[patch 09/31] x86/fpu: Do not inherit FPU context for CLONE_THREAD

Thomas Gleixner
2021-10-12T00:06:39Z

CLONE_THREAD does not have the guarantee of a true fork to inherit all
state. Especially the FPU state is meaningless for CLONE_THREAD.

Just wipe out the minimal required state so restore on return to user space
let's the thread start with a clean FPU.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
arch/x86/include/asm/fpu/internal.h | 2 +-
arch/x86/kernel/fpu/core.c | 8 +++++---
arch/x86/kernel/process.c | 2 +-
3 files changed, 7 insertions(+), 5 deletions(-)

--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -34,7 +34,7 @@ extern int fpu__exception_code(struct f
extern void fpu_sync_fpstate(struct fpu *fpu);

/* Clone and exit operations */
-extern int fpu_clone(struct task_struct *dst);
+extern int fpu_clone(struct task_struct *dst, unsigned long clone_flags);
extern void fpu_flush_thread(void);

/*
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -257,7 +257,7 @@ void fpstate_init(union fpregs_state *st
EXPORT_SYMBOL_GPL(fpstate_init);

/* Clone current's FPU state on fork */
-int fpu_clone(struct task_struct *dst)
+int fpu_clone(struct task_struct *dst, unsigned long clone_flags)
{
struct fpu *src_fpu = &current->thread.fpu;
struct fpu *dst_fpu = &dst->thread.fpu;
@@ -276,9 +276,11 @@ int fpu_clone(struct task_struct *dst)

/*
* No FPU state inheritance for kernel threads and IO
- * worker threads.
+ * worker threads. Neither CLONE_THREAD needs a copy
+ * of the FPU state.
*/
- if (dst->flags & (PF_KTHREAD | PF_IO_WORKER)) {
+ if (clone_flags & CLONE_THREAD ||
+ dst->flags & (PF_KTHREAD | PF_IO_WORKER)) {
/* Clear out the minimal state */
memcpy(&dst_fpu->state, &init_fpstate,
init_fpstate_copy_size());
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -154,7 +154,7 @@ int copy_thread(unsigned long clone_flag
frame->flags = X86_EFLAGS_FIXED;
#endif

- fpu_clone(p);
+ fpu_clone(p, clone_flags);

/* Kernel thread ? */
if (unlikely(p->flags & PF_KTHREAD)) {