Re: [PATCH] ata: libata-core: fix sloppy typing in ata_id_n_sectors()

[Sergey Shtylyov <s.shtylyov@omp.ru>]

Hello!

On 5/16/22 2:29 PM, Damien Le Moal wrote:

>> The code multiplying the # of cylinders/heads/sectors in ata_id_n_sectors()
>> to get a disk capacity implicitly uses the *int* type for that calculation
>> and casting the result to 'u64' before returning ensues a sign extension.
>> Explicitly casting the 'u16' typed multipliers to 'u32' results in avoiding
>> a sign extension instruction and so in a more compact code...
>>
>> Found by Linux Verification Center (linuxtesting.org) with the SVACE static
>> analysis tool.
>>
>> Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
>>
>> ---
>> This patch is against the 'for-next' branch of Damien's 'libata.git' repo.
>>
>>  drivers/ata/libata-core.c |   10 ++++++----
>>  1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> Index: libata/drivers/ata/libata-core.c
>> ===================================================================
>> --- libata.orig/drivers/ata/libata-core.c
>> +++ libata/drivers/ata/libata-core.c
>> @@ -1107,11 +1107,13 @@ static u64 ata_id_n_sectors(const u16 *i
>>  			return ata_id_u32(id, ATA_ID_LBA_CAPACITY);
>>  	} else {
>>  		if (ata_id_current_chs_valid(id))
>> -			return id[ATA_ID_CUR_CYLS] * id[ATA_ID_CUR_HEADS] *
>> -			       id[ATA_ID_CUR_SECTORS];
>> +			return (u32)id[ATA_ID_CUR_CYLS] *
>> +			       (u32)id[ATA_ID_CUR_HEADS] *
>> +			       (u32)id[ATA_ID_CUR_SECTORS];
>>  		else
> 
> While at it, you can drop this useless "else". The 2 else above this one are
> actually also useless...

   OK. But I think it's all a matter of a separate patch. I don't want to touch
the LBA branches in this same patch...

>> -			return id[ATA_ID_CYLS] * id[ATA_ID_HEADS] *
>> -			       id[ATA_ID_SECTORS];
>> +			return (u32)id[ATA_ID_CYLS] *
>> +			       (u32)id[ATA_ID_HEADS] *
>> +			       (u32)id[ATA_ID_SECTORS];
> 
> Given that the function returns an u64, I would cast everything to u64. That

   I don't think this is a good idea. Looking at the produced x86 32-bit code,
gcc produces an extra (3rd) multiplication instruction for no value.

> will avoid overflows too, which was possible before,

   No, it wasn't possible. Any possible CHS capacity always fits into 32 bits --
max # of sectors per track is 255, max # of heads is only 16.
   What actually seems to make sense is changing the order of multiplications
to first multiply # of sectors by # of heads and than multiply that by # of
cylinders...

> eventhough no problems seem
> to have been reported...

   Because there's not problem. :-)
   The current CHS capacity is stored in the words 57-58 (so 32-bit) and we
could read it from there instead of the multiplications... BUT I do remember
the disks (IIRC Fujitsu... but I'm not sure now -- that was back in 90s!)
that had totally wrong value in these words... so the code we have now is
a good thing! :-)

> Who uses CHS these days :)

   Indeed, the CHS days are long gone... :-)

[...]

MBR, Sergey