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

[Sourabh Jain <sourabhjain@linux.ibm.com>]

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.
> +	}
> +
>   	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.

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