Re: [PATCH v4 1/5] powerpc/rtas: Handle ibm,open-errinjct return format

[Narayana Murty N <nnmlinux@linux.ibm.com>]

Hi Sourabh,

Thanks for the detailed review.

On 01/09/26 2:52 PM, Sourabh Jain wrote:
> 
> On 31/08/26 12:24, Narayana Murty N wrote:
>> ibm,open-errinjct uses a non-standard RTAS return layout:
>>
>>    rets[0] = session token  (output parameter)
>>    rets[1] = status code
>>
>> Unlike all other RTAS functions which use:
>>
>>    rets[0] = status code
>>    rets[1..] = output parameters
>>
>> Add rtas_token_is_open_errinjct() to identify this call, and
>> rtas_status_from_args() to extract status from the correct position.
>>
>> Add an early guard in rtas_call() that rejects ibm,open-errinjct
>> invocations where nret < 2, since reading rets[1] would be out of
>> bounds:
>>
>>    if (rtas_token_is_open_errinjct(token) && nret < 2) {
>>            WARN_ON_ONCE(1);
>>            return RTAS_INVALID_PARAMETER;
>>    }
>>
>> Adjust the output-copy loop so that for ibm,open-errinjct:
>>
>>    return value = rets[1]   (status)
>>    outputs[0]   = rets[0]   (session token)
>>
>> For all other functions the existing convention is preserved:
>>
>>    return value = rets[0]   (status)
>>    outputs[0..] = rets[1..] (non-status outputs)
>>
>> Move the "/* A -1 return code... */" comment immediately before the
>> ret == -1 check so it documents the check it guards.
>>
>> Also fix sys_rtas() last-error status detection: ibm,open-errinjct
>> places status at rets[1], so the -1 sentinel check must use rets[1]
>> for that function rather than always using rets[0].
>>
>> Reference: OpenPOWER PAPR documentation
>>             https://files.openpower.foundation/s/XFgfMaqLMD5Bcm8
>> Signed-off-by: Narayana Murty N <nnmlinux@linux.ibm.com>
>> ---
>>   arch/powerpc/kernel/rtas.c | 78 +++++++++++++++++++++++++++++++++-----
>>   1 file changed, 68 insertions(+), 10 deletions(-)
>>
>> diff --git a/arch/powerpc/kernel/rtas.c b/arch/powerpc/kernel/rtas.c
>> index 8d81c1e7a8db..7131870655c6 100644
>> --- a/arch/powerpc/kernel/rtas.c
>> +++ b/arch/powerpc/kernel/rtas.c
>> @@ -1117,6 +1117,28 @@ static bool token_is_restricted_errinjct(s32 
>> token)
>>              token == rtas_function_token(RTAS_FN_IBM_ERRINJCT);
>>   }
>> +/*
>> + * ibm,open-errinjct uses a non-standard return layout:
>> + *   rets[0] = session token  (output parameter)
>> + *   rets[1] = status code
>> + *
>> + * All other RTAS functions use the standard layout:
>> + *   rets[0] = status code
>> + *   rets[1..] = output parameters
>> + */
>> +static inline bool rtas_token_is_open_errinjct(int token)
>> +{
>> +    return token == rtas_function_token(RTAS_FN_IBM_OPEN_ERRINJCT);
>> +}
>> +
>> +static int rtas_status_from_args(int token, struct rtas_args *args, 
>> int nret)
>> +{
>> +    if (rtas_token_is_open_errinjct(token))
>> +        return be32_to_cpu(args->rets[1]);
>> +
>> +    return nret > 0 ? be32_to_cpu(args->rets[0]) : 0;
>> +}
>> +
>>   /**
>>    * rtas_call() - Invoke an RTAS firmware function.
>>    * @token: Identifies the function being invoked.
>> @@ -1198,6 +1220,16 @@ int rtas_call(int token, int nargs, int nret, 
>> int *outputs, ...)
>>               return -1;
>>       }
>> +    /*
>> +     * ibm,open-errinjct returns rets[0]=session_token, rets[1]=status.
>> +     * We need nret >= 2 to read status from rets[1].  Reject early if
>> +     * the caller forgot to account for the extra return cell.
>> +     */
>> +    if (rtas_token_is_open_errinjct(token) && nret < 2) {
>> +        WARN_ON_ONCE(1);
>> +        return RTAS_INVALID_PARAMETER;
> 
> Nit: I would prefer -EINVAL instead. RTAS_INVALID_PARAMETER is RTAS 
> error code but here kernel is validating the parameter so I think - 
> EINVAL would be better.
I agree this is a kernel-side validation before entering RTAS, but for
rtas_call() I think keeping RTAS_INVALID_PARAMETER is safer.

The return value from rtas_call() is normally interpreted by callers as 
an RTAS status code. If we return -EINVAL from rtas_call(), the caller 
may still treat it as an RTAS return value. Since this is an invalid 
RTAS call layout for ibm,open-errinjct, I would prefer to keep the 
return value as RTAS_INVALID_PARAMETER in rtas_call().

>> +    }
>> +
>>       if ((mfmsr() & (MSR_IR|MSR_DR)) != (MSR_IR|MSR_DR)) {
>>           WARN_ON_ONCE(1);
>>           return -1;
>> @@ -1213,15 +1245,33 @@ int rtas_call(int token, int nargs, int nret, 
>> int *outputs, ...)
>>       va_rtas_call_unlocked(args, token, nargs, nret, list);
>>       va_end(list);
>> +    ret = rtas_status_from_args(token, args, nret);
>> +
>>       /* A -1 return code indicates that the last command couldn't
>> -       be completed due to a hardware error. */
>> -    if (be32_to_cpu(args->rets[0]) == -1)
>> +     * be completed due to a hardware error.
>> +     */
>> +    if (ret == -1)
>>           buff_copy = __fetch_rtas_last_error(NULL);
>> -    if (nret > 1 && outputs != NULL)
>> -        for (i = 0; i < nret-1; ++i)
>> -            outputs[i] = be32_to_cpu(args->rets[i + 1]);
>> -    ret = (nret > 0) ? be32_to_cpu(args->rets[0]) : 0;
>> +    /*
>> +     * Copy non-status outputs to the caller's buffer.
>> +     *
>> +     * For ibm,open-errinjct the layout is:
>> +     *   rets[0] = session token  -> outputs[0]
>> +     *   rets[1] = status         (returned, not copied)
>> +     *
>> +     * For all other RTAS functions:
>> +     *   rets[0] = status         (returned, not copied)
>> +     *   rets[1..nret-1] -> outputs[0..nret-2]
>> +     */
>> +    if (outputs != NULL) {
>> +        if (rtas_token_is_open_errinjct(token)) {
>> +            outputs[0] = be32_to_cpu(args->rets[0]);
>> +        } else if (nret > 1) {
>> +            for (i = 0; i < nret - 1; ++i)
>> +                outputs[i] = be32_to_cpu(args->rets[i + 1]);
>> +        }
>> +    }
>>       lockdep_unpin_lock(&rtas_lock, cookie);
>>       raw_spin_unlock_irqrestore(&rtas_lock, flags);
>> @@ -1942,10 +1992,18 @@ SYSCALL_DEFINE1(rtas, struct rtas_args __user 
>> *, uargs)
>>       do_enter_rtas(&rtas_args);
>>       args = rtas_args;
>> -    /* A -1 return code indicates that the last command couldn't
>> -       be completed due to a hardware error. */
>> -    if (be32_to_cpu(args.rets[0]) == -1)
>> -        errbuf = __fetch_rtas_last_error(buff_copy);
>> +    /*
>> +     * A -1 return code indicates that the last command couldn't
>> +     * be completed due to a hardware error.  ibm,open-errinjct
>> +     * places status at rets[1] rather than rets[0]; check the
>> +     * correct position for the -1 sentinel.
>> +     */
>> +    {
>> +        __be32 status_cell = (token == 
>> rtas_function_token(RTAS_FN_IBM_OPEN_ERRINJCT) &&
>> +                      nret >= 2) ? args.rets[1] : args.rets[0];
>> +        if (be32_to_cpu(status_cell) == -1)
>> +            errbuf = __fetch_rtas_last_error(buff_copy);
> 
> Do we know what happens when the ibm,open-errinjct RTAS call is made
> with nret < 2?
> 
> The reason I’m asking is that, even with the above changes,
> args.rets[0] is used as the return code if the ibm,open-errinjct call
> is made with nret < 2.
> 
> I like the approach you took in rtas_call() of pre-validating nret for
> the ibm,open-errinjct RTAS call and returning early if it is less than
> 2. I think we can use a similar approach here as well. If we do that,
> the above code changes will be much cleaner. In that case, we don't
> have to figure out how RTAS processes ibm,open-errinjct with nret < 2.
> 
> The only concern I have is that this change would alter the system
> call behavior. Right now, the kernel accepts nret < 2 for
> ibm,open-errinjct and makes the RTAS call, but with the above suggested
> change, the kernel would return early if nret < 2.
> 
> The prominent user of this system call is librtas, which passes
> nret = 2 for ibm,open-errinjct:
> 
> https://github.com/ibm-power-utilities/librtas/blob/ 
> d321a1f5ae3d528ba027fc748d1cc1123dd4ae29/librtas_src/syscall_calls.c#L488
> 
> Also, as per PAPR, users are supposed to pass nret = 2 for this RTAS
> call. So I think it should be fine to validate nret in sys_rtas for
> ibm,open-errinjct and return early if it is found to be less than 2.
> 
> Since this is a change in system call behavior, I want to be a
> little cautious. So, I’d like to hear your thoughts and would also
> like to know what others think about making the above change.
> 
Agreed.

For ibm,open-errinjct, rets[0] is the session token and rets[1] is the
status. So when nret < 2, we should not fall back to rets[0], because 
that can interpret a session token as a status code.

I will add an early validation in sys_rtas() before entering RTAS for 
this case and return -EINVAL there, since sys_rtas() is the userspace 
syscall boundary.

So the split will be:
rtas_call(): return RTAS_INVALID_PARAMETER for invalid RTAS return layout

sys_rtas(): return -EINVAL for invalid userspace syscall arguments

This should not affect the valid librtas path, since librtas already 
passes nret = 2 for ibm,open-errinjct, as required by PAPR.

Thanks,
Narayana

> Thanks, Sourabh Jain
>> +    }
>>       lockdep_unpin_lock(&rtas_lock, cookie);
>>       raw_spin_unlock_irqrestore(&rtas_lock, flags);
>