Re: [PATCH] riscv: lib: Fix ZBB strnlen wrap-around regression on huge counts

[David Laight <david.laight.linux@gmail.com>]

On Fri, 28 Aug 2026 14:51:52 +0800 (CST)
<shao.mingyin@zte.com.cn> wrote:

> From: Shao Mingyin <shao.mingyin@zte.com.cn>
> 
> commit 5d588c684833 ("riscv: lib: Fix ZBB strnlen reading past count
> boundary") computes the aligned scan boundary from the last valid byte
> (s + count - 1).  When count is huge (e.g. SIZE_MAX, which FORTIFY
> strcat/strlcat pass when the destination size is not known at compile
> time), s + count wraps around and the aligned boundary lands before s.
> The pre-loop guard "bgeu t0, t4, 2f" then always exits for aligned
> strings of 8 or more characters, and strnlen() returns 8 instead of
> the real length.
> 
> This silently truncates strings built by fortified strcat: the dm
> sysfs name attribute shows "live-bas" instead of "live-base", the
> truncated name pollutes the udev database, and blivet/anaconda (as
> well as LVM/dm-crypt/multipath userspace) break on RISC-V systems.
> 
> Detect the wrap-around and saturate the boundary to the top of the
> address space, making the scan equivalent to strlen().  Normal counts
> are unaffected.
> 
> Fixes: 5d588c684833 ("riscv: lib: Fix ZBB strnlen reading past count boundary")
> Signed-off-by: Shao Mingyin <shao.mingyin@zte.com.cn>
> ---
>  arch/riscv/lib/strnlen.S | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/arch/riscv/lib/strnlen.S b/arch/riscv/lib/strnlen.S
> index a8911605c248..2451289ed0f9 100644
> --- a/arch/riscv/lib/strnlen.S
> +++ b/arch/riscv/lib/strnlen.S
> @@ -87,9 +87,19 @@ strnlen_zbb:
>  	 * Aligned boundary.  Use the address of the last valid byte
>  	 * (s + count - 1) to avoid loading a word past the count
>  	 * boundary in the loop below.  count == 0 is handled above.
> +	 *
> +	 * Saturate the boundary when s + count wraps around (very large
> +	 * counts, e.g. SIZE_MAX passed by FORTIFY strcat/strlcat with a
> +	 * destination whose size is unknown at compile time).  Without
> +	 * this, the wrapped boundary lands before s and the pre-loop
> +	 * guard below always exits, returning a truncated length.
> +	 * Saturating makes the scan equivalent to strlen().
>  	 */
>  	add	t4, a0, a1
>  	addi	t4, t4, -1
> +	bgeu	t4, a0, 1f
> +	li	t4, -1
> +1:

Ugg....
That adds a taken branch to the normal path that is likely to get
mispredicted.
Might be better replaced with a cmp, dec, or sequence.

David 


>  	andi	t4, t4, -SZREG
> 
>  	/* Get the first word.  */