linux-kernel - Re: [PATCH v2 3/6] firmware: qcom: scm: Handle various probe ordering for qcom_scm_assign

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Message-ID: <o5oz3scbvsvutv43uqgh74xzfxituzieaa57o6mzomdyr3qixo@2j4ogsbu255o>
Date: Mon, 6 Jan 2025 18:42:35 -0600
From: Bjorn Andersson <andersson@...nel.org>
To: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc: Konrad Dybcio <konradybcio@...nel.org>, 
	Mukesh Ojha <quic_mojha@...cinc.com>, Dmitry Baryshkov <dmitry.baryshkov@...aro.org>, 
	Stephan Gerhold <stephan.gerhold@...aro.org>, Bartosz Golaszewski <bartosz.golaszewski@...aro.org>, 
	Kuldeep Singh <quic_kuldsing@...cinc.com>, Elliot Berman <quic_eberman@...cinc.com>, 
	Andrew Halaney <ahalaney@...hat.com>, Avaneesh Kumar Dwivedi <quic_akdwived@...cinc.com>, 
	Andy Gross <andy.gross@...aro.org>, linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/6] firmware: qcom: scm: Handle various probe
 ordering for qcom_scm_assign_mem()

On Mon, Dec 09, 2024 at 03:27:56PM +0100, Krzysztof Kozlowski wrote:
> The SCM driver can defer or fail probe, or just load a bit later so
> callers of qcom_scm_assign_mem() should defer if the device is not ready.
> 
> This fixes theoretical NULL pointer exception, triggered via introducing
> probe deferral in SCM driver with call trace:
> 
>   qcom_tzmem_alloc+0x70/0x1ac (P)
>   qcom_tzmem_alloc+0x64/0x1ac (L)
>   qcom_scm_assign_mem+0x78/0x194
>   qcom_rmtfs_mem_probe+0x2d4/0x38c
>   platform_probe+0x68/0xc8
> 
> Fixes: d82bd359972a ("firmware: scm: Add new SCM call API for switching memory ownership")
> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
> 
> ---
> 
> I am not sure about commit introducing it (Fixes tag) thus not Cc-ing
> stable.
> ---
>  drivers/firmware/qcom/qcom_scm.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/firmware/qcom/qcom_scm.c b/drivers/firmware/qcom/qcom_scm.c
> index 5d91b8e22844608f35432f1ba9c08d477d4ff762..93212c8f20ad65ecc44804b00f4b93e3eaaf8d95 100644
> --- a/drivers/firmware/qcom/qcom_scm.c
> +++ b/drivers/firmware/qcom/qcom_scm.c
> @@ -1075,6 +1075,9 @@ int qcom_scm_assign_mem(phys_addr_t mem_addr, size_t mem_sz,
>  	int ret, i, b;
>  	u64 srcvm_bits = *srcvm;
>  
> +	if (!qcom_scm_is_available())
> +		return -EPROBE_DEFER;

This API is generally called from places other than probe, making the
return of EPROBE_DEFER undesirable. While not pretty, a client depending
on the scm driver to be probed is expected to call
qcom_scm_is_available().

qcom_rmtfs_mem_probe() does this right before calling
qcom_scm_assign_mem(), am I misunderstanding the case you're describing?

Regards,
Bjorn

> +
>  	src_sz = hweight64(srcvm_bits) * sizeof(*src);
>  	mem_to_map_sz = sizeof(*mem_to_map);
>  	dest_sz = dest_cnt * sizeof(*destvm);
> 
> -- 
> 2.43.0
>