linux-kernel - Re: [PATCH v3 2/4] fsi: occ: Store the SBEFIFO FFDC in the user response buffer

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Message-ID: <48d997c0-5b2e-40d2-ef66-594b4275fd1a@linux.ibm.com>
Date:   Tue, 19 Oct 2021 15:16:22 -0500
From:   Eddie James <eajames@...ux.ibm.com>
To:     Joel Stanley <joel@....id.au>, Amitay Isaacs <amitay@...abs.org>
Cc:     linux-fsi@...ts.ozlabs.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-hwmon@...r.kernel.org, Jeremy Kerr <jk@...abs.org>,
        Alistair Popple <alistair@...ple.id.au>,
        Guenter Roeck <linux@...ck-us.net>,
        Jean Delvare <jdelvare@...e.com>
Subject: Re: [PATCH v3 2/4] fsi: occ: Store the SBEFIFO FFDC in the user
 response buffer


On 10/15/21 12:05 AM, Joel Stanley wrote:
> On Mon, 27 Sept 2021 at 15:59, Eddie James <eajames@...ux.ibm.com> wrote:
>> If the SBEFIFO response indicates an error, store the response in the
>> user buffer and return an error. Previously, the user had no way of
>> obtaining the SBEFIFO FFDC.
> How does this look for userspace?


The user's buffer now contains data in the event of a failure. No change 
in the event of a successful transfer.


>
> Will existing userspace handle this?


Yes, unless a poorly-designed application is relying on the data being 
the same after a failed transfer... In that case I would argue that the 
application should be fixed.


>
>> Signed-off-by: Eddie James <eajames@...ux.ibm.com>
>> ---
>> Changes since v1:
>>   - Don't store any magic value; only return non-zero resp_len in the error
>>     case if there is FFDC
>>
>>   drivers/fsi/fsi-occ.c | 66 ++++++++++++++++++++++++++++++-------------
>>   1 file changed, 47 insertions(+), 19 deletions(-)
>>
>> diff --git a/drivers/fsi/fsi-occ.c b/drivers/fsi/fsi-occ.c
>> index ace3ec7767e5..1d5f6fdc2a34 100644
>> --- a/drivers/fsi/fsi-occ.c
>> +++ b/drivers/fsi/fsi-occ.c
>> @@ -55,6 +55,9 @@ struct occ {
>>          int idx;
>>          u8 sequence_number;
>>          void *buffer;
>> +       void *client_buffer;
>> +       size_t client_buffer_size;
>> +       size_t client_response_size;
>>          enum versions version;
>>          struct miscdevice mdev;
>>          struct mutex occ_lock;
>> @@ -217,6 +220,20 @@ static const struct file_operations occ_fops = {
>>          .release = occ_release,
>>   };
>>
>> +static void occ_save_ffdc(struct occ *occ, __be32 *resp, size_t parsed_len,
>> +                         size_t resp_len)
>> +{
>> +       size_t dh = resp_len - parsed_len;
> Is there any chance that parsed_len is larger than resp_len?


No, based on the sbefifo_parse_status function.


>
>> +       size_t ffdc_len = (dh - 1) * 4;
>> +       __be32 *ffdc = &resp[resp_len - dh];
> Should you be checking that this number is sensible?


Do you mean ffdc_len or (resp_len - dh)? I was basing all this on the 
sbefifo_parse_status code, but I see that obviously:

resp_len - dh = resp_len - (resp_len - parsed_len) = parsed_len

So I will simplify.

As for ffdc_len, it is conceivable that dh is 0, so I will add a check 
for that.



Thanks Joel!

Eddie


>
>> +
>> +       if (ffdc_len > occ->client_buffer_size)
>> +               ffdc_len = occ->client_buffer_size;
>> +
>> +       memcpy(occ->client_buffer, ffdc, ffdc_len);
>> +       occ->client_response_size = ffdc_len;
>> +}