Sometimes we use seq_buf to format a string buffer, which
we then pass to printk(). However, in certain situations
the seq_buf string buffer can get too big, exceeding the
PRINTKRB_RECORD_MAX bytes limit, and causing printk() to
truncate the string.

Add a new seq_buf helper. This helper prints the seq_buf
string buffer line by line, using \n as a delimiter,
rather than passing the whole string buffer to printk()
at once.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
 include/linux/seq_buf.h |  2 ++
 lib/seq_buf.c           | 30 ++++++++++++++++++++++++++++++
 2 files changed, 32 insertions(+)

diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
index 5b31c5147969..80b78df89809 100644
--- a/include/linux/seq_buf.h
+++ b/include/linux/seq_buf.h
@@ -159,4 +159,6 @@ extern int
 seq_buf_bprintf(struct seq_buf *s, const char *fmt, const u32 *binary);
 #endif
 
+void seq_buf_printk(struct seq_buf *s, const char *lvl);
+
 #endif /* _LINUX_SEQ_BUF_H */
diff --git a/lib/seq_buf.c b/lib/seq_buf.c
index 0a68f7aa85d6..9d13004c2324 100644
--- a/lib/seq_buf.c
+++ b/lib/seq_buf.c
@@ -93,6 +93,36 @@ int seq_buf_printf(struct seq_buf *s, const char *fmt, ...)
 }
 EXPORT_SYMBOL_GPL(seq_buf_printf);
 
+/**
+ * seq_buf_printk - printk seq_buf line by line
+ * @s: seq_buf descriptor
+ * @lvl: printk level
+ *
+ * printk()-s a multi-line sequential buffer line by line
+ */
+void seq_buf_printk(struct seq_buf *s, const char *lvl)
+{
+	const char *start, *lf;
+	int len;
+
+	if (s->size == 0)
+		return;
+
+	start = s->buffer;
+	while ((lf = strchr(start, '\n'))) {
+		len = lf - start + 1;
+		printk("%s%.*s", lvl, len, start);
+		start = ++lf;
+	}
+
+	/* No trailing LF */
+	if (start < s->buffer + s->len) {
+		len = s->buffer + s->len - start;
+		printk("%s%.*s\n", lvl, len, start);
+	}
+}
+EXPORT_SYMBOL_GPL(seq_buf_printk);
+
 #ifdef CONFIG_BINARY_PRINTF
 /**
  * seq_buf_bprintf - Write the printf string from binary arguments
-- 
2.40.0.577.gac1e443424-goog

On Tue 2023-04-11 11:55:56, Sergey Senozhatsky wrote:
> Sometimes we use seq_buf to format a string buffer, which
> we then pass to printk(). However, in certain situations
> the seq_buf string buffer can get too big, exceeding the
> PRINTKRB_RECORD_MAX bytes limit, and causing printk() to
> truncate the string.
> 
> Add a new seq_buf helper. This helper prints the seq_buf
> string buffer line by line, using \n as a delimiter,
> rather than passing the whole string buffer to printk()
> at once.
> 
> Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
> ---
>  include/linux/seq_buf.h |  2 ++
>  lib/seq_buf.c           | 30 ++++++++++++++++++++++++++++++
>  2 files changed, 32 insertions(+)
> 
> diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
> index 5b31c5147969..80b78df89809 100644
> --- a/include/linux/seq_buf.h
> +++ b/include/linux/seq_buf.h
> @@ -159,4 +159,6 @@ extern int
>  seq_buf_bprintf(struct seq_buf *s, const char *fmt, const u32 *binary);
>  #endif
>  
> +void seq_buf_printk(struct seq_buf *s, const char *lvl);
> +
>  #endif /* _LINUX_SEQ_BUF_H */
> diff --git a/lib/seq_buf.c b/lib/seq_buf.c
> index 0a68f7aa85d6..9d13004c2324 100644
> --- a/lib/seq_buf.c
> +++ b/lib/seq_buf.c
> @@ -93,6 +93,36 @@ int seq_buf_printf(struct seq_buf *s, const char *fmt, ...)
>  }
>  EXPORT_SYMBOL_GPL(seq_buf_printf);
>  
> +/**
> + * seq_buf_printk - printk seq_buf line by line
> + * @s: seq_buf descriptor
> + * @lvl: printk level
> + *
> + * printk()-s a multi-line sequential buffer line by line
> + */
> +void seq_buf_printk(struct seq_buf *s, const char *lvl)

We might want to somehow distinguish that this is actually
printing (reading) the context of the buffer.

The name is similar to seq_buf_printf() and seq_buf_vprintf()
whose are wrinting into the buffer.

What about the following?

     + seq_buf_printf_seq() like the existing seq_buf_print_seq()
     + seq_buf_to_printk() like the existing seq_buf_to_user()

I personally prefer seq_buf_to_printk() because it looks more
selfexplaining to me.

Or maybe: seq_buf_to_printk_lvl() to allow later add
seq_buf_to_printk() that would us the default loglevel.


> +{
> +	const char *start, *lf;
> +	int len;
> +
> +	if (s->size == 0)
> +		return;
> +
> +	start = s->buffer;
> +	while ((lf = strchr(start, '\n'))) {

We should rather use strnchr(). It seems that the trailing '\0' is
not guaranteed. For example, seq_buf_putc() just adds the given
character at the end.

> +		len = lf - start + 1;
> +		printk("%s%.*s", lvl, len, start);
> +		start = ++lf;
> +	}
> +
> +	/* No trailing LF */
> +	if (start < s->buffer + s->len) {
> +		len = s->buffer + s->len - start;
> +		printk("%s%.*s\n", lvl, len, start);
> +	}
> +}
> +EXPORT_SYMBOL_GPL(seq_buf_printk);

Otherwise, it looks good to me.

Best Regards,
Petr

On (23/04/11 14:14), Petr Mladek wrote:
> We might want to somehow distinguish that this is actually
> printing (reading) the context of the buffer.
> 
> The name is similar to seq_buf_printf() and seq_buf_vprintf()
> whose are wrinting into the buffer.
> 
> What about the following?
> 
>      + seq_buf_printf_seq() like the existing seq_buf_print_seq()
>      + seq_buf_to_printk() like the existing seq_buf_to_user()
> 
> I personally prefer seq_buf_to_printk() because it looks more
> selfexplaining to me.

I like seq_buf_to_printk().

> > +{
> > +	const char *start, *lf;
> > +	int len;
> > +
> > +	if (s->size == 0)
> > +		return;
> > +
> > +	start = s->buffer;
> > +	while ((lf = strchr(start, '\n'))) {
> 
> We should rather use strnchr(). It seems that the trailing '\0' is
> not guaranteed. For example, seq_buf_putc() just adds the given
> character at the end.

Good point.

On Tue, 11 Apr 2023 14:14:30 +0200
Petr Mladek <pmladek@suse.com> wrote:

> > diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
> > index 5b31c5147969..80b78df89809 100644
> > --- a/include/linux/seq_buf.h
> > +++ b/include/linux/seq_buf.h
> > @@ -159,4 +159,6 @@ extern int
> >  seq_buf_bprintf(struct seq_buf *s, const char *fmt, const u32 *binary);
> >  #endif
> >  
> > +void seq_buf_printk(struct seq_buf *s, const char *lvl);
> > +
> >  #endif /* _LINUX_SEQ_BUF_H */
> > diff --git a/lib/seq_buf.c b/lib/seq_buf.c
> > index 0a68f7aa85d6..9d13004c2324 100644
> > --- a/lib/seq_buf.c
> > +++ b/lib/seq_buf.c
> > @@ -93,6 +93,36 @@ int seq_buf_printf(struct seq_buf *s, const char *fmt, ...)
> >  }
> >  EXPORT_SYMBOL_GPL(seq_buf_printf);
> >  
> > +/**
> > + * seq_buf_printk - printk seq_buf line by line
> > + * @s: seq_buf descriptor
> > + * @lvl: printk level
> > + *
> > + * printk()-s a multi-line sequential buffer line by line
> > + */
> > +void seq_buf_printk(struct seq_buf *s, const char *lvl)  
> 
> We might want to somehow distinguish that this is actually
> printing (reading) the context of the buffer.
> 
> The name is similar to seq_buf_printf() and seq_buf_vprintf()
> whose are wrinting into the buffer.
> 
> What about the following?
> 
>      + seq_buf_printf_seq() like the existing seq_buf_print_seq()
>      + seq_buf_to_printk() like the existing seq_buf_to_user()
> 
> I personally prefer seq_buf_to_printk() because it looks more
> selfexplaining to me.
> 
> Or maybe: seq_buf_to_printk_lvl() to allow later add
> seq_buf_to_printk() that would us the default loglevel.
> 

seq_buf came from trace_seq to become more generic. trace_seq is also part
of the libtraceevent library, and there we have:

  trace_seq_do_printf()

 https://www.trace-cmd.org/Documentation/libtraceevent/libtraceevent-tseq.html

That is to differentiate the trace_seq_printf() function. And does what
this function is doing (dumping the buffer).

Perhaps we should use the precedence of that function and have:

  seq_buf_do_printk()

> 
> > +{
> > +	const char *start, *lf;
> > +	int len;
> > +
> > +	if (s->size == 0)
> > +		return;
> > +
> > +	start = s->buffer;
> > +	while ((lf = strchr(start, '\n'))) {  
> 
> We should rather use strnchr(). It seems that the trailing '\0' is
> not guaranteed. For example, seq_buf_putc() just adds the given
> character at the end.

I think we should add a seq_buf_terminate() that adds the '\0' to the
buffer. There's one for user space (trace_seq_terminate()).

It's a nop if it already has the '\0'.

I think we should add that first, and then we can use that when entering
this function.

-- Steve

> 
> > +		len = lf - start + 1;
> > +		printk("%s%.*s", lvl, len, start);
> > +		start = ++lf;
> > +	}
> > +
> > +	/* No trailing LF */
> > +	if (start < s->buffer + s->len) {
> > +		len = s->buffer + s->len - start;
> > +		printk("%s%.*s\n", lvl, len, start);
> > +	}
> > +}
> > +EXPORT_SYMBOL_GPL(seq_buf_printk);  
> 
> Otherwise, it looks good to me.
> 
> Best Regards,
> Petr

On (23/04/11 10:10), Steven Rostedt wrote:
[..]
> seq_buf came from trace_seq to become more generic. trace_seq is also part
> of the libtraceevent library, and there we have:
> 
>   trace_seq_do_printf()
> 
>  https://www.trace-cmd.org/Documentation/libtraceevent/libtraceevent-tseq.html
> 
> That is to differentiate the trace_seq_printf() function. And does what
> this function is doing (dumping the buffer).
> 
> Perhaps we should use the precedence of that function and have:
> 
>   seq_buf_do_printk()

OK, I'll rename the helper to seq_buf_do_printk().

> > We should rather use strnchr(). It seems that the trailing '\0' is
> > not guaranteed. For example, seq_buf_putc() just adds the given
> > character at the end.
> 
> I think we should add a seq_buf_terminate() that adds the '\0' to the
> buffer. There's one for user space (trace_seq_terminate()).
> 
> It's a nop if it already has the '\0'.
> 
> I think we should add that first, and then we can use that when entering
> this function.

Sounds good. This also should address Petr's concern.

On Mon, Apr 10, 2023 at 7:56 PM Sergey Senozhatsky
<senozhatsky@chromium.org> wrote:
>
> Sometimes we use seq_buf to format a string buffer, which
> we then pass to printk(). However, in certain situations
> the seq_buf string buffer can get too big, exceeding the
> PRINTKRB_RECORD_MAX bytes limit, and causing printk() to
> truncate the string.
>
> Add a new seq_buf helper. This helper prints the seq_buf
> string buffer line by line, using \n as a delimiter,
> rather than passing the whole string buffer to printk()
> at once.
>
> Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>


I tested this with a local patch that I am currently working on that
benefits from this helper, works great.

Tested-by: Yosry Ahmed <yosryahmed@google.com>

>
> ---
>  include/linux/seq_buf.h |  2 ++
>  lib/seq_buf.c           | 30 ++++++++++++++++++++++++++++++
>  2 files changed, 32 insertions(+)
>
> diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
> index 5b31c5147969..80b78df89809 100644
> --- a/include/linux/seq_buf.h
> +++ b/include/linux/seq_buf.h
> @@ -159,4 +159,6 @@ extern int
>  seq_buf_bprintf(struct seq_buf *s, const char *fmt, const u32 *binary);
>  #endif
>
> +void seq_buf_printk(struct seq_buf *s, const char *lvl);
> +
>  #endif /* _LINUX_SEQ_BUF_H */
> diff --git a/lib/seq_buf.c b/lib/seq_buf.c
> index 0a68f7aa85d6..9d13004c2324 100644
> --- a/lib/seq_buf.c
> +++ b/lib/seq_buf.c
> @@ -93,6 +93,36 @@ int seq_buf_printf(struct seq_buf *s, const char *fmt, ...)
>  }
>  EXPORT_SYMBOL_GPL(seq_buf_printf);
>
> +/**
> + * seq_buf_printk - printk seq_buf line by line
> + * @s: seq_buf descriptor
> + * @lvl: printk level
> + *
> + * printk()-s a multi-line sequential buffer line by line
> + */
> +void seq_buf_printk(struct seq_buf *s, const char *lvl)
> +{
> +       const char *start, *lf;
> +       int len;
> +
> +       if (s->size == 0)
> +               return;
> +
> +       start = s->buffer;
> +       while ((lf = strchr(start, '\n'))) {
> +               len = lf - start + 1;
> +               printk("%s%.*s", lvl, len, start);
> +               start = ++lf;
> +       }
> +
> +       /* No trailing LF */
> +       if (start < s->buffer + s->len) {
> +               len = s->buffer + s->len - start;
> +               printk("%s%.*s\n", lvl, len, start);
> +       }
> +}
> +EXPORT_SYMBOL_GPL(seq_buf_printk);
> +
>  #ifdef CONFIG_BINARY_PRINTF
>  /**
>   * seq_buf_bprintf - Write the printf string from binary arguments
> --
> 2.40.0.577.gac1e443424-goog
>