x86, math-emu: fix init_fpu for task != current

Impact: fix math-emu related crash while using GDB/ptrace

init_fpu() calls finit to initialize a task's xstate, while finit always
works on the current task. If we use PTRACE_GETFPREGS on another
process and both processes did not already use floating point, we get
a null pointer exception in finit.

This patch creates a new function finit_task that takes a task_struct
parameter. finit becomes a wrapper that simply calls finit_task with
current. On the plus side this avoids many calls to get_current which
would each resolve to an inline assembler mov instruction.

An empty finit_task has been added to i387.h to avoid linker errors in
case the compiler still emits the call in init_fpu when
CONFIG_MATH_EMULATION is not defined.

The declaration of finit in i387.h has been removed as the remaining
code using this function gets its prototype from fpu_proto.h.

Signed-off-by: Daniel Glöckner <dg@emlix.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: "Pallipadi Venkatesh" <venkatesh.pallipadi@intel.com>
Cc: Arjan van de Ven <arjan@infradead.org>
Cc: Bill Metzenthen <billm@melbpc.org.au>
LKML-Reference: <E1Lew31-0004il-Fg@mailer.emlix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/include/asm/i387.h

@@ -172,7 +172,13 @@ static inline void __save_init_fpu(struct task_struct *tsk)
 
 #else  /* CONFIG_X86_32 */
 
-extern void finit(void);
+#ifdef CONFIG_MATH_EMULATION
+extern void finit_task(struct task_struct *tsk);
+#else
+static inline void finit_task(struct task_struct *tsk)
+{
+}
+#endif
 
 static inline void tolerant_fwait(void)
 {

arch/x86/kernel/i387.c

@@ -136,7 +136,7 @@ int init_fpu(struct task_struct *tsk)
 #ifdef CONFIG_X86_32
 	if (!HAVE_HWFP) {
 		memset(tsk->thread.xstate, 0, xstate_size);
-		finit();
+		finit_task(tsk);
 		set_stopped_child_used_math(tsk);
 		return 0;
 	}

arch/x86/math-emu/fpu_aux.c

@@ -30,20 +30,29 @@ static void fclex(void)
 }
 
 /* Needs to be externally visible */
-void finit(void)
+void finit_task(struct task_struct *tsk)
 {
-	control_word = 0x037f;
-	partial_status = 0;
-	top = 0;		/* We don't keep top in the status word internally. */
-	fpu_tag_word = 0xffff;
+	struct i387_soft_struct *soft = &tsk->thread.xstate->soft;
+	struct address *oaddr, *iaddr;
+	soft->cwd = 0x037f;
+	soft->swd = 0;
+	soft->ftop = 0;	/* We don't keep top in the status word internally. */
+	soft->twd = 0xffff;
 	/* The behaviour is different from that detailed in
 	   Section 15.1.6 of the Intel manual */
-	operand_address.offset = 0;
-	operand_address.selector = 0;
-	instruction_address.offset = 0;
-	instruction_address.selector = 0;
-	instruction_address.opcode = 0;
-	no_ip_update = 1;
+	oaddr = (struct address *)&soft->foo;
+	oaddr->offset = 0;
+	oaddr->selector = 0;
+	iaddr = (struct address *)&soft->fip;
+	iaddr->offset = 0;
+	iaddr->selector = 0;
+	iaddr->opcode = 0;
+	soft->no_update = 1;
+}
+
+void finit(void)
+{
+	finit_task(current);
 }
 
 /*