Re: [RFC][PATCH 03/12 v3] tracing: Create seq_buf layer in trace_seq

[Petr Mladek]

On Tue 2014-11-04 10:52:40, Steven Rostedt wrote:
> From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>
> 
> Create a seq_buf layer that trace_seq sits on. The seq_buf will not
> be limited to page size. This will allow other usages of seq_buf
> instead of a hard set PAGE_SIZE one that trace_seq has.
> 
> Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> ---
>  include/linux/seq_buf.h              |  72 ++++++++
>  include/linux/trace_seq.h            |  10 +-
>  kernel/trace/Makefile                |   1 +
>  kernel/trace/seq_buf.c               | 341 +++++++++++++++++++++++++++++++++++
>  kernel/trace/trace.c                 |  39 ++--
>  kernel/trace/trace_events.c          |   6 +-
>  kernel/trace/trace_functions_graph.c |   6 +-
>  kernel/trace/trace_seq.c             | 184 +++++++++----------
>  8 files changed, 534 insertions(+), 125 deletions(-)
>  create mode 100644 include/linux/seq_buf.h
>  create mode 100644 kernel/trace/seq_buf.c
> 
> diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
> new file mode 100644
> index 000000000000..97872154d51c
> --- /dev/null
> +++ b/include/linux/seq_buf.h
> @@ -0,0 +1,72 @@
> +#ifndef _LINUX_SEQ_BUF_H
> +#define _LINUX_SEQ_BUF_H
> +
> +#include <linux/fs.h>
> +
> +#include <asm/page.h>
> +
> +/*
> + * Trace sequences are used to allow a function to call several other functions
> + * to create a string of data to use.
> + */
> +
> +/**
> + * seq_buf - seq buffer structure
> + * @buffer:	pointer to the buffer
> + * @size:	size of the buffer
> + * @len:	the amount of data inside the buffer
> + * @readpos:	The next position to read in the buffer.
> + * @overflow:	Set if more bytes should have been written to buffer

There is no @overflow flag in the end.

Also I would add an explanation of the overall logic. If I get it
correctly from the code, it is:

/*
 * The last byte of the buffer is used to detect an overflow in some
 * operations. Therefore, the buffer offers (@size - 1) bytes for valid
 * data.
 */

> + */
> +struct seq_buf {
> +	unsigned char		*buffer;
> +	unsigned int		size;
> +	unsigned int		len;
> +	unsigned int		readpos;
> +};
> +

[...]

> diff --git a/kernel/trace/seq_buf.c b/kernel/trace/seq_buf.c
> new file mode 100644
> index 000000000000..2bf582753902
> --- /dev/null
> +++ b/kernel/trace/seq_buf.c
> @@ -0,0 +1,341 @@
> +/*
> + * seq_buf.c
> + *
> + * Copyright (C) 2014 Red Hat Inc, Steven Rostedt <srostedt@xxxxxxxxxx>
> + *
> + * The seq_buf 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 seq_buf must be initialized with seq_buf_init().
> + * This will set up the counters within the descriptor. You can call
> + * seq_buf_init() more than once to reset the seq_buf to start
> + * from scratch.
> + * 

^ trailing whitespace :-)

> + */
> +#include <linux/uaccess.h>
> +#include <linux/seq_file.h>
> +#include <linux/seq_buf.h>
> +
> +/* How much buffer is left on the seq_buf? */

I would write the following to explain the -1:

/* How much buffer is left for valid data */

> +#define SEQ_BUF_LEFT(s) (((s)->size - 1) - (s)->len)

Hmm, it might overflow when the buffer has overflown (s->len == s->size)
or when the buffer is not initialized (s->size == 0). Note that the
result should be unsigned int.

I can't find any cool solution as a macro at the moment. It might be
better to define an inline function for this.


> +/* How much buffer is written? */

I would write the following to explain the -1:

/* How much buffer is written with valid data */

> +#define SEQ_BUF_USED(s) min((s)->len, (s)->size - 1)

[...]

> +
> +/**
> + * seq_buf_bitmask - write a bitmask array in its ASCII representation
> + * @s:		seq_buf 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 the number of bytes written.

The text should be:

 * Returns zero on success, -1 on overflow.

> + */
> +int seq_buf_bitmask(struct seq_buf *s, const unsigned long *maskp,
> +		    int nmaskbits)
> +{
> +	unsigned int len = SEQ_BUF_LEFT(s);
>
> +	int ret;
> +
> +	WARN_ON(s->size == 0);
> +
> +	if (s->len < s->size) {
> +		ret = bitmap_scnprintf(s->buffer, len, maskp, nmaskbits);

It writes to the beginning of the buffer. It should be

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


> +		if (s->len + ret < s->size) {

This will always happen because bitmap_scnprintf() is limited by SEQ_BUF_LEFT(s)
and it currently returns the remaining size - len - 1.

You might want to use "s->size - s->len" instead of SEQ_BUF_LEFT(s).


> +			s->len += ret;
> +			return 0;
> +		}
> +	}
> +	seq_buf_set_overflow(s);
> +	return -1;
> +}
> +
> +/**
> + * seq_buf_bprintf - Write the printf string from binary arguments
> + * @s: seq_buf 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 zero on success, -1 on overflow.
> + */
> +int seq_buf_bprintf(struct seq_buf *s, const char *fmt, const u32 *binary)
> +{
> +	unsigned int len = SEQ_BUF_LEFT(s);
> +	int ret;
> +
> +	WARN_ON(s->size == 0);
> +
> +	if (s->len < s->size) {

Always true. It is the same problem as in seq_buf_bitmask().

> +		ret = bstr_printf(s->buffer + s->len, len, fmt, binary);
> +		if (s->len + ret < s->size) {
> +			s->len += ret;
> +			return 0;
> +		}
> +	}
> +	seq_buf_set_overflow(s);
> +	return -1;
> +}
> +
> +/**
> + * seq_buf_puts - sequence printing of simple string
> + * @s: seq_buf descriptor
> + * @str: simple string to record
> + *
> + * Copy a simple string into the sequence buffer.
> + *
> + * Returns zero on success, -1 on overflow
> + */
> +int seq_buf_puts(struct seq_buf *s, const char *str)
> +{
> +	unsigned int len = strlen(str);
> +
> +	WARN_ON(s->size == 0);
> +
> +	if (s->len + len < s->size) {
> +		memcpy(s->buffer + s->len, str, len);
> +		s->len += len;
> +		return 0;
> +	}

We might want to copy the maximum possible number of bytes.
It will then behave the same as the other functions.

> +	seq_buf_set_overflow(s);
> +	return -1;
> +}
> +

[...]

> +
> +/**
> + * seq_buf_putmem - write raw data into the sequenc buffer
> + * @s: seq_buf 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 zero on success, -1 on overflow
> + */
> +int seq_buf_putmem(struct seq_buf *s, const void *mem, unsigned int len)
> +{
> +	WARN_ON(s->size == 0);
> +
> +	if (s->len + len < s->size) {
> +		memcpy(s->buffer + s->len, mem, len);
> +		s->len += len;
> +		return 0;
> +	}

Same as seq_buf_puts(). Do we want to always copy the possible number of
bytes?

> +	seq_buf_set_overflow(s);
> +	return -1;
> +}
> +
> +#define MAX_MEMHEX_BYTES	8U
> +#define HEX_CHARS		(MAX_MEMHEX_BYTES*2 + 1)
> +
> +/**
> + * seq_buf_putmem_hex - write raw memory into the buffer in ASCII hex
> + * @s: seq_buf 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 seq_buf_putmem() except instead of just copying the
> + * raw memory into the buffer it writes its ASCII representation of it
> + * in hex characters.
> + *
> + * Returns zero on success, -1 on overflow
> + */
> +int seq_buf_putmem_hex(struct seq_buf *s, const void *mem,
> +		       unsigned int len)
> +{
> +	unsigned char hex[HEX_CHARS];
> +	const unsigned char *data = mem;
> +	unsigned int start_len;
> +	int i, j;
> +
> +	WARN_ON(s->size == 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++] = ' ';
> +
> +		seq_buf_putmem(s, hex, j);
> +		if (seq_buf_has_overflowed(s))

We might want to use the seq_buf_putmem() return value here.

> +			return -1;
> +	}
> +	return 0;
> +}
> +

[...]

> +
> +/**
> + * seq_buf_to_user - copy the squence buffer to user space
> + * @s: seq_buf 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).

sequenc -> sequence

> + *
> + * Returns -EFAULT if the copy to userspace fails.
> + */
> +int seq_buf_to_user(struct seq_buf *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;
> +}

[...]

> diff --git a/kernel/trace/trace_seq.c b/kernel/trace/trace_seq.c
> index 1f24ed99dca2..960ccfb2f50c 100644
> --- a/kernel/trace/trace_seq.c
> +++ b/kernel/trace/trace_seq.c
> @@ -27,10 +27,19 @@
>  #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)
> +#define TRACE_SEQ_BUF_LEFT(s) ((PAGE_SIZE - 1) - (s)->seq.len)

This might overflow when s->len == PAGE_SIZE. I think that it
newer happenes because we always check s->full before. The question
is if we really want to depend on this.

>  /* How much buffer is written? */
> -#define TRACE_SEQ_BUF_USED(s) min((s)->len, (unsigned int)(PAGE_SIZE - 1))
> +#define TRACE_SEQ_BUF_USED(s) min((s)->seq.len, (unsigned int)(PAGE_SIZE - 1))
> +
> +/*
> + * trace_seq should work with being initialized with 0s.
> + */
> +static inline void __trace_seq_init(struct trace_seq *s)
> +{
> +	if (unlikely(!s->seq.size))
> +		trace_seq_init(s);
> +}
>  
>  /**
>   * trace_print_seq - move the contents of trace_seq into a seq_file
> @@ -43,10 +52,11 @@
>   */
>  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);
> +	__trace_seq_init(s);
> +
> +	ret = seq_buf_print_seq(m, &s->seq);
>  
>  	/*
>  	 * Only reset this buffer if we successfully wrote to the
> @@ -77,25 +87,25 @@ int trace_print_seq(struct seq_file *m, struct trace_seq *s)
>   */
>  int trace_seq_printf(struct trace_seq *s, const char *fmt, ...)
>  {
> -	unsigned int len = TRACE_SEQ_BUF_LEFT(s);
> +	unsigned int save_len = s->seq.len;
>  	va_list ap;
> -	int ret;
>  
> -	if (s->full || !len)
> +	if (s->full)
>  		return 0;
>  
> +	__trace_seq_init(s);
> +
>  	va_start(ap, fmt);
> -	ret = vsnprintf(s->buffer + s->len, len, fmt, ap);
> +	seq_buf_vprintf(&s->seq, fmt, ap);
>  	va_end(ap);
>  
>  	/* If we can't write it all, don't bother writing anything */
> -	if (ret >= len) {
> +	if (unlikely(seq_buf_has_overflowed(&s->seq))) {

We might check the return value from seq_buf_vprintf() here.

We could do similar thing also in the other functions. We even
already store the ret value in some of them.

Best Regards,
Petr
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