation of
  * @len: The length of the raw memory to copy (in bytes)
  *
  * This is similar to trace_seq_putmem() except instead of just copying the
  * raw memory into the buffer it writes its ASCII representation of it
  * in hex characters.
  *
  * Returns how much it wrote to the buffer.
  */
 int trace_seq_putmem_hex(struct trace_seq *s, const void *mem,
 			 unsigned int len)
 {
 	unsigned char hex[HEX_CHARS];
 	const unsigned char *data = mem;
 	unsigned int start_len;
 	int i, j;
 	int cnt = 0;

 	if (s->full)
 		return 0;

 	while (len) {
 		start_len = min(len, HEX_CHARS - 1);
 #ifdef __BIG_ENDIAN
 		for (i = 0, j = 0; i < start_len; i++) {
 #else
 		for (i = start_len-1, j = 0; i >= 0; i--) {
 #endif
 			hex[j++] = hex_asc_hi(data[i]);
 			hex[j++] = hex_asc_lo(data[i]);
 		}
 		if (WARN_ON_ONCE(j == 0 || j/2 > len))
 			break;

 		/* j increments twice per loop */
 		len -= j / 2;
 		hex[j++] = ' ';

 		cnt += trace_seq_putmem(s, hex, j);
 	}
 	return cnt;
 }
 EXPORT_SYMBOL_GPL(trace_seq_putmem_hex);

 /**
  * trace_seq_path - copy a path into the sequence buffer
  * @s: trace sequence descriptor
  * @path: path to write into the sequence buffer.
  *
  * Write a path name into the sequence buffer.
  *
  * Returns 1 if we successfully written all the contents to
  *   the buffer.
  * Returns 0 if we the length to write is bigger than the
  *   reserved buffer space. In this case, nothing gets written.
  */
 int trace_seq_path(struct trace_seq *s, const struct path *path)
 {
 	unsigned char *p;

 	if (s->full)
 		return 0;

 	if (TRACE_SEQ_BUF_LEFT(s) < 1) {
 		s->full = 1;
 		return 0;
 	}

 	p = d_path(path, s->buffer + s->len, PAGE_SIZE - s->len);
 	if (!IS_ERR(p)) {
 		p = mangle_path(s->buffer + s->len, p, "\n");
 		if (p) {
 			s->len = p - s->buffer;
 			return 1;
 		}
 	} else {
 		s->buffer[s->len++] = '?';
 		return 1;
 	}

 	s->full = 1;
 	return 0;
 }
 EXPORT_SYMBOL_GPL(trace_seq_path);

 /**
  * trace_seq_to_user - copy the squence buffer to user space
  * @s: trace sequence descriptor
  * @ubuf: The userspace memory location to copy to
  * @cnt: The amount to copy
  *
  * Copies the sequence buffer into the userspace memory pointed to
  * by @ubuf. It starts from the last read position (@s->readpos)
  * and writes up to @cnt characters or till it reaches the end of
  * the content in the buffer (@s->len), which ever comes first.
  *
  * On success, it returns a positive number of the number of bytes
  * it copied.
  *
  * On failure it returns -EBUSY if all of the content in the
  * sequence has been already read, which includes nothing in the
  * sequenc (@s->len == @s->readpos).
  *
  * Returns -EFAULT if the copy to userspace fails.
  */
 int trace_seq_to_user(struct trace_seq *s, char __user *ubuf, int cnt)
 {
 	int len;
 	int ret;

 	if (!cnt)
 		return 0;

 	if (s->len <= s->readpos)
 		return -EBUSY;

 	len = s->len - s->readpos;
 	if (cnt > len)
 		cnt = len;
 	ret = copy_to_user(ubuf, s->buffer + s->readpos, cnt);
 	if (ret == cnt)
 		return -EFAULT;

 	cnt -= ret;

 	s->readpos += cnt;
 	return cnt;
 }
 EXPORT_SYMBOL_GPL(trace_seq_to_user);
	/*
	* trace_seq.c
	*
	* Copyright (C) 2008-2014 Red Hat Inc, Steven Rostedt <srostedt@redhat.com>
	*
	* The trace_seq is a handy tool that allows you to pass a descriptor around
	* to a buffer that other functions can write to. It is similar to the
	* seq_file functionality but has some differences.
	*
	* To use it, the trace_seq must be initialized with trace_seq_init().
	* This will set up the counters within the descriptor. You can call
	* trace_seq_init() more than once to reset the trace_seq to start
	* from scratch.
	*
	* The buffer size is currently PAGE_SIZE, although it may become dynamic
	* in the future.
	*
	* A write to the buffer will either succed or fail. That is, unlike
	* sprintf() there will not be a partial write (well it may write into
	* the buffer but it wont update the pointers). This allows users to
	* try to write something into the trace_seq buffer and if it fails
	* they can flush it and try again.
	*
	*/
	#include <linux/uaccess.h>
	#include <linux/seq_file.h>
	#include <linux/trace_seq.h>

	/* How much buffer is left on the trace_seq? */
	#define TRACE_SEQ_BUF_LEFT(s) ((PAGE_SIZE - 1) - (s)->len)

	/* How much buffer is written? */
	#define TRACE_SEQ_BUF_USED(s) min((s)->len, (unsigned int)(PAGE_SIZE - 1))

	/**
	* trace_print_seq - move the contents of trace_seq into a seq_file
	* @m: the seq_file descriptor that is the destination
	* @s: the trace_seq descriptor that is the source.
	*
	* Returns 0 on success and non zero on error. If it succeeds to
	* write to the seq_file it will reset the trace_seq, otherwise
	* it does not modify the trace_seq to let the caller try again.
	*/
	int trace_print_seq(struct seq_file m, struct trace_seq s)
	{
	unsigned int len = TRACE_SEQ_BUF_USED(s);
	int ret;

	ret = seq_write(m, s->buffer, len);

	/*
	* Only reset this buffer if we successfully wrote to the
	* seq_file buffer. This lets the caller try again or
	* do something else with the contents.
	*/
	if (!ret)
	trace_seq_init(s);

	return ret;
	}

	/**
	* trace_seq_printf - sequence printing of trace information
	* @s: trace sequence descriptor
	* @fmt: printf format string
	*
	* The tracer may use either sequence operations or its own
	* copy to user routines. To simplify formating of a trace
	* trace_seq_printf() is used to store strings into a special
	* buffer (@s). Then the output may be either used by
	* the sequencer or pulled into another buffer.
	*
	* Returns 1 if we successfully written all the contents to
	* the buffer.
	* Returns 0 if we the length to write is bigger than the
	* reserved buffer space. In this case, nothing gets written.
	*/
	int trace_seq_printf(struct trace_seq s, const char fmt, ...)
	{
	unsigned int len = TRACE_SEQ_BUF_LEFT(s);
	va_list ap;
	int ret;

	if (s->full \|\| !len)
	return 0;

	va_start(ap, fmt);
	ret = vsnprintf(s->buffer + s->len, len, fmt, ap);
	va_end(ap);

	/* If we can't write it all, don't bother writing anything */
	if (ret >= len) {
	s->full = 1;
	return 0;
	}

	s->len += ret;

	return 1;
	}
	EXPORT_SYMBOL_GPL(trace_seq_printf);

	/**
	* trace_seq_bitmask - write a bitmask array in its ASCII representation
	* @s: trace sequence descriptor
	* @maskp: points to an array of unsigned longs that represent a bitmask
	* @nmaskbits: The number of bits that are valid in @maskp
	*
	* Writes a ASCII representation of a bitmask string into @s.
	*
	* Returns 1 if we successfully written all the contents to
	* the buffer.
	* Returns 0 if we the length to write is bigger than the
	* reserved buffer space. In this case, nothing gets written.
	*/
	int trace_seq_bitmask(struct trace_seq s, const unsigned long maskp,
	int nmaskbits)
	{
	unsigned int len = TRACE_SEQ_BUF_LEFT(s);
	int ret;

	if (s->full \|\| !len)
	return 0;

	ret = bitmap_scnprintf(s->buffer, len, maskp, nmaskbits);
	s->len += ret;

	return 1;
	}
	EXPORT_SYMBOL_GPL(trace_seq_bitmask);

	/**
	* trace_seq_vprintf - sequence printing of trace information
	* @s: trace sequence descriptor
	* @fmt: printf format string
	*
	* The tracer may use either sequence operations or its own
	* copy to user routines. To simplify formating of a trace
	* trace_seq_printf is used to store strings into a special
	* buffer (@s). Then the output may be either used by
	* the sequencer or pulled into another buffer.
	*
	* Returns how much it wrote to the buffer.
	*/
	int trace_seq_vprintf(struct trace_seq s, const char fmt, va_list args)
	{
	unsigned int len = TRACE_SEQ_BUF_LEFT(s);
	int ret;

	if (s->full \|\| !len)
	return 0;

	ret = vsnprintf(s->buffer + s->len, len, fmt, args);

	/* If we can't write it all, don't bother writing anything */
	if (ret >= len) {
	s->full = 1;
	return 0;
	}

	s->len += ret;

	return len;
	}
	EXPORT_SYMBOL_GPL(trace_seq_vprintf);

	/**
	* trace_seq_bprintf - Write the printf string from binary arguments
	* @s: trace sequence descriptor
	* @fmt: The format string for the @binary arguments
	* @binary: The binary arguments for @fmt.
	*
	* When recording in a fast path, a printf may be recorded with just
	* saving the format and the arguments as they were passed to the
	* function, instead of wasting cycles converting the arguments into
	* ASCII characters. Instead, the arguments are saved in a 32 bit
	* word array that is defined by the format string constraints.
	*
	* This function will take the format and the binary array and finish
	* the conversion into the ASCII string within the buffer.
	*
	* Returns how much it wrote to the buffer.
	*/
	int trace_seq_bprintf(struct trace_seq s, const char fmt, const u32 *binary)
	{
	unsigned int len = TRACE_SEQ_BUF_LEFT(s);
	int ret;

	if (s->full \|\| !len)
	return 0;

	ret = bstr_printf(s->buffer + s->len, len, fmt, binary);

	/* If we can't write it all, don't bother writing anything */
	if (ret >= len) {
	s->full = 1;
	return 0;
	}

	s->len += ret;

	return len;
	}
	EXPORT_SYMBOL_GPL(trace_seq_bprintf);

	/**
	* trace_seq_puts - trace sequence printing of simple string
	* @s: trace sequence descriptor
	* @str: simple string to record
	*
	* The tracer may use either the sequence operations or its own
	* copy to user routines. This function records a simple string
	* into a special buffer (@s) for later retrieval by a sequencer
	* or other mechanism.
	*
	* Returns how much it wrote to the buffer.
	*/
	int trace_seq_puts(struct trace_seq s, const char str)
	{
	unsigned int len = strlen(str);

	if (s->full)
	return 0;

	if (len > TRACE_SEQ_BUF_LEFT(s)) {
	s->full = 1;
	return 0;
	}

	memcpy(s->buffer + s->len, str, len);
	s->len += len;

	return len;
	}
	EXPORT_SYMBOL_GPL(trace_seq_puts);

	/**
	* trace_seq_putc - trace sequence printing of simple character
	* @s: trace sequence descriptor
	* @c: simple character to record
	*
	* The tracer may use either the sequence operations or its own
	* copy to user routines. This function records a simple charater
	* into a special buffer (@s) for later retrieval by a sequencer
	* or other mechanism.
	*
	* Returns how much it wrote to the buffer.
	*/
	int trace_seq_putc(struct trace_seq *s, unsigned char c)
	{
	if (s->full)
	return 0;

	if (TRACE_SEQ_BUF_LEFT(s) < 1) {
	s->full = 1;
	return 0;
	}

	s->buffer[s->len++] = c;

	return 1;
	}
	EXPORT_SYMBOL_GPL(trace_seq_putc);

	/**
	* trace_seq_putmem - write raw data into the trace_seq buffer
	* @s: trace sequence descriptor
	* @mem: The raw memory to copy into the buffer
	* @len: The length of the raw memory to copy (in bytes)
	*
	* There may be cases where raw memory needs to be written into the
	* buffer and a strcpy() would not work. Using this function allows
	* for such cases.
	*
	* Returns how much it wrote to the buffer.
	*/
	int trace_seq_putmem(struct trace_seq s, const void mem, unsigned int len)
	{
	if (s->full)
	return 0;

	if (len > TRACE_SEQ_BUF_LEFT(s)) {
	s->full = 1;
	return 0;
	}

	memcpy(s->buffer + s->len, mem, len);
	s->len += len;

	return len;
	}
	EXPORT_SYMBOL_GPL(trace_seq_putmem);

	#define MAX_MEMHEX_BYTES 8U
	#define HEX_CHARS (MAX_MEMHEX_BYTES*2 + 1)

	/**
	* trace_seq_putmem_hex - write raw memory into the buffer in ASCII hex
	* @s: trace sequence descriptor
	* @mem: The raw memory to write its hex ASCII representation of
	* @len: The length of the raw memory to copy (in bytes)
	*
	* This is similar to trace_seq_putmem() except instead of just copying the
	* raw memory into the buffer it writes its ASCII representation of it
	* in hex characters.
	*
	* Returns how much it wrote to the buffer.
	*/
	int trace_seq_putmem_hex(struct trace_seq s, const void mem,
	unsigned int len)
	{
	unsigned char hex[HEX_CHARS];
	const unsigned char *data = mem;
	unsigned int start_len;
	int i, j;
	int cnt = 0;

	if (s->full)
	return 0;

	while (len) {
	start_len = min(len, HEX_CHARS - 1);
	#ifdef __BIG_ENDIAN
	for (i = 0, j = 0; i < start_len; i++) {
	#else
	for (i = start_len-1, j = 0; i >= 0; i--) {
	#endif
	hex[j++] = hex_asc_hi(data[i]);
	hex[j++] = hex_asc_lo(data[i]);
	}
	if (WARN_ON_ONCE(j == 0 \|\| j/2 > len))
	break;

	/* j increments twice per loop */
	len -= j / 2;
	hex[j++] = ' ';

	cnt += trace_seq_putmem(s, hex, j);
	}
	return cnt;
	}
	EXPORT_SYMBOL_GPL(trace_seq_putmem_hex);

	/**
	* trace_seq_path - copy a path into the sequence buffer
	* @s: trace sequence descriptor
	* @path: path to write into the sequence buffer.
	*
	* Write a path name into the sequence buffer.
	*
	* Returns 1 if we successfully written all the contents to
	* the buffer.
	* Returns 0 if we the length to write is bigger than the
	* reserved buffer space. In this case, nothing gets written.
	*/
	int trace_seq_path(struct trace_seq s, const struct path path)
	{
	unsigned char *p;

	if (s->full)
	return 0;

	if (TRACE_SEQ_BUF_LEFT(s) < 1) {
	s->full = 1;
	return 0;
	}

	p = d_path(path, s->buffer + s->len, PAGE_SIZE - s->len);
	if (!IS_ERR(p)) {
	p = mangle_path(s->buffer + s->len, p, "\n");
	if (p) {
	s->len = p - s->buffer;
	return 1;
	}
	} else {
	s->buffer[s->len++] = '?';
	return 1;
	}

	s->full = 1;
	return 0;
	}
	EXPORT_SYMBOL_GPL(trace_seq_path);

	/**
	* trace_seq_to_user - copy the squence buffer to user space
	* @s: trace sequence descriptor
	* @ubuf: The userspace memory location to copy to
	* @cnt: The amount to copy
	*
	* Copies the sequence buffer into the userspace memory pointed to
	* by @ubuf. It starts from the last read position (@s->readpos)
	* and writes up to @cnt characters or till it reaches the end of
	* the content in the buffer (@s->len), which ever comes first.
	*
	* On success, it returns a positive number of the number of bytes
	* it copied.
	*
	* On failure it returns -EBUSY if all of the content in the
	* sequence has been already read, which includes nothing in the
	* sequenc (@s->len == @s->readpos).
	*
	* Returns -EFAULT if the copy to userspace fails.
	*/
	int trace_seq_to_user(struct trace_seq s, char __user ubuf, int cnt)
	{
	int len;
	int ret;

	if (!cnt)
	return 0;

	if (s->len <= s->readpos)
	return -EBUSY;

	len = s->len - s->readpos;
	if (cnt > len)
	cnt = len;
	ret = copy_to_user(ubuf, s->buffer + s->readpos, cnt);
	if (ret == cnt)
	return -EFAULT;

	cnt -= ret;

	s->readpos += cnt;
	return cnt;
	}
	EXPORT_SYMBOL_GPL(trace_seq_to_user);