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From: Konstantin Belousov <kib@FreeBSD.org>
Date: Thu, 27 May 2010 18:33:00 +0000 (UTC)
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Subject: svn commit: r208604 - head/sys/i386/i386
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Author: kib
Date: Thu May 27 18:33:00 2010
New Revision: 208604
URL: http://svn.freebsd.org/changeset/base/208604

Log:
  Clarify a potential issue in get_fpcontext() use.
  
  MFC after:	1 week

Modified:
  head/sys/i386/i386/machdep.c

Modified: head/sys/i386/i386/machdep.c
==============================================================================
--- head/sys/i386/i386/machdep.c	Thu May 27 18:07:20 2010	(r208603)
+++ head/sys/i386/i386/machdep.c	Thu May 27 18:33:00 2010	(r208604)
@@ -633,6 +633,13 @@ sendsig(sig_t catcher, ksiginfo_t *ksi, 
 	sf.sf_uc.uc_mcontext.mc_gs = rgs();
 	bcopy(regs, &sf.sf_uc.uc_mcontext.mc_fs, sizeof(*regs));
 	sf.sf_uc.uc_mcontext.mc_len = sizeof(sf.sf_uc.uc_mcontext); /* magic */
+
+	/*
+	 * The get_fpcontext() call must be placed before assignments
+	 * to mc_fsbase and mc_gsbase due to the alignment-override
+	 * code in get_fpcontext() that possibly clobbers 12 bytes of
+	 * mcontext after mc_fpstate.
+	 */
 	get_fpcontext(td, &sf.sf_uc.uc_mcontext);
 	fpstate_drop(td);
 	/*
@@ -3240,6 +3247,13 @@ get_mcontext(struct thread *td, mcontext
 	mcp->mc_esp = tp->tf_esp;
 	mcp->mc_ss = tp->tf_ss;
 	mcp->mc_len = sizeof(*mcp);
+
+	/*
+	 * The get_fpcontext() call must be placed before assignments
+	 * to mc_fsbase and mc_gsbase due to the alignment-override
+	 * code in get_fpcontext() that possibly clobbers 12 bytes of
+	 * mcontext after mc_fpstate.
+	 */
 	get_fpcontext(td, mcp);
 	sdp = &td->td_pcb->pcb_gsd;
 	mcp->mc_fsbase = sdp->sd_hibase << 24 | sdp->sd_lobase;