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Subject[PATCH 06/24] x86/fpu: Don't save fxregs for ia32 frames in copy_fpstate_to_sigframe()
Date
In commit

72a671ced66db ("x86, fpu: Unify signal handling code paths for x86 and x86_64 kernels")

the 32bit and 64bit path of the signal delivery code were merged. The 32bit version:
|int save_i387_xstate_ia32(void __user *buf)
|…
| if (cpu_has_xsave)
| return save_i387_xsave(fp);
| if (cpu_has_fxsr)
| return save_i387_fxsave(fp);

The 64bit version:
|int save_i387_xstate(void __user *buf)
|…
| if (user_has_fpu()) {
| if (use_xsave())
| err = xsave_user(buf);
| else
| err = fxsave_user(buf);
|
| if (unlikely(err)) {
| __clear_user(buf, xstate_size);
| return err;

The merge:
|int save_xstate_sig(void __user *buf, void __user *buf_fx, int size)
|…
| if (user_has_fpu()) {
| /* Save the live register state to the user directly. */
| if (save_user_xstate(buf_fx))
| return -1;
| /* Update the thread's fxstate to save the fsave header. */
| if (ia32_fxstate)
| fpu_fxsave(&tsk->thread.fpu);

I don't think that we needed to save the FPU registers to ->thread.fpu because
the registers were stored in `buf_fx'. Today the state will be restored from
`buf_fx' after the signal was handled (I assume that this was also the case
with lazy-FPU). Since commit

66463db4fc560 ("x86, fpu: shift drop_init_fpu() from save_xstate_sig() to handle_signal()")

it is ensured that the signal handler starts with clear/fresh set of FPU
registers which means that the previous store is futile.

Remove copy_fxregs_to_kernel() because task's FPU state is cleared later in
handle_signal() via fpu__clear().

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
arch/x86/kernel/fpu/signal.c | 4 ----
1 file changed, 4 deletions(-)

diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index de83d0ed9e14e..2f044021fde2b 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -155,7 +155,6 @@ static inline int copy_fpregs_to_sigframe(struct xregs_state __user *buf)
*/
int copy_fpstate_to_sigframe(void __user *buf, void __user *buf_fx, int size)
{
- struct fpu *fpu = &current->thread.fpu;
struct task_struct *tsk = current;
int ia32_fxstate = (buf != buf_fx);

@@ -173,9 +172,6 @@ int copy_fpstate_to_sigframe(void __user *buf, void __user *buf_fx, int size)
/* Save the live register state to the user directly. */
if (copy_fpregs_to_sigframe(buf_fx))
return -1;
- /* Update the thread's fxstate to save the fsave header. */
- if (ia32_fxstate)
- copy_fxregs_to_kernel(fpu);

/* Save the fsave header for the 32-bit frames. */
if ((ia32_fxstate || !use_fxsr()) && save_fsave_header(tsk, buf))
--
2.20.1
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