Re: [PATCH] riscv: lib: Fix address overflow due to large count values in strnlen ZBB path

[gao.rui]

Hi Aurelien,

>On 2026-08-19 16:18, gao.rui@xxxxxxxxxx wrote:

>> Hi all,

>> 

>> This patch fixes an issue in the RISC-V ZBB optimized strnlen implementation.

>> 

>> Problem description: When using the ZBB optimized strnlen implementation, passing very large count values (such as SIZE_MAX) can cause address overflow and return incorrect results.

>> 

>> This issue was observed in device-mapper tests:

>> 

>> sh-5.2# dmsetup create testname9 --table "0 8 zero"

>> sh-5.2# cat /sys/block/dm-*/dm/name

>> testname

>> sh-5.2# dmsetup remove testname9

>> 

>> The overflow in strnlen caused failures in string handling during device-mapper operations.

>> 

>> Patch summary:

>> - Explicitly introduce the strnlen_generic label.

>> - Simplify the generic implementation loop.

>

>This part should be in a separate patch, separated from the bug fix, and 

>if possible with some benchmark.

I agree, I will split the generic implementation changes into a separate patch and provide benchmark results 

to show the performance impact.

>> - Add fallback logic in strnlen_zbb to redirect to the generic path when a0 + a1 overflows.

>> This ensures correct behavior for large count values and SIZE_MAX cases.

>

>Is there a way to instead to fix the strnlen_zbb to avoid the fallback?

>

It is possible to make strnlen_zbb work correctly without fallback, I will reconsider this point:  if the performance loss 

of the non-fallback version turns out to be small, then using the non-fallback code may be acceptable.

Regards,  

Rui