linux-kernel - Re: [PATCH v2] scsi: 3w-9xxx: Fix endianness issues found by sparse

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Date:   Sun, 2 Aug 2020 22:43:19 -0500
From:   Samuel Holland <samuel@...lland.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Adam Radford <aradford@...il.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-scsi <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] scsi: 3w-9xxx: Fix endianness issues found by sparse

On 7/31/20 2:29 AM, Arnd Bergmann wrote:
> On Fri, Jul 31, 2020 at 12:07 AM Samuel Holland <samuel@...lland.org> wrote:
>>
>> The main issue observed was at the call to scsi_set_resid, where the
>> byteswapped parameter would eventually trigger the alignment check at
>> drivers/scsi/sd.c:2009. At that point, the kernel would continuously
>> complain about an "Unaligned partial completion", and no further I/O
>> could occur.
>>
>> This gets the controller working on big endian powerpc64.
>>
>> Signed-off-by: Samuel Holland <samuel@...lland.org>
>> ---
>>
>> Changes since v1:
>>  - Include changes to use __le?? types in command structures
>>  - Use an object literal for the intermediate "schedulertime" value
>>  - Use local "error" variable to avoid repeated byte swapping
>>  - Create a local "length" variable to avoid very long lines
>>  - Move byte swapping to TW_REQ_LUN_IN/TW_LUN_OUT to avoid long lines
>>
> 
> Looks much better, thanks for the update. I see one more issue here
>>  /* Command Packet */
>>  typedef struct TW_Command {
>> -       unsigned char opcode__sgloffset;
>> -       unsigned char size;
>> -       unsigned char request_id;
>> -       unsigned char unit__hostid;
>> +       u8      opcode__sgloffset;
>> +       u8      size;
>> +       u8      request_id;
>> +       u8      unit__hostid;
>>         /* Second DWORD */
>> -       unsigned char status;
>> -       unsigned char flags;
>> +       u8      status;
>> +       u8      flags;
>>         union {
>> -               unsigned short block_count;
>> -               unsigned short parameter_count;
>> +               __le16  block_count;
>> +               __le16  parameter_count;
>>         } byte6_offset;
>>         union {
>>                 struct {
>> -                       u32 lba;
>> -                       TW_SG_Entry sgl[TW_ESCALADE_MAX_SGL_LENGTH];
>> -                       dma_addr_t padding;
>> +                       __le32          lba;
>> +                       TW_SG_Entry     sgl[TW_ESCALADE_MAX_SGL_LENGTH];
>> +                       dma_addr_t      padding;
> 
> 
> The use of dma_addr_t here seems odd, since this is neither endian-safe nor
> fixed-length. I see you replaced the dma_addr_t in TW_SG_Entry with
> a variable-length fixed-endian word. I guess there is a chance that this is
> correct, but it is really confusing. On top of that, it seems that there is
> implied padding in the structure when built with a 64-bit dma_addr_t
> on most architectures but not on x86-32 (which uses 32-bit alignment for
> 64-bit integers). I don't know what the hardware definition is for TW_Command,
> but ideally this would be expressed using only fixed-endian fixed-length
> members and explicit padding.

All of the command structures are packed, due to the "#pragma pack(1)" earlier
in the file. So alignment is not an issue. This dma_addr_t member _is_ the
explicit padding to make sizeof(TW_Command) -
sizeof(TW_Command.byte8_offset.{io,param}.sgl) equal TW_COMMAND_SIZE * 4. And
indeed the structure is expected to be a different size depending on
sizeof(dma_addr_t).

I left the padding member alone to avoid the #ifdef; since it's never accessed,
the endianness doesn't matter. In fact, since in both cases it's at the end of
the structure, it could probably be removed entirely. I don't see
sizeof(TW_Command) being used anywhere, but I'm not 100% certain. The downside
of removing it would be TW_COMMAND_SIZE becoming a slightly more magic number.

Regards,
Samuel