x86/fpu: Make task_struct::thread constant size

Turn thread.fpu into a pointer. Since most FPU code internals work by passing
around the FPU pointer already, the code generation impact is small.

This allows us to remove the old kludge of task_struct being variable size:

  struct task_struct {

       ...
       /*
        * New fields for task_struct should be added above here, so that
        * they are included in the randomized portion of task_struct.
        */
       randomized_struct_fields_end

       /* CPU-specific state of this task: */
       struct thread_struct            thread;

       /*
        * WARNING: on x86, 'thread_struct' contains a variable-sized
        * structure.  It *MUST* be at the end of 'task_struct'.
        *
        * Do not put anything below here!
        */
  };

... which creates a number of problems, such as requiring thread_struct to be
the last member of the struct - not allowing it to be struct-randomized, etc.

But the primary motivation is to allow the decoupling of task_struct from
hardware details (<asm/processor.h> in particular), and to eventually allow
the per-task infrastructure:

   DECLARE_PER_TASK(type, name);
   ...
   per_task(current, name) = val;

... which requires task_struct to be a constant size struct.

The fpu_thread_struct_whitelist() quirk to hardened usercopy can be removed,
now that the FPU structure is not embedded in the task struct anymore, which
reduces text footprint a bit.

Fixed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Chang S. Bae <chang.seok.bae@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250409211127.3544993-4-mingo@kernel.org

1 files changed, 9 insertions, 11 deletions

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 2f631e0adea3..5ea7e5d2c4de 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -516,21 +516,19 @@ struct thread_struct {
 #endif
 
 	/* Floating point and extended processor state */
-	struct fpu		fpu;
-	/*
-	 * WARNING: 'fpu' is dynamically-sized.  It *MUST* be at
-	 * the end.
-	 */
+	struct fpu		*fpu;
 };
 
-#define x86_task_fpu(task) (&(task)->thread.fpu)
-
-extern void fpu_thread_struct_whitelist(unsigned long *offset, unsigned long *size);
+#define x86_task_fpu(task) ((task)->thread.fpu)
 
-static inline void arch_thread_struct_whitelist(unsigned long *offset,
-						unsigned long *size)
+/*
+ * X86 doesn't need any embedded-FPU-struct quirks:
+ */
+static inline void
+arch_thread_struct_whitelist(unsigned long *offset, unsigned long *size)
 {
-	fpu_thread_struct_whitelist(offset, size);
+	*offset = 0;
+	*size = 0;
 }
 
 static inline void