[PATCH bpf-next 0/2] bpf: Allow access to const void pointer arguments in tracing programs

[PATCH bpf-next 0/2] bpf: Allow access to const void pointer arguments in tracing programs

[KaFai Wan]

hi, 
Tracing programs can access arguments via BTF [1]. Currently we allow 
tracing programs to access pointers to string (char pointer), 
void pointers, pointers to structs, and int pointers [2].

If we try to access argument which is pointer to const void like 2nd 
argument in kfree, it's an UNKNOWN type, verifier will fail to load. 
typedef void (*btf_trace_kfree)(void *, long unsigned int, const void *);

[1] https://lore.kernel.org/bpf/20191016032505.2089704-7-ast@kernel.org/
[2] https://lore.kernel.org/bpf/20211208193245.172141-1-jolsa@kernel.org/
---
KaFai Wan (2):
  bpf: Allow access to const void pointer arguments in tracing programs
  selftests/bpf: Add test to access const void pointer argument in
    tracing program

 kernel/bpf/btf.c                                       | 10 +++++++++-
 .../selftests/bpf/progs/verifier_btf_ctx_access.c      |  9 +++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

-- 
2.43.0

[PATCH bpf-next 1/2] bpf: Allow access to const void pointer arguments in tracing programs

[KaFai Wan]

Adding support to access arguments with const void pointer arguments
in tracing programs.

Currently we allow tracing programs to access void pointers. If we try to
access argument which is pointer to const void like 2nd argument in kfree,
verifier will fail to load the program with;

0: R1=ctx() R10=fp0
; asm volatile ("r2 = *(u64 *)(r1 + 8); ");
0: (79) r2 = *(u64 *)(r1 +8)
func 'kfree' arg1 type UNKNOWN is not a struct

Adding is_void_ptr to generic void  pointer check.

Cc: Leon Hwang <leon.hwang@linux.dev>
Signed-off-by: KaFai Wan <kafai.wan@hotmail.com>
---
 kernel/bpf/btf.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 16ba36f34dfa..e11d3afd0562 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -6383,6 +6383,14 @@ struct btf *bpf_prog_get_target_btf(const struct bpf_prog *prog)
 		return prog->aux->attach_btf;
 }
 
+static bool is_void_ptr(struct btf *btf, const struct btf_type *t)
+{
+	/* skip modifiers */
+	t = btf_type_skip_modifiers(btf, t->type, NULL);
+
+	return t->type == 0;
+}
+
 static bool is_int_ptr(struct btf *btf, const struct btf_type *t)
 {
 	/* skip modifiers */
@@ -6776,7 +6784,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 		}
 	}
 
-	if (t->type == 0)
+	if (is_void_ptr(btf, t))
 		/* This is a pointer to void.
 		 * It is the same as scalar from the verifier safety pov.
 		 * No further pointer walking is allowed.
-- 
2.43.0

[PATCH bpf-next 2/2] selftests/bpf: Add test to access const void pointer argument in tracing program

[KaFai Wan]

Adding verifier test for accessing const void pointer argument in
tracing programs.

The test program loads 2nd argument of kfree tp_btf which is
const void pointer and checks that verifier allows that.

Signed-off-by: KaFai Wan <kafai.wan@hotmail.com>
---
 .../selftests/bpf/progs/verifier_btf_ctx_access.c        | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_btf_ctx_access.c b/tools/testing/selftests/bpf/progs/verifier_btf_ctx_access.c
index 28b939572cda..a6cec7f73dcd 100644
--- a/tools/testing/selftests/bpf/progs/verifier_btf_ctx_access.c
+++ b/tools/testing/selftests/bpf/progs/verifier_btf_ctx_access.c
@@ -65,4 +65,13 @@ __naked void ctx_access_u32_pointer_reject_8(void)
 "	::: __clobber_all);
 }
 
+SEC("tp_btf/kfree")
+__description("btf_ctx_access const void pointer accept")
+int ctx_access_const_void_pointer_accept(void)
+{
+	/* load 2nd argument value (const void pointer) */
+	asm volatile ("r2 = *(u64 *)(r1 + 8); ");
+	return 0;
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.43.0

Re: [PATCH bpf-next 1/2] bpf: Allow access to const void pointer arguments in tracing programs

[Jiri Olsa]

On Sun, Apr 13, 2025 at 01:06:25AM +0800, KaFai Wan wrote:
> Adding support to access arguments with const void pointer arguments
> in tracing programs.
> 
> Currently we allow tracing programs to access void pointers. If we try to
> access argument which is pointer to const void like 2nd argument in kfree,
> verifier will fail to load the program with;
> 
> 0: R1=ctx() R10=fp0
> ; asm volatile ("r2 = *(u64 *)(r1 + 8); ");
> 0: (79) r2 = *(u64 *)(r1 +8)
> func 'kfree' arg1 type UNKNOWN is not a struct
> 
> Adding is_void_ptr to generic void  pointer check.
> 
> Cc: Leon Hwang <leon.hwang@linux.dev>
> Signed-off-by: KaFai Wan <kafai.wan@hotmail.com>
> ---
>  kernel/bpf/btf.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 16ba36f34dfa..e11d3afd0562 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -6383,6 +6383,14 @@ struct btf *bpf_prog_get_target_btf(const struct bpf_prog *prog)
>  		return prog->aux->attach_btf;
>  }
>  
> +static bool is_void_ptr(struct btf *btf, const struct btf_type *t)
> +{
> +	/* skip modifiers */
> +	t = btf_type_skip_modifiers(btf, t->type, NULL);
> +
> +	return t->type == 0;

I think you can use btf_type_is_void in here

> +}
> +
>  static bool is_int_ptr(struct btf *btf, const struct btf_type *t)
>  {
>  	/* skip modifiers */
> @@ -6776,7 +6784,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>  		}
>  	}
>  
> -	if (t->type == 0)
> +	if (is_void_ptr(btf, t))

lgtm,

nit, the is_void_ptr name suggest there's also ptr check in the helper function,
which is not the case. I understand it follows is_int_ptr name, but perhaps we
could rename both helpers to is_void and is_int ... feel free to ignore ;-)

jirka


>  		/* This is a pointer to void.
>  		 * It is the same as scalar from the verifier safety pov.
>  		 * No further pointer walking is allowed.
> -- 
> 2.43.0
> 

Re: [PATCH bpf-next 2/2] selftests/bpf: Add test to access const void pointer argument in tracing program

[Jiri Olsa]

On Sun, Apr 13, 2025 at 01:06:26AM +0800, KaFai Wan wrote:
> Adding verifier test for accessing const void pointer argument in
> tracing programs.
> 
> The test program loads 2nd argument of kfree tp_btf which is
> const void pointer and checks that verifier allows that.
> 
> Signed-off-by: KaFai Wan <kafai.wan@hotmail.com>
> ---
>  .../selftests/bpf/progs/verifier_btf_ctx_access.c        | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/tools/testing/selftests/bpf/progs/verifier_btf_ctx_access.c b/tools/testing/selftests/bpf/progs/verifier_btf_ctx_access.c
> index 28b939572cda..a6cec7f73dcd 100644
> --- a/tools/testing/selftests/bpf/progs/verifier_btf_ctx_access.c
> +++ b/tools/testing/selftests/bpf/progs/verifier_btf_ctx_access.c
> @@ -65,4 +65,13 @@ __naked void ctx_access_u32_pointer_reject_8(void)
>  "	::: __clobber_all);
>  }
>  
> +SEC("tp_btf/kfree")
> +__description("btf_ctx_access const void pointer accept")
> +int ctx_access_const_void_pointer_accept(void)
> +{
> +	/* load 2nd argument value (const void pointer) */
> +	asm volatile ("r2 = *(u64 *)(r1 + 8); ");

I think we should follow formatting of other tests in the file,
a do smth like:

	asm volatile ("				\
	r2 = *(u64 *)(r1 + 8); ");   /* load 2nd argument value (const void pointer) */\
	...

thanks,
jirka


> +	return 0;
> +}
> +
>  char _license[] SEC("license") = "GPL";
> -- 
> 2.43.0
> 

Re: [PATCH bpf-next 2/2] selftests/bpf: Add test to access const void pointer argument in tracing program

[Kafai Wan]

On Mon, Apr 14, 2025 at 6:35 PM Jiri Olsa <olsajiri@gmail.com> wrote:
>
> On Sun, Apr 13, 2025 at 01:06:26AM +0800, KaFai Wan wrote:
> > Adding verifier test for accessing const void pointer argument in
> > tracing programs.
> >
> > The test program loads 2nd argument of kfree tp_btf which is
> > const void pointer and checks that verifier allows that.
> >
> > Signed-off-by: KaFai Wan <kafai.wan@hotmail.com>
> > ---
> >  .../selftests/bpf/progs/verifier_btf_ctx_access.c        | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> >
> > diff --git a/tools/testing/selftests/bpf/progs/verifier_btf_ctx_access.c b/tools/testing/selftests/bpf/progs/verifier_btf_ctx_access.c
> > index 28b939572cda..a6cec7f73dcd 100644
> > --- a/tools/testing/selftests/bpf/progs/verifier_btf_ctx_access.c
> > +++ b/tools/testing/selftests/bpf/progs/verifier_btf_ctx_access.c
> > @@ -65,4 +65,13 @@ __naked void ctx_access_u32_pointer_reject_8(void)
> >  "    ::: __clobber_all);
> >  }
> >
> > +SEC("tp_btf/kfree")
> > +__description("btf_ctx_access const void pointer accept")
> > +int ctx_access_const_void_pointer_accept(void)
> > +{
> > +     /* load 2nd argument value (const void pointer) */
> > +     asm volatile ("r2 = *(u64 *)(r1 + 8); ");
>
> I think we should follow formatting of other tests in the file,
> a do smth like:
>
>         asm volatile ("                         \
>         r2 = *(u64 *)(r1 + 8); ");   /* load 2nd argument value (const void pointer) */\
>         ...

I will fix it. and I find out the kernel does not support test_run of tp_btf, I
will change to fentry.

>
> thanks,
> jirka
>
>
> > +     return 0;
> > +}
> > +
> >  char _license[] SEC("license") = "GPL";
> > --
> > 2.43.0
> >

thanks,
kafai

Re: [PATCH bpf-next 1/2] bpf: Allow access to const void pointer arguments in tracing programs

[Kafai Wan]

On Mon, Apr 14, 2025 at 6:35 PM Jiri Olsa <olsajiri@gmail.com> wrote:
>
> On Sun, Apr 13, 2025 at 01:06:25AM +0800, KaFai Wan wrote:
> > Adding support to access arguments with const void pointer arguments
> > in tracing programs.
> >
> > Currently we allow tracing programs to access void pointers. If we try to
> > access argument which is pointer to const void like 2nd argument in kfree,
> > verifier will fail to load the program with;
> >
> > 0: R1=ctx() R10=fp0
> > ; asm volatile ("r2 = *(u64 *)(r1 + 8); ");
> > 0: (79) r2 = *(u64 *)(r1 +8)
> > func 'kfree' arg1 type UNKNOWN is not a struct
> >
> > Adding is_void_ptr to generic void  pointer check.
> >
> > Cc: Leon Hwang <leon.hwang@linux.dev>
> > Signed-off-by: KaFai Wan <kafai.wan@hotmail.com>
> > ---
> >  kernel/bpf/btf.c | 10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index 16ba36f34dfa..e11d3afd0562 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -6383,6 +6383,14 @@ struct btf *bpf_prog_get_target_btf(const struct bpf_prog *prog)
> >               return prog->aux->attach_btf;
> >  }
> >
> > +static bool is_void_ptr(struct btf *btf, const struct btf_type *t)
> > +{
> > +     /* skip modifiers */
> > +     t = btf_type_skip_modifiers(btf, t->type, NULL);
> > +
> > +     return t->type == 0;
>
> I think you can use btf_type_is_void in here
>

Yes, I will use btf_type_is_void.

> > +}
> > +
> >  static bool is_int_ptr(struct btf *btf, const struct btf_type *t)
> >  {
> >       /* skip modifiers */
> > @@ -6776,7 +6784,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
> >               }
> >       }
> >
> > -     if (t->type == 0)
> > +     if (is_void_ptr(btf, t))
>
> lgtm,
>
> nit, the is_void_ptr name suggest there's also ptr check in the helper function,
> which is not the case. I understand it follows is_int_ptr name, but perhaps we
> could rename both helpers to is_void and is_int ... feel free to ignore ;-)

you are right, I will rename it. but I'm not sure if it's possible to merge
these two functions into one like is_void_or_int, they both skip modifiers.

>
> jirka
>
>
> >               /* This is a pointer to void.
> >                * It is the same as scalar from the verifier safety pov.
> >                * No further pointer walking is allowed.
> > --
> > 2.43.0
> >

thanks,
kafai