[PATCH 3/1] x86/fpu: Remove init_task FPU state dependencies, add debugging warning

[Ingo Molnar <mingo@kernel.org>]

* Ingo Molnar <mingo@kernel.org> wrote:

> >    	- exit_thread() - trivial
> > 
> > 	- arch_dup_task_struct() clears thread.fpu, but I guess this is unnecessary
> > 
> > 	- fpu_clone() initializes dst->thread.fpu, no longer needed
> > 
> > 	- finally, fpu__init_system_early_generic() which initializes the
> > 	  init_task.thread.fpu pointer.
> > 
> > 	  But this doesn't look difficult? Although I don't understand the magic in
> > 	  arch/x86/kernel/vmlinux.lds.S ...
> 
> Yeah.
> 
> Something like the patch below, on top of my previous patch.
> 
> The main/only fishy bit left is:
> 
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -172,6 +172,10 @@ SECTIONS
>                 /* init_task */
>                 INIT_TASK_DATA(THREAD_SIZE)
>  
> +               __x86_init_fpu_begin = .;
> +               . = __x86_init_fpu_begin + 128*PAGE_SIZE;
> +               __x86_init_fpu_end = .;
> 
> ... as we don't know the FPU context switch size in advance, and modern 
> vector CPUs can be huge. Plus CPU vendors like to push more crap into 
> the FPU context area (XSAVE area), because the rather opaque 
> enumeration and variable-size format makes OS support easier for them. 
> :-/
> 
> And in principle init_task should never use its task-fpu context switch 
> area for real, but I couldn't convince myself (yet) whether that's true 
> under all circumstances - and getting this wrong would be a memory 
> corruptor ...
> 
> kernel_fpu_begin_mask() has this:
> 
>         this_cpu_write(in_kernel_fpu, true);
> 
>         if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER)) &&
>             !test_thread_flag(TIF_NEED_FPU_LOAD)) {
>                 set_thread_flag(TIF_NEED_FPU_LOAD);
>                 save_fpregs_to_fpstate(x86_task_fpu(current));
>         }
>         __cpu_invalidate_fpregs_state();
> 
> and the save_fpregs_to_fpstate() inside the branch should never execute 
> for init_task.

So there were a few places, the patch below should cure all warnings 
that I was able to trigger.

 - x86_64 boots to user-space in KVM/qemu
 - i386 math-emu boots to init (not sure if it works though)

Note that we weren't actually using src_fpu for anything useful in 
fpu_clone() anymore.

So this now looks like something that looks useful and clean, but the 
changes are quite a bit more scary than I wanted them to be ...

Thanks,

	Ingo


======================>
From: Ingo Molnar <mingo@kernel.org>
Date: Fri, 29 Mar 2024 10:33:27 +0100
Subject: [PATCH] x86/fpu: Remove init_task FPU state dependencies, add debugging warning

Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/include/asm/processor.h |  6 +++++-
 arch/x86/kernel/fpu/core.c       | 12 +++++++++---
 arch/x86/kernel/fpu/init.c       |  5 ++---
 arch/x86/kernel/fpu/xstate.c     |  3 ---
 arch/x86/kernel/vmlinux.lds.S    |  4 ----
 5 files changed, 16 insertions(+), 14 deletions(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index fb78720774ea..d90c31e87b96 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -491,7 +491,11 @@ struct thread_struct {
 #endif
 };
 
-#define x86_task_fpu(task) ((struct fpu *)((void *)task + sizeof(*task)))
+#ifdef CONFIG_X86_DEBUG_FPU
+extern struct fpu *x86_task_fpu(struct task_struct *task);
+#else
+# define x86_task_fpu(task) ((struct fpu *)((void *)task + sizeof(*task)))
+#endif
 
 /*
  * X86 doesn't need any embedded-FPU-struct quirks:
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 0ccabcd3bf62..fdc3b227800d 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -51,6 +51,15 @@ static DEFINE_PER_CPU(bool, in_kernel_fpu);
  */
 DEFINE_PER_CPU(struct fpu *, fpu_fpregs_owner_ctx);
 
+#ifdef CONFIG_X86_DEBUG_FPU
+struct fpu *x86_task_fpu(struct task_struct *task)
+{
+	WARN_ON_ONCE(task == &init_task);
+
+	return (void *)task + sizeof(*task);
+}
+#endif
+
 /*
  * Can we use the FPU in kernel mode with the
  * whole "kernel_fpu_begin/end()" sequence?
@@ -591,10 +600,8 @@ int fpu_clone(struct task_struct *dst, unsigned long clone_flags, bool minimal,
 	 * This is safe because task_struct size is a multiple of cacheline size.
 	 */
 	struct fpu *dst_fpu = (void *)dst + sizeof(*dst);
-	struct fpu *src_fpu = x86_task_fpu(current);
 
 	BUILD_BUG_ON(sizeof(*dst) % SMP_CACHE_BYTES != 0);
-	BUG_ON(!src_fpu);
 
 	/* The new task's FPU state cannot be valid in the hardware. */
 	dst_fpu->last_cpu = -1;
@@ -657,7 +664,6 @@ int fpu_clone(struct task_struct *dst, unsigned long clone_flags, bool minimal,
 	if (update_fpu_shstk(dst, ssp))
 		return 1;
 
-	trace_x86_fpu_copy_src(src_fpu);
 	trace_x86_fpu_copy_dst(dst_fpu);
 
 	return 0;
diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
index 11aa31410df2..53580e59e5db 100644
--- a/arch/x86/kernel/fpu/init.c
+++ b/arch/x86/kernel/fpu/init.c
@@ -38,7 +38,7 @@ static void fpu__init_cpu_generic(void)
 	/* Flush out any pending x87 state: */
 #ifdef CONFIG_MATH_EMULATION
 	if (!boot_cpu_has(X86_FEATURE_FPU))
-		fpstate_init_soft(&x86_task_fpu(current)->fpstate->regs.soft);
+		;
 	else
 #endif
 		asm volatile ("fninit");
@@ -164,7 +164,7 @@ static void __init fpu__init_task_struct_size(void)
 	 * Subtract off the static size of the register state.
 	 * It potentially has a bunch of padding.
 	 */
-	task_size -= sizeof(x86_task_fpu(current)->__fpstate.regs);
+	task_size -= sizeof(union fpregs_state);
 
 	/*
 	 * Add back the dynamically-calculated register state
@@ -209,7 +209,6 @@ static void __init fpu__init_system_xstate_size_legacy(void)
 	fpu_kernel_cfg.default_size = size;
 	fpu_user_cfg.max_size = size;
 	fpu_user_cfg.default_size = size;
-	fpstate_reset(x86_task_fpu(current));
 }
 
 /*
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index dade16d92484..75196e4d40bd 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -844,9 +844,6 @@ void __init fpu__init_system_xstate(unsigned int legacy_size)
 	if (err)
 		goto out_disable;
 
-	/* Reset the state for the current task */
-	fpstate_reset(x86_task_fpu(current));
-
 	/*
 	 * Update info used for ptrace frames; use standard-format size and no
 	 * supervisor xstates:
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index d21331187b20..56451fd2099e 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -172,10 +172,6 @@ SECTIONS
 		/* init_task */
 		INIT_TASK_DATA(THREAD_SIZE)
 
-		__x86_init_fpu_begin = .;
-		. = __x86_init_fpu_begin + 128*PAGE_SIZE;
-		__x86_init_fpu_end = .;
-
 #ifdef CONFIG_X86_32
 		/* 32 bit has nosave before _edata */
 		NOSAVE_DATA