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@xxxxxx>
>>
>> ---
>> 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
