mips: Call lose_fpu(0) before initializing fcr31 in mips_set_personality_nan

If we still own the FPU after initializing fcr31, when we are preempted
the dirty value in the FPU will be read out and stored into fcr31,
clobbering our setting.  This can cause an improper floating-point
environment after execve().  For example:

    zsh% cat measure.c
    #include <fenv.h>
    int main() { return fetestexcept(FE_INEXACT); }
    zsh% cc measure.c -o measure -lm
    zsh% echo $((1.0/3)) # raising FE_INEXACT
    0.33333333333333331
    zsh% while ./measure; do ; done
    (stopped in seconds)

Call lose_fpu(0) before setting fcr31 to prevent this.

Closes: https://lore.kernel.org/linux-mips/7a6aa1bbdbbe2e63ae96ff163fab0349f58f1b9e.camel@xry111.site/
Fixes: 9b26616 ("MIPS: Respect the ISA level in FCSR handling")
Cc: stable@vger.kernel.org
Signed-off-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

Author: xry111

arch/mips/kernel/elf.c

@@ -11,6 +11,7 @@
 
 #include <asm/cpu-features.h>
 #include <asm/cpu-info.h>
+#include <asm/fpu.h>
 
 #ifdef CONFIG_MIPS_FP_SUPPORT
 
@@ -309,6 +310,11 @@ void mips_set_personality_nan(struct arch_elf_state *state)
 	struct cpuinfo_mips *c = &boot_cpu_data;
 	struct task_struct *t = current;
 
+	/* Do this early so t->thread.fpu.fcr31 won't be clobbered in case
+	 * we are preempted before the lose_fpu(0) in start_thread.
+	 */
+	lose_fpu(0);
+
 	t->thread.fpu.fcr31 = c->fpu_csr31;
 	switch (state->nan_2008) {
 	case 0: