[RFC PATCH 15/22] x86/fpu/xstate: Support ptracer-induced xstate area expansion

From: "Chang S. Bae" <chang.seok.bae@intel.com>
To: tglx@linutronix.de, mingo@kernel.org, bp@suse.de,
	luto@kernel.org, x86@kernel.org
Cc: len.brown@intel.com, dave.hansen@intel.com, jing2.liu@intel.com,
	ravi.v.shankar@intel.com, linux-kernel@vger.kernel.org,
	chang.seok.bae@intel.com
Subject: [RFC PATCH 15/22] x86/fpu/xstate: Support ptracer-induced xstate area expansion
Date: Thu,  1 Oct 2020 13:39:06 -0700	[thread overview]
Message-ID: <20201001203913.9125-16-chang.seok.bae@intel.com> (raw)
In-Reply-To: <20201001203913.9125-1-chang.seok.bae@intel.com>

ptrace() may request an update to task->fpu that has not yet been
allocated. Detect this case and allocate task->fpu to support the request.
Also, disable the (now unnecessary) associated first-use fault.

No functional change until the kernel supports dynamic user states.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
---
 arch/x86/kernel/fpu/regset.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/arch/x86/kernel/fpu/regset.c b/arch/x86/kernel/fpu/regset.c
index 8d863240b9c6..6b9d0c0a266d 100644
--- a/arch/x86/kernel/fpu/regset.c
+++ b/arch/x86/kernel/fpu/regset.c
@@ -125,6 +125,35 @@ int xstateregs_set(struct task_struct *target, const struct user_regset *regset,
 
 	xsave = __xsave(fpu);
 
+	/*
+	 * When a ptracer attempts to write any state in task->fpu but not allocated,
+	 * it dynamically expands the xstate area of fpu->state_ptr.
+	 */
+	if (count > get_xstate_size(fpu->state_mask)) {
+		unsigned int offset, size;
+		struct xstate_header hdr;
+		u64 mask;
+
+		offset = offsetof(struct xregs_state, header);
+		size = sizeof(hdr);
+
+		/* Retrieve XSTATE_BV */
+		if (kbuf) {
+			memcpy(&hdr, kbuf + offset, size);
+		} else {
+			ret = __copy_from_user(&hdr, ubuf + offset, size);
+			if (ret)
+				return ret;
+		}
+
+		mask = hdr.xfeatures & xfeatures_mask_user_dynamic;
+		if (!mask) {
+			ret = alloc_xstate_area(fpu, mask, NULL);
+			if (ret)
+				return ret;
+		}
+	}
+
 	fpu__prepare_write(fpu);
 
 	if (using_compacted_format()) {
-- 
2.17.1

