Re: [PATCH v2] mips: Call lose_fpu(0) before initializing fcr31 in mips_set_personality_nan

On Sat, Jan 27, 2024 at 05:05:57AM +0800, Xi Ruoyao wrote:

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.... Fixes: 9b26616c8d9d ("MIPS: Respect the ISA level in FCSR handling") Cc: stable@vger.kernel.org Signed-off-by: Xi Ruoyao xry111@xry111.site

---

v1 -> v2: Fix stable list address in Cc line.

arch/mips/kernel/elf.c | 6 ++++++ 1 file changed, 6 insertions(+)

applied to mips-fixes.

Thomas.