On Fri, 18 Sep 2026 17:27:03 -0700 Kees Cook <[email protected]> wrote:
> Several strlcat() call sites being converted to seq_buf need behavior > seq_buf doesn't currently provide. The return from seq_buf_used() is > not the length of the string in a seq_buf. Once the buffer is full or > has overflowed it returns the buffer size, which counts the byte that > seq_buf_str() replaces with the NUL, so a caller that needs the string > and its length has to call seq_buf_str() and then walk the string with > strlen(). > > Move the termination out of seq_buf_str() into a helper that returns > where it put the NUL, and add seq_buf_strlen(), which terminates the > buffer in the same way and returns that offset. > Let's make a appropriate function to terminate the string in the seq_buf and not have it be an internal helper function (This has been on my todo list for some time). > Add tests comparing seq_buf_strlen() against strlen() of seq_buf_str() > for empty, appended, truncated, exactly full, and overflowed buffers, > and checking that seq_buf_strlen() alone terminates a full buffer. > > Tests passed under qemu on ARCH=x86_64 with GCC 16.2.0 and CONFIG_KASAN=y, > and on big-endian ARCH=s390 with GCC s390x-linux-gnu 16.1.0. > > Assisted-by: LLM > Reviewed-by: Andy Shevchenko <[email protected]> > Signed-off-by: Kees Cook <[email protected]> > --- > Cc: "Matthew Wilcox (Oracle)" <[email protected]> > Cc: Andrew Morton <[email protected]> > Cc: Andy Shevchenko <[email protected]> > Cc: David Gow <[email protected]> > Cc: Petr Mladek <[email protected]> > Cc: Shuvam Pandey <[email protected]> > Cc: Steven Rostedt <[email protected]> > --- > include/linux/seq_buf.h | 57 +++++++++++++++++-- > lib/tests/seq_buf_kunit.c | 114 ++++++++++++++++++++++++++++++++++++++ > 2 files changed, 166 insertions(+), 5 deletions(-) > > diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h > index 0c0a0db04b09..7f025c7a68be 100644 > --- a/include/linux/seq_buf.h > +++ b/include/linux/seq_buf.h > @@ -89,6 +89,27 @@ static inline unsigned int seq_buf_used(struct seq_buf *s) > return min(s->len, s->size); > } > > +/* > + * NUL-terminate the buffer in @s: directly after the data when there is > + * room for it, otherwise in the last byte of the buffer. @s->size must not > + * be zero. > + * > + * Returns: the offset of the NUL. > + */ > +static inline size_t __seq_buf_terminate(struct seq_buf *s) > +{ > + size_t end; > + > + if (seq_buf_buffer_left(s)) > + end = s->len; > + else > + end = s->size - 1; > + > + s->buffer[end] = 0; > + > + return end; > +} That is, make this a separate patch to introduce a "seq_buf_terminate()" function as there's several places in the kernel that could replace seq_buf_str() with it. Thanks, -- Steve
