linux-kernel - Re: [PATCH v15 05/27] riscv: usercfi state for task and save/restore of CSR

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Message-Id: <D9OZVNOGLU4T.2XOUPX27HN0W8@ventanamicro.com>
Date: Tue, 06 May 2025 12:10:34 +0200
From: Radim Krčmář <rkrcmar@...tanamicro.com>
To: "Deepak Gupta" <debug@...osinc.com>, "Thomas Gleixner"
 <tglx@...utronix.de>, "Ingo Molnar" <mingo@...hat.com>, "Borislav Petkov"
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Cc: <linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
 <linux-mm@...ck.org>, <linux-riscv@...ts.infradead.org>,
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 <broonie@...nel.org>, <rick.p.edgecombe@...el.com>,
 <rust-for-linux@...r.kernel.org>, "Zong Li" <zong.li@...ive.com>,
 "linux-riscv" <linux-riscv-bounces@...ts.infradead.org>
Subject: Re: [PATCH v15 05/27] riscv: usercfi state for task and
 save/restore of CSR_SSP on trap entry/exit

[Ah, I missed v13 and v14, feel free to Cc me on next versions.]

2025-05-02T16:30:36-07:00, Deepak Gupta <debug@...osinc.com>:
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> @@ -91,6 +91,32 @@
> +.macro save_userssp tmp, status
> +	ALTERNATIVE("nops(4)",
> +		__stringify(				\
> +		andi \tmp, \status, SR_SPP;		\
> +		bnez \tmp, skip_ssp_save;		\
> +		csrrw \tmp, CSR_SSP, x0;		\
> +		REG_S \tmp, TASK_TI_USER_SSP(tp);	\
> +		skip_ssp_save:),
> +		0,
> +		RISCV_ISA_EXT_ZICFISS,
> +		CONFIG_RISCV_USER_CFI)
> +.endm
> +
> +.macro restore_userssp tmp
> +	ALTERNATIVE("nops(2)",
> +		__stringify(				\
> +		REG_L \tmp, TASK_TI_USER_SSP(tp);	\
> +		csrw CSR_SSP, \tmp),
> +		0,
> +		RISCV_ISA_EXT_ZICFISS,
> +		CONFIG_RISCV_USER_CFI)
> +.endm

Do we need to emit the nops when CONFIG_RISCV_USER_CFI isn't selected?

(Why not put #ifdef CONFIG_RISCV_USER_CFI around the ALTERNATIVES?)

Thanks.