Re: [PATCH v5 05/12] seq_buf: Clear what a writer did not claim when a seq_buf overflows

[Steven Rostedt]

On Mon,  5 Oct 2026 08:56:55 -0700
Kees Cook <kees@xxxxxxxxxx> wrote:

> Using seq_buf_set_overflow() would leave the bytes between "len"
> and "size" untouched, so if seq_buf_str() is used on an overflowed
> seq_buf, those bytes may be exposed. For any paths that don't claim
> partially written bytes, by setting "len = size" before calling
> seq_buf_set_overflow(), wipe the unclaimed bytes. The seq_buf_puts()
> and related APIs already claim those bytes now, so only the unclaimed
> cases remain. A specific example of this was seq_buf_path() which uses
> d_path() and would write to the tail before discovering it was out
> of space, and would correctly mark a seq_buf as overflowed, but the
> path fragment would be left over.
> 
> Clear from len to the end of the buffer in seq_buf_set_overflow(), which
> every overflow goes through, including seq_buf_commit() with a negative
> count.
> 
> Add a test that fills a seq_buf, leaves it too little room for a path, and
> checks that nothing of the path is left in the buffer. The tests run before
> anything writable is mounted, so it takes its file from shmem.
> 
> seq_buf_path() was not exported, so the test failed to link as a
> module. Export it.
> 
> 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 16.2.0, and the
> test builds as a module (CONFIG_SEQ_BUF_KUNIT_TEST=m).
> 
> Assisted-by: LLM
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> Signed-off-by: Kees Cook <kees@xxxxxxxxxx>
> ---

Reviewed-by: Steven Rostedt <rostedt@xxxxxxxxxxx>

-- Steve