diff mbox series

[v3] tracepoint: Fix CFI failures with tp_sub_func

Message ID 20230324123142.7463-1-mathieu.desnoyers@efficios.com
State New
Headers
Series [v3] tracepoint: Fix CFI failures with tp_sub_func |

Commit Message

Mathieu Desnoyers March 24, 2023, 12:31 p.m. UTC 
  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 simplified 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>
Tested-by: Mark Rutland <mark.rutlnand@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.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>
---
Changes since v2:
- Fix typo in commit message.
- Add acked-by, tested-by.
---
 include/linux/tracepoint.h | 14 ++++++++++++--
 kernel/tracepoint.c        | 20 +++++++-------------
 2 files changed, 19 insertions(+), 15 deletions(-)

Comments

Mathieu Desnoyers March 24, 2023, 12:33 p.m. UTC | #1 
On 2023-03-24 08:31, Mathieu Desnoyers wrote:
> 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 simplified 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>
> Tested-by: Mark Rutland <mark.rutlnand@arm.com>

It appears there was a typo in this Tested-by address from Mark. Steven, 
should I resend or can you correct this as you pick it up ?

Thanks,

Mathieu

> Acked-by: Mark Rutland <mark.rutland@arm.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>
> ---
> Changes since v2:
> - Fix typo in commit message.
> - Add acked-by, tested-by.
> ---
>   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;
>   		}
Mark Rutland April 4, 2023, 10:59 a.m. UTC | #2 
Hi Steve,

On Fri, Mar 24, 2023 at 08:33:35AM -0400, Mathieu Desnoyers wrote:
> On 2023-03-24 08:31, Mathieu Desnoyers wrote:
> > 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.

[...]

> > Reported-by: Mark Rutland <mark.rutland@arm.com>
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > Tested-by: Mark Rutland <mark.rutlnand@arm.com>
> 
> It appears there was a typo in this Tested-by address from Mark. Steven,
> should I resend or can you correct this as you pick it up ?

Sorry to ping, but are you planning to pick this up, and/or did you want this
resent with the Testing-by typo fixed?

I couldn't spot it in the tracing tree, and I'm not sure if it's still on your
TODO list for review or has just been missed.

Thanks,
Mark.

> 
> Thanks,
> 
> Mathieu
> 
> > Acked-by: Mark Rutland <mark.rutland@arm.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>
> > ---
> > Changes since v2:
> > - Fix typo in commit message.
> > - Add acked-by, tested-by.
> > ---
> >   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;
> >   		}
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
>
Steven Rostedt April 4, 2023, 8:22 p.m. UTC | #3 
On Tue, 4 Apr 2023 11:59:26 +0100
Mark Rutland <mark.rutland@arm.com> wrote:

> Sorry to ping, but are you planning to pick this up, and/or did you want this
> resent with the Testing-by typo fixed?
> 
> I couldn't spot it in the tracing tree, and I'm not sure if it's still on your
> TODO list for review or has just been missed.

Actually, we may be going back to your original patches, as having a stub
function for every trace event will allow us to attach fprobes to them. And
we actually have a need to do that (connecting to tracepoints to get more
info than the trace event gives).

Masami is currently working on this.

-- Steve
Masami Hiramatsu (Google) April 8, 2023, 2:03 p.m. UTC | #4 
On Tue, 4 Apr 2023 16:22:41 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Tue, 4 Apr 2023 11:59:26 +0100
> Mark Rutland <mark.rutland@arm.com> wrote:
> 
> > Sorry to ping, but are you planning to pick this up, and/or did you want this
> > resent with the Testing-by typo fixed?
> > 
> > I couldn't spot it in the tracing tree, and I'm not sure if it's still on your
> > TODO list for review or has just been missed.
> 
> Actually, we may be going back to your original patches, as having a stub
> function for every trace event will allow us to attach fprobes to them. And
> we actually have a need to do that (connecting to tracepoints to get more
> info than the trace event gives).
> 
> Masami is currently working on this.

I think we need another stub function similar to the original patch, because
the __tracestub_* functions are usually not called back. Even if I enabled
the static key to enable the stub callback, if user sets another callback
handler to the tracepoint (e.g. enable trace event on it), the __tracestub_*
function is not called anymore.

Thus I think we just pick this version and add another patch to introduce
__probestub_* functions in macro, and fprobe on it. This new stub function is
something like the original one so that it doesn't break CFI. And it will be
registered to tracepoint when the new fprobe based dynamic event is enabled.


Thank you,
Steven Rostedt April 11, 2023, 5:14 p.m. UTC | #5 
On Sat, 8 Apr 2023 23:03:44 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> I think we need another stub function similar to the original patch, because
> the __tracestub_* functions are usually not called back. Even if I enabled
> the static key to enable the stub callback, if user sets another callback
> handler to the tracepoint (e.g. enable trace event on it), the __tracestub_*
> function is not called anymore.
> 
> Thus I think we just pick this version and add another patch to introduce
> __probestub_* functions in macro, and fprobe on it. This new stub function is
> something like the original one so that it doesn't break CFI. And it will be
> registered to tracepoint when the new fprobe based dynamic event is enabled.
> 

If we are going to add *another* stub, then no, I don't want to use it. But
I don't see why we can't use one stub for both?

What's the issue you are having?

-- Steve
diff mbox series

Patch

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;
 		}