Re: [PATCH v2] seq_buf: add seq_buf_strlen(), seq_buf_init_append(), seq_buf_puts_trunc()

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 16 Sep 2026 22:21:25 +0000 Bill Wendling <morbo@google.com> wrote:

> Converting some strlcat() call sites seq_buf need behavior seq_buf doesn't
> currently provide, either directly or without introducing subtle bugs:
> 
>  - seq_buf_strlen(): seq_buf_used() reports the full buffer size when the
>    buffer is completely filled, even though seq_buf_str() then overwrites the
>    final byte with a NUL terminator, leaving only size - 1 bytes of
>    actual content. Callers that need the true length of the
>    NUL-terminated string have to fall back to strlen(seq_buf_str(s)).
>    seq_buf_strlen() mirrors seq_buf_str()'s NUL-termination logic but returns
>    the resulting string's length directly.
> 
>  - seq_buf_init_append(): seq_buf_init() always clears the buffer it's given
>    via seq_buf_clear(). Code migrating from strlcat(buf, ...), which
>    appends to whatever @buf already contains, can't use seq_buf_init()
>    without silently discarding that existing content. seq_buf_init_append()
>    preserves it and positions the seq_buf to append after it.
> 
>  - seq_buf_puts_trunc(): seq_buf_puts() (like seq_buf_printf() and friends)
>    writes nothing at all if the string doesn't fully fit, whereas strlcat()
>    always copies as much of the source as there is room for. Converting a
>    strlcat() call site that relied on that partial-copy behavior to
>    plain seq_buf_puts() can silently drop content that used to survive
>    truncated. seq_buf_puts_trunc() keeps the leading bytes of the string
>    that fit.
> 
> ...
>
>  include/linux/seq_buf.h | 60 +++++++++++++++++++++++++++++++++++++++++
>  lib/seq_buf.c           | 35 ++++++++++++++++++++++++
>  2 files changed, 95 insertions(+)

This seems a lot of code for enabling some strlcat removals.  How many
is "some"?  If "3" then perhaps do something different at those
callsites?

Sashiko had a couple of comments:
	https://sashiko.dev/#/patchset/20260916222125.1259631-1-morbo@google.com

Should these new functions be added to lib/tests/seq_buf_kunit.c?