linux-kernel - Re: [PATCH v4 6/8] arm64: uprobes: Add GCS support to uretprobes

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Message-ID: <aIC06oEF5FV99ukl@arm.com>
Date: Wed, 23 Jul 2025 11:09:46 +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,
	Steve Capper <steve.capper@....com>
Subject: Re: [PATCH v4 6/8] arm64: uprobes: Add GCS support to uretprobes

On Fri, Jul 18, 2025 at 11:37:38PM -0500, Jeremy Linton wrote:
> @@ -159,11 +160,41 @@ arch_uretprobe_hijack_return_addr(unsigned long trampoline_vaddr,
>  				  struct pt_regs *regs)
>  {
>  	unsigned long orig_ret_vaddr;
> +	unsigned long gcs_ret_vaddr;
> +	int err = 0;
> +	u64 gcspr;
>  
>  	orig_ret_vaddr = procedure_link_pointer(regs);
> +
> +	if (task_gcs_el0_enabled(current)) {
> +		gcspr = read_sysreg_s(SYS_GCSPR_EL0);
> +		gcs_ret_vaddr = load_user_gcs((unsigned long __user *)gcspr, &err);
> +		if (err) {
> +			force_sig(SIGSEGV);
> +			goto out;
> +		}

Nit: add an empty line here, I find it easier to read.

> +		/*
> +		 * If the LR and GCS entry don't match, then some kind of PAC/control
> +		 * flow happened. Likely because the user is attempting to retprobe

I don't full get the first sentence.

> +		 * on something that isn't a function boundary or inside a leaf
> +		 * function. Explicitly abort this retprobe because it will generate
> +		 * a GCS exception.
> +		 */
> +		if (gcs_ret_vaddr != orig_ret_vaddr)	{
> +			orig_ret_vaddr = -1;
> +			goto out;
> +		}

Nit: another empty line here.

> +		put_user_gcs(trampoline_vaddr, (unsigned long __user *) gcspr, &err);

Nit: (... *)gcspr (no space after cast).

> +		if (err) {
> +			force_sig(SIGSEGV);
> +			goto out;
> +		}
> +	}
> +
>  	/* Replace the return addr with trampoline addr */
>  	procedure_link_pointer_set(regs, trampoline_vaddr);
>  
> +out:
>  	return orig_ret_vaddr;
>  }

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