[PATCH v3 02/11] seq_buf: Do not pop from an overflowed seq_buf

[sashiko-bot at kernel.org]

> When a seq_buf has overflowed, its len is size + 1, so seq_buf_pop()
> decrements len to size and reads buffer[size], one byte past the end of
> the buffer. It also leaves len equal to size, which no longer counts as
> overflowed, so a truncated seq_buf then looks like a complete, full one.
> 
> An overflowed seq_buf logically has no last character to pop: the
> length of what was written has been lost, and the last byte of the
> buffer may be the NUL written by vsnprintf() or bytes that were never
> committed. Return -1 for an overflowed seq_buf, as for an empty one,
> and leave it overflowed, as the rest of the seq_buf API does until
> seq_buf_clear() or seq_buf_init().
> 
> The current callers do not reach this, e.g. trace_syscalls only calls
> trace_seq_pop() when the trace_seq it pops from has not overflowed, and
> kernel/bpf/diagnostics.c sets the length from strnlen() before popping.
> 
> Add tests for the pop corner cases.
> 
> 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.
> 
> Fixes: 32e0f607ac6a2 ("tracing: Add trace_seq_pop() and seq_buf_pop()")
> Assisted-by: LLM
> Reviewed-by: Andy Shevchenko <andriy.shevchenko at linux.intel.com>
> Signed-off-by: Kees Cook <kees at kernel.org>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260930235231.out.387-kees@kernel.org?part=2