linux-kernel - Re: [PATCH 2/3] x86: Move CPU feature test out of uaccess region

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Date:   Wed, 29 May 2019 07:29:01 -0700
From:   hpa@...or.com
To:     Marco Elver <elver@...gle.com>, peterz@...radead.org,
        aryabinin@...tuozzo.com, dvyukov@...gle.com, glider@...gle.com,
        andreyknvl@...gle.com, mark.rutland@....com
CC:     corbet@....net, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        x86@...nel.org, arnd@...db.de, jpoimboe@...hat.com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, kasan-dev@...glegroups.com
Subject: Re: [PATCH 2/3] x86: Move CPU feature test out of uaccess region

On May 29, 2019 7:15:00 AM PDT, Marco Elver <elver@...gle.com> wrote:
>This patch is a pre-requisite for enabling KASAN bitops
>instrumentation:
>moves boot_cpu_has feature test out of the uaccess region, as
>boot_cpu_has uses test_bit. With instrumentation, the KASAN check would
>otherwise be flagged by objtool.
>
>This approach is preferred over adding the explicit kasan_check_*
>functions to the uaccess whitelist of objtool, as the case here appears
>to be the only one.
>
>Signed-off-by: Marco Elver <elver@...gle.com>
>---
>v1:
>* This patch replaces patch: 'tools/objtool: add kasan_check_* to
>  uaccess whitelist'
>---
> arch/x86/ia32/ia32_signal.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
>diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
>index 629d1ee05599..12264e3c9c43 100644
>--- a/arch/x86/ia32/ia32_signal.c
>+++ b/arch/x86/ia32/ia32_signal.c
>@@ -333,6 +333,7 @@ int ia32_setup_rt_frame(int sig, struct ksignal
>*ksig,
> 	void __user *restorer;
> 	int err = 0;
> 	void __user *fpstate = NULL;
>+	bool has_xsave;
> 
> 	/* __copy_to_user optimizes that into a single 8 byte store */
> 	static const struct {
>@@ -352,13 +353,19 @@ int ia32_setup_rt_frame(int sig, struct ksignal
>*ksig,
> 	if (!access_ok(frame, sizeof(*frame)))
> 		return -EFAULT;
> 
>+	/*
>+	 * Move non-uaccess accesses out of uaccess region if not strictly
>+	 * required; this also helps avoid objtool flagging these accesses
>with
>+	 * instrumentation enabled.
>+	 */
>+	has_xsave = boot_cpu_has(X86_FEATURE_XSAVE);
> 	put_user_try {
> 		put_user_ex(sig, &frame->sig);
> 		put_user_ex(ptr_to_compat(&frame->info), &frame->pinfo);
> 		put_user_ex(ptr_to_compat(&frame->uc), &frame->puc);
> 
> 		/* Create the ucontext.  */
>-		if (boot_cpu_has(X86_FEATURE_XSAVE))
>+		if (has_xsave)
> 			put_user_ex(UC_FP_XSTATE, &frame->uc.uc_flags);
> 		else
> 			put_user_ex(0, &frame->uc.uc_flags);

This was meant to use static_cpu_has(). Why did that get dropped?
-- 
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