cpu_thread_new_kstack: Introduce new MD callback

Previously, the cpu_thread_alloc callback was invoked each time a
kernel stack was allocated for a thread.  This included thread
creation, but it was also invoked if a recycled thread had to allocate
a new kstack.  This means that cpu_thread_alloc could be called
multiple times for a single thread, but cpu_thread_free is only called
once.  Not only that, but the cpu_thread_alloc callback can't tell if
it is being invoked on a new thread object, or a recycled thread.
Calling *_alloc multiple times on an object is also atypical for
kernel APIs.

As a result of this confusion, amd64 was potentially leaking an XSAVE
buffer each time a new kstack was allocated for an existing thread,
since cpu_thread_alloc for amd64 always allocated a new XSAVE buffer.
In practice, this edge case is probably rare.  A process object needs
to be recycled where either the new or old process is a kernel process
with a non-default kernel stack size.

Nevertheless, to ease the confusion, redefine cpu_thread_alloc to only
be called once when a new thread is allocated.  The new callback,
cpu_thread_new_kstack is invoked each time a kstack is allocated for a
thread, including both at thread creation time and if a recycled
thread allocates a new kstack.  The new callback should set any fields
whose value is dependent on td_kstack (e.g. the user frame in
td_frame, or td_pcb if the PCB is allocated on the kstack).

Reviewed by:	kib, andrew (arm changes)
Sponsored by:	AFRL, DARPA
Pull Request:	https://ron-dev.freebsd.org/FreeBSD/src/pulls/23

share/man/man9/Makefile

@@ -943,6 +943,7 @@ MLINKS+=cpu_machdep.9 cpu_copy_thread.9 \
 	cpu_machdep.9 cpu_thread_clean.9 \
 	cpu_machdep.9 cpu_thread_exit.9 \
 	cpu_machdep.9 cpu_thread_free.9 \
+	cpu_machdep.9 cpu_thread_new_kstack.9 \
 	cpu_machdep.9 cpu_throw.9 \
 	cpu_machdep.9 cpu_update_pcb.9
 MLINKS+=cpuset.9 CPUSET_T_INITIALIZER.9 \

share/man/man9/cpu_machdep.9

@@ -8,7 +8,7 @@
 .\" Technology), and Capabilities Limited under Defense Advanced Research
 .\" Projects Agency (DARPA) Contract No. FA8750-24-C-B047 ("DEC").
 .\"
-.Dd January 31, 2025
+.Dd May 27, 2026
 .Dt cpu_machdep 9
 .Os
 .Sh NAME
@@ -31,6 +31,7 @@
 .Nm cpu_thread_clean ,
 .Nm cpu_thread_exit ,
 .Nm cpu_thread_free ,
+.Nm cpu_thread_new_kstack ,
 .Nm cpu_throw ,
 .Nm cpu_update_pcb
 .Nd machine-dependent interfaces to handle CPU and thread state
@@ -84,6 +85,8 @@
 .Ft void
 .Fn cpu_thread_free "struct thread *td"
 .Ft void
+.Fn cpu_thread_new_kstack "struct thread *td"
+.Ft void
 .Fn cpu_throw "struct thread *old" "struct thread *new"
 .Ft void
 .Fn cpu_update_pcb "struct thread *td"
@@ -366,19 +369,25 @@ When the process object is later reused for a new process in
 .Xr fork 2 ,
 the kernel recycles that last thread object and uses it as the initial
 thread in the new process.
-When a thread is recycled, some of the steps in the thread allocation
+When a thread is recycled, a new kernel stack may be allocated if
-and free cycle are skipped as an optimization.
+the existing kernel stack is not suitable for the new process.
 .Pp
 .Fn cpu_thread_alloc
 initializes machine-dependent fields in
 .Fa td
+when allocating a new thread object.
+.Pp
+.Fn cpu_thread_new_kstack
+initializes kernel stack-related machine-dependent fields in
+.Fa td
 after allocating a new kernel stack.
 This function typically sets the
 .Fa td_pcb
+.Pq on architectures which store the pcb in the kernel stack
 and initial
 .Fa td_frame
 pointers.
-.Fn cpu_thread_alloc
+.Fn cpu_thread_new_kstack
 is called both when allocating a new thread object and
 when a recycled thread allocates a new kernel stack.
 Note that this function is
@@ -386,12 +395,19 @@ Note that this function is
 called if a recycled thread reuses its existing kernel stack.
 .Pp
 .Fn cpu_thread_clean
-releases any machine-dependent resources for the last thread in a
+releases machine-dependent resources for the last thread in a
 process during
 .Xr wait 2 .
-The thread is a candidate for recycling so should be reset to run as a
+Since the thread is a candidate for recycling,
+machine-dependent fields should be reset to run as a
 new thread in case it is recycled by a future
 .Xr fork 2 .
+In particular,
+if the thread reuses its existing kernel stack,
+no other
+.Fn cpu_thread_*
+function will be invoked before the thread is reused as the main
+thread of a new process.
 .Pp
 .Fn cpu_thread_exit
 cleans any machine-dependent state in

sys/amd64/amd64/vm_machdep.c

@@ -376,13 +376,18 @@ cpu_thread_alloc(struct thread *td)
 {
 	struct pcb *pcb;
 
-	set_top_of_stack_td(td);
 	td->td_pcb = pcb = get_pcb_td(td);
-	td->td_frame = (struct trapframe *)td->td_md.md_stack_base - 1;
 	td->td_md.md_usr_fpu_save = fpu_save_area_alloc();
 	pcb->pcb_save = get_pcb_user_save_pcb(pcb);
 }
 
+void
+cpu_thread_new_kstack(struct thread *td)
+{
+	set_top_of_stack_td(td);
+	td->td_frame = (struct trapframe *)td->td_md.md_stack_base - 1;
+}
+
 void
 cpu_thread_free(struct thread *td)
 {

sys/arm/arm/vm_machdep.c

@@ -240,6 +240,11 @@ cpu_thread_exit(struct thread *td)
 
 void
 cpu_thread_alloc(struct thread *td)
+{
+}
+
+void
+cpu_thread_new_kstack(struct thread *td)
 {
 	td->td_pcb = (struct pcb *)(td->td_kstack + td->td_kstack_pages *
 	    PAGE_SIZE) - 1;

sys/arm64/arm64/vm_machdep.c

@@ -260,12 +260,16 @@ cpu_thread_exit(struct thread *td)
 void
 cpu_thread_alloc(struct thread *td)
 {
+	ptrauth_thread_alloc(td);
+}
 
+void
+cpu_thread_new_kstack(struct thread *td)
+{
 	td->td_pcb = (struct pcb *)(td->td_kstack +
 	    td->td_kstack_pages * PAGE_SIZE) - 1;
 	td->td_frame = (struct trapframe *)STACKALIGN(
 	    (struct trapframe *)td->td_pcb - 1);
-	ptrauth_thread_alloc(td);
 }
 
 void

sys/i386/i386/vm_machdep.c

@@ -371,6 +371,11 @@ cpu_thread_clean(struct thread *td)
 
 void
 cpu_thread_alloc(struct thread *td)
+{
+}
+
+void
+cpu_thread_new_kstack(struct thread *td)
 {
 	struct pcb *pcb;
 

sys/kern/kern_thread.c

@@ -803,6 +803,7 @@ thread_alloc(int pages)
 	kasan_thread_alloc(td);
 	kmsan_thread_alloc(td);
 	cpu_thread_alloc(td);
+	cpu_thread_new_kstack(td);
 	EVENTHANDLER_DIRECT_INVOKE(thread_ctor, td);
 	return (td);
 }
@@ -815,7 +816,7 @@ thread_recycle(struct thread *td, int pages)
 			vm_thread_dispose(td);
 		if (!vm_thread_new(td, pages))
 			return (ENOMEM);
-		cpu_thread_alloc(td);
+		cpu_thread_new_kstack(td);
 	}
 	kasan_thread_alloc(td);
 	kmsan_thread_alloc(td);

sys/powerpc/powerpc/exec_machdep.c

@@ -1079,6 +1079,11 @@ cpu_thread_clean(struct thread *td)
 
 void
 cpu_thread_alloc(struct thread *td)
+{
+}
+
+void
+cpu_thread_new_kstack(struct thread *td)
 {
 	struct pcb *pcb;
 

sys/riscv/riscv/vm_machdep.c

@@ -58,8 +58,8 @@
 #define	TP_OFFSET	16	/* sizeof(struct tcb) */
 #endif
 
-static void
+void
-cpu_set_pcb_frame(struct thread *td)
+cpu_thread_new_kstack(struct thread *td)
 {
 	td->td_pcb = (struct pcb *)(td->td_kstack +
 	    td->td_kstack_pages * PAGE_SIZE) - 1;
@@ -228,7 +228,6 @@ cpu_thread_exit(struct thread *td)
 void
 cpu_thread_alloc(struct thread *td)
 {
-	cpu_set_pcb_frame(td);
 }
 
 void

sys/sys/proc.h

@@ -1246,6 +1246,7 @@ void	cpu_thread_alloc(struct thread *);
 void	cpu_thread_clean(struct thread *);
 void	cpu_thread_exit(struct thread *);
 void	cpu_thread_free(struct thread *);
+void	cpu_thread_new_kstack(struct thread *);
 struct	thread *thread_alloc(int pages);
 int	thread_check_susp(struct thread *td, bool sleep);
 void	thread_cow_get_proc(struct thread *newtd, struct proc *p);