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From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     linux-kernel@vger.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        "Jose E . Marchesi" <jose.marchesi@oracle.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        Peter Zijlstra <peterz@infradead.org>
Subject: [RFC PATCH] tracepoint: Fix CFI failures with tp_sub_func (v2)
Date:   Thu, 23 Mar 2023 17:28:32 -0400
Message-Id: <20230323212832.5691-1-mathieu.desnoyers@efficios.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

When CLANG_CFI is in use, using tracing will occasionally result in
CFI failures, e.g.

| CFI failure at syscall_trace_enter+0x66c/0x7d0 (target: tp_stub_func+0x0/0x2c; expected type: 0x4877830c)
| Internal error: Oops - CFI: 00000000f200823a [#1] PREEMPT SMP
| CPU: 2 PID: 118 Comm: klogd Not tainted 6.3.0-rc3-00002-gc242ea6f2f98 #2
| Hardware name: linux,dummy-virt (DT)
| pstate: 30400005 (nzCV daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
| pc : syscall_trace_enter+0x66c/0x7d0
| lr : syscall_trace_enter+0x5e8/0x7d0
| sp : ffff800015ce7d80
| x29: ffff800015ce7d80 x28: ffff000017538000 x27: 0000000000000003
| x26: ffff8000084c9454 x25: ffff00001182bd10 x24: dfff800000000000
| x23: 1fffe00002ea7001 x22: ffff00001182bd10 x21: ffff000017538008
| x20: ffff000017538000 x19: ffff800015ce7eb0 x18: 0000000000000000
| x17: 000000004877830c x16: 00000000a540670c x15: 0000000000000000
| x14: 0000000000000000 x13: 0000000000000000 x12: ff80800008039d8c
| x11: ff80800008039dd0 x10: 0000000000000000 x9 : 0000000000000000
| x8 : 0000000000000000 x7 : 0000000000000000 x6 : 0000000000000000
| x5 : 0000000000000000 x4 : 0000000000000000 x3 : 0000000000000000
| x2 : 00000000000000ce x1 : ffff800015ce7eb0 x0 : ffff800013d55000
| Call trace:
|  syscall_trace_enter+0x66c/0x7d0
|  el0_svc_common+0x1dc/0x268
|  do_el0_svc+0x70/0x1a4
|  el0_svc+0x58/0x14c
|  el0t_64_sync_handler+0x84/0xf0
|  el0t_64_sync+0x190/0x194
| Code: 72906191 72a90ef1 6b11021f 54000040 (d4304740)
| ---[ end trace 0000000000000000 ]---

This happens because the function prototype of tp_sub_func doesn't match
the prototype of the tracepoint function. As each tracepoint may have a
distinct prototype, it's not possible to share a common stub function.

Avoid this by comparing the tracepoint function pointer to the value 1
before calling each function.

Prior to this change, it_func_ptr->func was loaded twice per loop
iteration:

    it_func = READ_ONCE((it_func_ptr)->func);
[...]
  } while ((++it_func_ptr)->func);

With this change, the loaded it_func is used for comparisons with 0 (end
of loop), 1 (skip) and as a function pointer, thus saving a load per
loop.

The comparison with <= TRACEPOINT_FUNC_SKIP is done on purpose to branch
over both comparisons with 0 and 1 with a single branch for each loop
and an extra conditional branch on the last loop to compare with 0.

A simplied example of this compiled on godbolt.org:

  #include <stdint.h>

  typedef void (*f_t)(void);
  volatile f_t *arr;

  void fct(void)
  {
      volatile f_t *iter = &arr[0];

      for (;;) {
          f_t l_iter = *iter;
          if (((uintptr_t)l_iter <= 0x1)) {
              if ((uintptr_t)l_iter == 0x1)
                  continue;
              else
                  break;
          }
          l_iter();
          iter++;
      }
  }

Generates:

x86-64 gcc 12.2 -O2:

  fct:
          push    rbx
          mov     rbx, QWORD PTR arr[rip]
  .L29:
          mov     rax, QWORD PTR [rbx]
          cmp     rax, 1
          jbe     .L38
  .L30:
          call    rax
          mov     rax, QWORD PTR [rbx+8]
          add     rbx, 8
          cmp     rax, 1
          ja      .L30
  .L38:
          je      .L29
          pop     rbx
          ret

clang 16.0.0 -O2:

  fct:                                    # @fct
          push    rbx
          mov     rbx, qword ptr [rip + arr]
  .LBB0_1:                                # =>This Inner Loop Header: Depth=1
          mov     rax, qword ptr [rbx]
          cmp     rax, 1
          ja      .LBB0_4
          je      .LBB0_1
          jmp     .LBB0_3
  .LBB0_4:                                #   in Loop: Header=BB0_1 Depth=1
          call    rax
          add     rbx, 8
          jmp     .LBB0_1
  .LBB0_3:
          pop     rbx
          ret
  arr:
          .quad   0

On x86, the result of the "cmp" instruction is used to conditionally
branch based on both inequality (jbe, ja) and equality (je), thus
keeping the code size small and limiting the number of instructions to
execute on this tracepoint instrumentation fast path.

Reported-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Jose E. Marchesi <jose.marchesi@oracle.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Sami Tolvanen <samitolvanen@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
---
 include/linux/tracepoint.h | 14 ++++++++++++--
 kernel/tracepoint.c        | 20 +++++++-------------
 2 files changed, 19 insertions(+), 15 deletions(-)

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 4b33b95eb8be..0aeac249d412 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -33,6 +33,9 @@ struct trace_eval_map {
 
 #define TRACEPOINT_DEFAULT_PRIO	10
 
+/* Reserved value for tracepoint callback. */
+#define TRACEPOINT_FUNC_SKIP	((void *) 0x1)
+
 extern struct srcu_struct tracepoint_srcu;
 
 extern int
@@ -314,11 +317,18 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 		it_func_ptr =						\
 			rcu_dereference_raw((&__tracepoint_##_name)->funcs); \
 		if (it_func_ptr) {					\
-			do {						\
+			for (;;) {					\
 				it_func = READ_ONCE((it_func_ptr)->func); \
+				if ((uintptr_t) it_func <= (uintptr_t) TRACEPOINT_FUNC_SKIP) { \
+					if (it_func == TRACEPOINT_FUNC_SKIP) \
+						continue;		\
+					else				\
+						break;			\
+				}					\
 				__data = (it_func_ptr)->data;		\
 				((void(*)(void *, proto))(it_func))(__data, args); \
-			} while ((++it_func_ptr)->func);		\
+				it_func_ptr++;				\
+			}						\
 		}							\
 		return 0;						\
 	}								\
diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index f23144af5743..2fa108ddbbc2 100644
--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -98,12 +98,6 @@ struct tp_probes {
 	struct tracepoint_func probes[];
 };
 
-/* Called in removal of a func but failed to allocate a new tp_funcs */
-static void tp_stub_func(void)
-{
-	return;
-}
-
 static inline void *allocate_probes(int count)
 {
 	struct tp_probes *p  = kmalloc(struct_size(p, probes, count),
@@ -193,8 +187,8 @@ func_add(struct tracepoint_func **funcs, struct tracepoint_func *tp_func,
 	if (old) {
 		/* (N -> N+1), (N != 0, 1) probes */
 		for (iter_probes = 0; old[iter_probes].func; iter_probes++) {
-			if (old[iter_probes].func == tp_stub_func)
-				continue;	/* Skip stub functions. */
+			if (old[iter_probes].func == TRACEPOINT_FUNC_SKIP)
+				continue;
 			if (old[iter_probes].func == tp_func->func &&
 			    old[iter_probes].data == tp_func->data)
 				return ERR_PTR(-EEXIST);
@@ -208,7 +202,7 @@ func_add(struct tracepoint_func **funcs, struct tracepoint_func *tp_func,
 	if (old) {
 		nr_probes = 0;
 		for (iter_probes = 0; old[iter_probes].func; iter_probes++) {
-			if (old[iter_probes].func == tp_stub_func)
+			if (old[iter_probes].func == TRACEPOINT_FUNC_SKIP)
 				continue;
 			/* Insert before probes of lower priority */
 			if (pos < 0 && old[iter_probes].prio < prio)
@@ -246,7 +240,7 @@ static void *func_remove(struct tracepoint_func **funcs,
 		for (nr_probes = 0; old[nr_probes].func; nr_probes++) {
 			if ((old[nr_probes].func == tp_func->func &&
 			     old[nr_probes].data == tp_func->data) ||
-			    old[nr_probes].func == tp_stub_func)
+			    old[nr_probes].func == TRACEPOINT_FUNC_SKIP)
 				nr_del++;
 		}
 	}
@@ -269,7 +263,7 @@ static void *func_remove(struct tracepoint_func **funcs,
 			for (i = 0; old[i].func; i++) {
 				if ((old[i].func != tp_func->func ||
 				     old[i].data != tp_func->data) &&
-				    old[i].func != tp_stub_func)
+				    old[i].func != TRACEPOINT_FUNC_SKIP)
 					new[j++] = old[i];
 			}
 			new[nr_probes - nr_del].func = NULL;
@@ -277,12 +271,12 @@ static void *func_remove(struct tracepoint_func **funcs,
 		} else {
 			/*
 			 * Failed to allocate, replace the old function
-			 * with calls to tp_stub_func.
+			 * with TRACEPOINT_FUNC_SKIP.
 			 */
 			for (i = 0; old[i].func; i++) {
 				if (old[i].func == tp_func->func &&
 				    old[i].data == tp_func->data)
-					WRITE_ONCE(old[i].func, tp_stub_func);
+					WRITE_ONCE(old[i].func, TRACEPOINT_FUNC_SKIP);
 			}
 			*funcs = old;
 		}
-- 
2.25.1