[PATCH] riscv, lib: Fix Zbb strncmp

[Palmer Dabbelt]

On Tue, 28 Feb 2023 13:18:12 PST (-0800), linux at roeck-us.net wrote:
> On 2/28/23 10:42, Björn Töpel wrote:
>> From: Björn Töpel <bjorn at rivosinc.com>
>>
>> The Zbb optimized strncmp has two parts; a fast path that does XLEN/8B
>> per iteration, and a slow that does one byte per iteration.
>>
>> The idea is to compare aligned XLEN chunks for most of strings, and do
>> the remainder tail in the slow path.
>>
>> The Zbb strncmp has two issues in the fast path:
>>
>> Incorrect remainder handling (wrong compare): Assume that the string
>> length is 9. On 64b systems, the fast path should do one iteration,
>> and one iteration in the slow path. Instead, both were done in the
>> fast path, which lead to incorrect results. An example:
>>
>>    strncmp("/dev/vda", "/dev/", 5);
>>
>> Correct by changing "bgt" to "bge".
>>
>> Missing NULL checks in the second string: This could lead to incorrect
>> results for:
>>
>>    strncmp("/dev/vda", "/dev/vda\0", 8);
>>
>> Correct by adding an additional check.
>>
>> Fixes: b6fcdb191e36 ("RISC-V: add zbb support to string functions")
>> Suggested-by: Heiko Stuebner <heiko.stuebner at vrull.eu>
>> Signed-off-by: Björn Töpel <bjorn at rivosinc.com>
>
> Tested-by: Guenter Roeck <linux at roeck-us.net>

Thanks guys, I'm going to apply this ASAP as it's a pretty concrete 
breakage in new code from this merge window.  I just stuck it in my 
staging branch, it should show up on for-next after the next batch of 
tests.

>
>> ---
>>   arch/riscv/lib/strncmp.S | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/riscv/lib/strncmp.S b/arch/riscv/lib/strncmp.S
>> index ee49595075be..efd3f3150a54 100644
>> --- a/arch/riscv/lib/strncmp.S
>> +++ b/arch/riscv/lib/strncmp.S
>> @@ -78,11 +78,13 @@ strncmp_zbb:
>>   	/* Main loop for aligned string.  */
>>   	.p2align 3
>>   1:
>> -	bgt	a0, t6, 3f
>> +	bge	a0, t6, 3f
>>   	REG_L	t0, 0(a0)
>>   	REG_L	t1, 0(a1)
>>   	orc.b	t3, t0
>>   	bne	t3, t5, 2f
>> +	orc.b	t3, t1
>> +	bne	t3, t5, 2f
>>   	addi	a0, a0, SZREG
>>   	addi	a1, a1, SZREG
>>   	beq	t0, t1, 1b
>>
>> base-commit: eb9be8310c58c166f9fae3b71c0ad9d6741b4897