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