linux-kernel - Re: [PATCH v4 4/8] arm64: uaccess: Add additional userspace GCS accessors

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Message-ID: <aICwaQZxK5Spjj8G@arm.com>
Date: Wed, 23 Jul 2025 10:50:33 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Jeremy Linton <jeremy.linton@....com>
Cc: linux-trace-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
	mhiramat@...nel.org, oleg@...hat.com, peterz@...radead.org,
	acme@...nel.org, namhyung@...nel.org, mark.rutland@....com,
	alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
	irogers@...gle.com, adrian.hunter@...el.com,
	kan.liang@...ux.intel.com, thiago.bauermann@...aro.org,
	broonie@...nel.org, yury.khrustalev@....com,
	kristina.martsenko@....com, liaochang1@...wei.com, will@...nel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 4/8] arm64: uaccess: Add additional userspace GCS
 accessors

On Fri, Jul 18, 2025 at 11:37:36PM -0500, Jeremy Linton wrote:
> +/*
> + * Unlike put_user_gcs() above, the use of copy_from_user() may provide
> + * an opening for non GCS pages to be used to source data. Therefore this
> + * should only be used in contexts where that is acceptable.
> + */

Even in user space, the GCS pages can be read with normal loads, so
already usable as a data source if one wants to (not that it's of much
use). So not sure the comment here is needed.

> +static inline u64 load_user_gcs(unsigned long __user *addr, int *err)

Nitpick: name this get_user_gcs() for symmetry with put_user_gcs().

> +{
> +	unsigned long ret;
> +	u64 load = 0;
> +
> +	gcsb_dsync();

Might be worth a comment here, see the one in gcs_restore_signal().

> +	ret = copy_from_user(&load, addr, sizeof(load));
> +	if (ret != 0)
> +		*err = ret;
> +	return load;
> +}

Otherwise the patch looks fine:

Reviewed-by: Catalin Marinas <catalin.marinas@....com>