[PATCH bpf-next v2] libbpf: Add support for dynamic program attach target

[PATCH bpf-next v2] libbpf: Add support for dynamic program attach target

[Eelco Chaudron]

Currently when you want to attach a trace program to a bpf program
the section name needs to match the tracepoint/function semantics.

However the addition of the bpf_program__set_attach_target() API
allows you to specify the tracepoint/function dynamically.

The call flow would look something like this:

  xdp_fd = bpf_prog_get_fd_by_id(id);
  trace_obj = bpf_object__open_file("func.o", NULL);
  prog = bpf_object__find_program_by_title(trace_obj,
                                           "fentry/myfunc");
  bpf_program__set_expected_attach_type(prog, BPF_TRACE_FENTRY);
  bpf_program__set_attach_target(prog, xdp_fd,
                                 "xdpfilt_blk_all");
  bpf_object__load(trace_obj)

Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
---
v1 -> v2: Remove requirement for attach type name in API

 tools/lib/bpf/libbpf.c   |   33 +++++++++++++++++++++++++++++++--
 tools/lib/bpf/libbpf.h   |    4 ++++
 tools/lib/bpf/libbpf.map |    1 +
 3 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 514b1a524abb..9b8cab995580 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -4939,8 +4939,8 @@ int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
 {
 	int err = 0, fd, i, btf_id;
 
-	if (prog->type == BPF_PROG_TYPE_TRACING ||
-	    prog->type == BPF_PROG_TYPE_EXT) {
+	if ((prog->type == BPF_PROG_TYPE_TRACING ||
+	     prog->type == BPF_PROG_TYPE_EXT) && !prog->attach_btf_id) {
 		btf_id = libbpf_find_attach_btf_id(prog);
 		if (btf_id <= 0)
 			return btf_id;
@@ -8132,6 +8132,35 @@ void bpf_program__bpil_offs_to_addr(struct bpf_prog_info_linear *info_linear)
 	}
 }
 
+int bpf_program__set_attach_target(struct bpf_program *prog,
+				   int attach_prog_fd,
+				   const char *attach_func_name)
+{
+	int btf_id;
+
+	if (!prog || attach_prog_fd < 0 || !attach_func_name)
+		return -EINVAL;
+
+	if (attach_prog_fd)
+		btf_id = libbpf_find_prog_btf_id(attach_func_name,
+						 attach_prog_fd);
+	else
+		btf_id = __find_vmlinux_btf_id(prog->obj->btf_vmlinux,
+					       attach_func_name,
+					       prog->expected_attach_type);
+
+	if (btf_id <= 0) {
+		if (!attach_prog_fd)
+			pr_warn("%s is not found in vmlinux BTF\n",
+				attach_func_name);
+		return btf_id;
+	}
+
+	prog->attach_btf_id = btf_id;
+	prog->attach_prog_fd = attach_prog_fd;
+	return 0;
+}
+
 int parse_cpu_mask_str(const char *s, bool **mask, int *mask_sz)
 {
 	int err = 0, n, len, start, end = -1;
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 3fe12c9d1f92..02fc58a21a7f 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -334,6 +334,10 @@ LIBBPF_API void
 bpf_program__set_expected_attach_type(struct bpf_program *prog,
 				      enum bpf_attach_type type);
 
+LIBBPF_API int
+bpf_program__set_attach_target(struct bpf_program *prog, int attach_prog_fd,
+			       const char *attach_func_name);
+
 LIBBPF_API bool bpf_program__is_socket_filter(const struct bpf_program *prog);
 LIBBPF_API bool bpf_program__is_tracepoint(const struct bpf_program *prog);
 LIBBPF_API bool bpf_program__is_raw_tracepoint(const struct bpf_program *prog);
diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index b035122142bb..8aba5438a3f0 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -230,6 +230,7 @@ LIBBPF_0.0.7 {
 		bpf_program__name;
 		bpf_program__is_extension;
 		bpf_program__is_struct_ops;
+		bpf_program__set_attach_target;
 		bpf_program__set_extension;
 		bpf_program__set_struct_ops;
 		btf__align_of;

Re: [PATCH bpf-next v2] libbpf: Add support for dynamic program attach target

[Toke Høiland-Jørgensen]

Eelco Chaudron <echaudro@redhat.com> writes:

> Currently when you want to attach a trace program to a bpf program
> the section name needs to match the tracepoint/function semantics.
>
> However the addition of the bpf_program__set_attach_target() API
> allows you to specify the tracepoint/function dynamically.
>
> The call flow would look something like this:
>
>   xdp_fd = bpf_prog_get_fd_by_id(id);
>   trace_obj = bpf_object__open_file("func.o", NULL);
>   prog = bpf_object__find_program_by_title(trace_obj,
>                                            "fentry/myfunc");
>   bpf_program__set_expected_attach_type(prog, BPF_TRACE_FENTRY);
>   bpf_program__set_attach_target(prog, xdp_fd,
>                                  "xdpfilt_blk_all");
>   bpf_object__load(trace_obj)
>
> Signed-off-by: Eelco Chaudron <echaudro@redhat.com>

Hmm, one question about the attach_prog_fd usage:

> +int bpf_program__set_attach_target(struct bpf_program *prog,
> +				   int attach_prog_fd,
> +				   const char *attach_func_name)
> +{
> +	int btf_id;
> +
> +	if (!prog || attach_prog_fd < 0 || !attach_func_name)
> +		return -EINVAL;
> +
> +	if (attach_prog_fd)
> +		btf_id = libbpf_find_prog_btf_id(attach_func_name,
> +						 attach_prog_fd);
> +	else
> +		btf_id = __find_vmlinux_btf_id(prog->obj->btf_vmlinux,
> +					       attach_func_name,
> +					       prog->expected_attach_type);

This implies that no one would end up using fd 0 as a legitimate prog
fd. This already seems to be the case for the existing code, but is that
really a safe assumption? Couldn't a caller that closes fd 0 (for
instance while forking) end up having it reused? Seems like this could
result in weird hard-to-debug bugs?

-Toke

Re: [PATCH bpf-next v2] libbpf: Add support for dynamic program attach target

[Eelco Chaudron]

On 13 Feb 2020, at 16:32, Toke Høiland-Jørgensen wrote:

> Eelco Chaudron <echaudro@redhat.com> writes:
>
>> Currently when you want to attach a trace program to a bpf program
>> the section name needs to match the tracepoint/function semantics.
>>
>> However the addition of the bpf_program__set_attach_target() API
>> allows you to specify the tracepoint/function dynamically.
>>
>> The call flow would look something like this:
>>
>>   xdp_fd = bpf_prog_get_fd_by_id(id);
>>   trace_obj = bpf_object__open_file("func.o", NULL);
>>   prog = bpf_object__find_program_by_title(trace_obj,
>>                                            "fentry/myfunc");
>>   bpf_program__set_expected_attach_type(prog, BPF_TRACE_FENTRY);
>>   bpf_program__set_attach_target(prog, xdp_fd,
>>                                  "xdpfilt_blk_all");
>>   bpf_object__load(trace_obj)
>>
>> Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
>
> Hmm, one question about the attach_prog_fd usage:
>
>> +int bpf_program__set_attach_target(struct bpf_program *prog,
>> +				   int attach_prog_fd,
>> +				   const char *attach_func_name)
>> +{
>> +	int btf_id;
>> +
>> +	if (!prog || attach_prog_fd < 0 || !attach_func_name)
>> +		return -EINVAL;
>> +
>> +	if (attach_prog_fd)
>> +		btf_id = libbpf_find_prog_btf_id(attach_func_name,
>> +						 attach_prog_fd);
>> +	else
>> +		btf_id = __find_vmlinux_btf_id(prog->obj->btf_vmlinux,
>> +					       attach_func_name,
>> +					       prog->expected_attach_type);
>
> This implies that no one would end up using fd 0 as a legitimate prog
> fd. This already seems to be the case for the existing code, but is 
> that
> really a safe assumption? Couldn't a caller that closes fd 0 (for
> instance while forking) end up having it reused? Seems like this could
> result in weird hard-to-debug bugs?


Yes, in theory, this can happen but it has nothing to do with this 
specific patch. The existing code already assumes that attach_prog_fd == 
0 means attach to a kernel function :(

Re: [PATCH bpf-next v2] libbpf: Add support for dynamic program attach target

[Toke Høiland-Jørgensen]

"Eelco Chaudron" <echaudro@redhat.com> writes:

> On 13 Feb 2020, at 16:32, Toke Høiland-Jørgensen wrote:
>
>> Eelco Chaudron <echaudro@redhat.com> writes:
>>
>>> Currently when you want to attach a trace program to a bpf program
>>> the section name needs to match the tracepoint/function semantics.
>>>
>>> However the addition of the bpf_program__set_attach_target() API
>>> allows you to specify the tracepoint/function dynamically.
>>>
>>> The call flow would look something like this:
>>>
>>>   xdp_fd = bpf_prog_get_fd_by_id(id);
>>>   trace_obj = bpf_object__open_file("func.o", NULL);
>>>   prog = bpf_object__find_program_by_title(trace_obj,
>>>                                            "fentry/myfunc");
>>>   bpf_program__set_expected_attach_type(prog, BPF_TRACE_FENTRY);
>>>   bpf_program__set_attach_target(prog, xdp_fd,
>>>                                  "xdpfilt_blk_all");
>>>   bpf_object__load(trace_obj)
>>>
>>> Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
>>
>> Hmm, one question about the attach_prog_fd usage:
>>
>>> +int bpf_program__set_attach_target(struct bpf_program *prog,
>>> +				   int attach_prog_fd,
>>> +				   const char *attach_func_name)
>>> +{
>>> +	int btf_id;
>>> +
>>> +	if (!prog || attach_prog_fd < 0 || !attach_func_name)
>>> +		return -EINVAL;
>>> +
>>> +	if (attach_prog_fd)
>>> +		btf_id = libbpf_find_prog_btf_id(attach_func_name,
>>> +						 attach_prog_fd);
>>> +	else
>>> +		btf_id = __find_vmlinux_btf_id(prog->obj->btf_vmlinux,
>>> +					       attach_func_name,
>>> +					       prog->expected_attach_type);
>>
>> This implies that no one would end up using fd 0 as a legitimate prog
>> fd. This already seems to be the case for the existing code, but is 
>> that
>> really a safe assumption? Couldn't a caller that closes fd 0 (for
>> instance while forking) end up having it reused? Seems like this could
>> result in weird hard-to-debug bugs?
>
>
> Yes, in theory, this can happen but it has nothing to do with this 
> specific patch. The existing code already assumes that attach_prog_fd == 
> 0 means attach to a kernel function :(

Yup, I do realise you're just sticking to the existing behaviour. Seems
even the kernel does that check for fd != 0, so I guess that's ABI now.
Still not sure I believe this will not trip anyone up, though... :/

-Toke

Re: [PATCH bpf-next v2] libbpf: Add support for dynamic program attach target

[Toke Høiland-Jørgensen]

Eelco Chaudron <echaudro@redhat.com> writes:

> Currently when you want to attach a trace program to a bpf program
> the section name needs to match the tracepoint/function semantics.
>
> However the addition of the bpf_program__set_attach_target() API
> allows you to specify the tracepoint/function dynamically.
>
> The call flow would look something like this:
>
>   xdp_fd = bpf_prog_get_fd_by_id(id);
>   trace_obj = bpf_object__open_file("func.o", NULL);
>   prog = bpf_object__find_program_by_title(trace_obj,
>                                            "fentry/myfunc");
>   bpf_program__set_expected_attach_type(prog, BPF_TRACE_FENTRY);
>   bpf_program__set_attach_target(prog, xdp_fd,
>                                  "xdpfilt_blk_all");
>   bpf_object__load(trace_obj)
>
> Signed-off-by: Eelco Chaudron <echaudro@redhat.com>

Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>

Re: [PATCH bpf-next v2] libbpf: Add support for dynamic program attach target

[Andrii Nakryiko]

On Thu, Feb 13, 2020 at 7:05 AM Eelco Chaudron <echaudro@redhat.com> wrote:
>
> Currently when you want to attach a trace program to a bpf program
> the section name needs to match the tracepoint/function semantics.
>
> However the addition of the bpf_program__set_attach_target() API
> allows you to specify the tracepoint/function dynamically.
>
> The call flow would look something like this:
>
>   xdp_fd = bpf_prog_get_fd_by_id(id);
>   trace_obj = bpf_object__open_file("func.o", NULL);
>   prog = bpf_object__find_program_by_title(trace_obj,
>                                            "fentry/myfunc");
>   bpf_program__set_expected_attach_type(prog, BPF_TRACE_FENTRY);
>   bpf_program__set_attach_target(prog, xdp_fd,
>                                  "xdpfilt_blk_all");
>   bpf_object__load(trace_obj)
>
> Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
> ---

API-wise this looks good, thanks! Please address feedback below and
re-submit once bpf-next opens. Can you please also convert one of
existing selftests using open_opts's attach_prog_fd to use this API
instead to have a demonstration there?

> v1 -> v2: Remove requirement for attach type name in API
>
>  tools/lib/bpf/libbpf.c   |   33 +++++++++++++++++++++++++++++++--
>  tools/lib/bpf/libbpf.h   |    4 ++++
>  tools/lib/bpf/libbpf.map |    1 +
>  3 files changed, 36 insertions(+), 2 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 514b1a524abb..9b8cab995580 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -4939,8 +4939,8 @@ int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
>  {
>         int err = 0, fd, i, btf_id;
>
> -       if (prog->type == BPF_PROG_TYPE_TRACING ||
> -           prog->type == BPF_PROG_TYPE_EXT) {
> +       if ((prog->type == BPF_PROG_TYPE_TRACING ||
> +            prog->type == BPF_PROG_TYPE_EXT) && !prog->attach_btf_id) {
>                 btf_id = libbpf_find_attach_btf_id(prog);
>                 if (btf_id <= 0)
>                         return btf_id;
> @@ -8132,6 +8132,35 @@ void bpf_program__bpil_offs_to_addr(struct bpf_prog_info_linear *info_linear)
>         }
>  }
>
> +int bpf_program__set_attach_target(struct bpf_program *prog,
> +                                  int attach_prog_fd,
> +                                  const char *attach_func_name)
> +{
> +       int btf_id;
> +
> +       if (!prog || attach_prog_fd < 0 || !attach_func_name)
> +               return -EINVAL;
> +
> +       if (attach_prog_fd)
> +               btf_id = libbpf_find_prog_btf_id(attach_func_name,
> +                                                attach_prog_fd);
> +       else
> +               btf_id = __find_vmlinux_btf_id(prog->obj->btf_vmlinux,
> +                                              attach_func_name,
> +                                              prog->expected_attach_type);
> +
> +       if (btf_id <= 0) {
> +               if (!attach_prog_fd)
> +                       pr_warn("%s is not found in vmlinux BTF\n",
> +                               attach_func_name);

libbpf_find_attach_btf_id's error reporting is misleading (it always
reports as if error happened with vmlinux BTF, even if attach_prog_fd
> 0). Could you please fix that and add better error reporting here
for attach_prog_fd>0 case here?

> +               return btf_id;
> +       }
> +
> +       prog->attach_btf_id = btf_id;
> +       prog->attach_prog_fd = attach_prog_fd;
> +       return 0;
> +}
> +
>  int parse_cpu_mask_str(const char *s, bool **mask, int *mask_sz)
>  {
>         int err = 0, n, len, start, end = -1;
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 3fe12c9d1f92..02fc58a21a7f 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -334,6 +334,10 @@ LIBBPF_API void
>  bpf_program__set_expected_attach_type(struct bpf_program *prog,
>                                       enum bpf_attach_type type);
>
> +LIBBPF_API int
> +bpf_program__set_attach_target(struct bpf_program *prog, int attach_prog_fd,
> +                              const char *attach_func_name);
> +
>  LIBBPF_API bool bpf_program__is_socket_filter(const struct bpf_program *prog);
>  LIBBPF_API bool bpf_program__is_tracepoint(const struct bpf_program *prog);
>  LIBBPF_API bool bpf_program__is_raw_tracepoint(const struct bpf_program *prog);
> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> index b035122142bb..8aba5438a3f0 100644
> --- a/tools/lib/bpf/libbpf.map
> +++ b/tools/lib/bpf/libbpf.map
> @@ -230,6 +230,7 @@ LIBBPF_0.0.7 {
>                 bpf_program__name;
>                 bpf_program__is_extension;
>                 bpf_program__is_struct_ops;
> +               bpf_program__set_attach_target;

This will have to go into LIBBPF_0.0.8 once bpf-next opens. Please
rebase and re-send then.

>                 bpf_program__set_extension;
>                 bpf_program__set_struct_ops;
>                 btf__align_of;
>

Re: [PATCH bpf-next v2] libbpf: Add support for dynamic program attach target

[Eelco Chaudron]

On 13 Feb 2020, at 18:42, Andrii Nakryiko wrote:

> On Thu, Feb 13, 2020 at 7:05 AM Eelco Chaudron <echaudro@redhat.com> 
> wrote:
>>
>> Currently when you want to attach a trace program to a bpf program
>> the section name needs to match the tracepoint/function semantics.
>>
>> However the addition of the bpf_program__set_attach_target() API
>> allows you to specify the tracepoint/function dynamically.
>>
>> The call flow would look something like this:
>>
>>   xdp_fd = bpf_prog_get_fd_by_id(id);
>>   trace_obj = bpf_object__open_file("func.o", NULL);
>>   prog = bpf_object__find_program_by_title(trace_obj,
>>                                            "fentry/myfunc");
>>   bpf_program__set_expected_attach_type(prog, BPF_TRACE_FENTRY);
>>   bpf_program__set_attach_target(prog, xdp_fd,
>>                                  "xdpfilt_blk_all");
>>   bpf_object__load(trace_obj)
>>
>> Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
>> ---
>
> API-wise this looks good, thanks! Please address feedback below and
> re-submit once bpf-next opens. Can you please also convert one of
> existing selftests using open_opts's attach_prog_fd to use this API
> instead to have a demonstration there?

Yes will update the one I added for bfp2bpf testing…

>> v1 -> v2: Remove requirement for attach type name in API
>>
>>  tools/lib/bpf/libbpf.c   |   33 +++++++++++++++++++++++++++++++--
>>  tools/lib/bpf/libbpf.h   |    4 ++++
>>  tools/lib/bpf/libbpf.map |    1 +
>>  3 files changed, 36 insertions(+), 2 deletions(-)
>>
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index 514b1a524abb..9b8cab995580 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
>> @@ -4939,8 +4939,8 @@ int bpf_program__load(struct bpf_program *prog, 
>> char *license, __u32 kern_ver)
>>  {
>>         int err = 0, fd, i, btf_id;
>>
>> -       if (prog->type == BPF_PROG_TYPE_TRACING ||
>> -           prog->type == BPF_PROG_TYPE_EXT) {
>> +       if ((prog->type == BPF_PROG_TYPE_TRACING ||
>> +            prog->type == BPF_PROG_TYPE_EXT) && 
>> !prog->attach_btf_id) {
>>                 btf_id = libbpf_find_attach_btf_id(prog);
>>                 if (btf_id <= 0)
>>                         return btf_id;
>> @@ -8132,6 +8132,35 @@ void bpf_program__bpil_offs_to_addr(struct 
>> bpf_prog_info_linear *info_linear)
>>         }
>>  }
>>
>> +int bpf_program__set_attach_target(struct bpf_program *prog,
>> +                                  int attach_prog_fd,
>> +                                  const char *attach_func_name)
>> +{
>> +       int btf_id;
>> +
>> +       if (!prog || attach_prog_fd < 0 || !attach_func_name)
>> +               return -EINVAL;
>> +
>> +       if (attach_prog_fd)
>> +               btf_id = libbpf_find_prog_btf_id(attach_func_name,
>> +                                                attach_prog_fd);
>> +       else
>> +               btf_id = 
>> __find_vmlinux_btf_id(prog->obj->btf_vmlinux,
>> +                                              attach_func_name,
>> +                                              
>> prog->expected_attach_type);
>> +
>> +       if (btf_id <= 0) {
>> +               if (!attach_prog_fd)
>> +                       pr_warn("%s is not found in vmlinux BTF\n",
>> +                               attach_func_name);
>
> libbpf_find_attach_btf_id's error reporting is misleading (it always
> reports as if error happened with vmlinux BTF, even if attach_prog_fd
> 0). Could you please fix that and add better error reporting here
> for attach_prog_fd>0 case here?
>

I did not add log messages for the btf_id > 0 case as they are covered 
in the libbpf_find_prog_btf_id() function. Please let me know if this is 
not enough.

>> +               return btf_id;
>> +       }
>> +
>> +       prog->attach_btf_id = btf_id;
>> +       prog->attach_prog_fd = attach_prog_fd;
>> +       return 0;
>> +}
>> +
>>  int parse_cpu_mask_str(const char *s, bool **mask, int *mask_sz)
>>  {
>>         int err = 0, n, len, start, end = -1;
>> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
>> index 3fe12c9d1f92..02fc58a21a7f 100644
>> --- a/tools/lib/bpf/libbpf.h
>> +++ b/tools/lib/bpf/libbpf.h
>> @@ -334,6 +334,10 @@ LIBBPF_API void
>>  bpf_program__set_expected_attach_type(struct bpf_program *prog,
>>                                       enum bpf_attach_type type);
>>
>> +LIBBPF_API int
>> +bpf_program__set_attach_target(struct bpf_program *prog, int 
>> attach_prog_fd,
>> +                              const char *attach_func_name);
>> +
>>  LIBBPF_API bool bpf_program__is_socket_filter(const struct 
>> bpf_program *prog);
>>  LIBBPF_API bool bpf_program__is_tracepoint(const struct bpf_program 
>> *prog);
>>  LIBBPF_API bool bpf_program__is_raw_tracepoint(const struct 
>> bpf_program *prog);
>> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
>> index b035122142bb..8aba5438a3f0 100644
>> --- a/tools/lib/bpf/libbpf.map
>> +++ b/tools/lib/bpf/libbpf.map
>> @@ -230,6 +230,7 @@ LIBBPF_0.0.7 {
>>                 bpf_program__name;
>>                 bpf_program__is_extension;
>>                 bpf_program__is_struct_ops;
>> +               bpf_program__set_attach_target;
>
> This will have to go into LIBBPF_0.0.8 once bpf-next opens. Please
> rebase and re-send then.

Will do…

>>                 bpf_program__set_extension;
>>                 bpf_program__set_struct_ops;
>>                 btf__align_of;
>>

Re: [PATCH bpf-next v2] libbpf: Add support for dynamic program attach target

[Andrii Nakryiko]

On Thu, Feb 13, 2020 at 11:34 PM Eelco Chaudron <echaudro@redhat.com> wrote:
>
>
>
> On 13 Feb 2020, at 18:42, Andrii Nakryiko wrote:
>
> > On Thu, Feb 13, 2020 at 7:05 AM Eelco Chaudron <echaudro@redhat.com>
> > wrote:
> >>
> >> Currently when you want to attach a trace program to a bpf program
> >> the section name needs to match the tracepoint/function semantics.
> >>
> >> However the addition of the bpf_program__set_attach_target() API
> >> allows you to specify the tracepoint/function dynamically.
> >>
> >> The call flow would look something like this:
> >>
> >>   xdp_fd = bpf_prog_get_fd_by_id(id);
> >>   trace_obj = bpf_object__open_file("func.o", NULL);
> >>   prog = bpf_object__find_program_by_title(trace_obj,
> >>                                            "fentry/myfunc");
> >>   bpf_program__set_expected_attach_type(prog, BPF_TRACE_FENTRY);
> >>   bpf_program__set_attach_target(prog, xdp_fd,
> >>                                  "xdpfilt_blk_all");
> >>   bpf_object__load(trace_obj)
> >>
> >> Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
> >> ---
> >
> > API-wise this looks good, thanks! Please address feedback below and
> > re-submit once bpf-next opens. Can you please also convert one of
> > existing selftests using open_opts's attach_prog_fd to use this API
> > instead to have a demonstration there?
>
> Yes will update the one I added for bfp2bpf testing…
>
> >> v1 -> v2: Remove requirement for attach type name in API
> >>
> >>  tools/lib/bpf/libbpf.c   |   33 +++++++++++++++++++++++++++++++--
> >>  tools/lib/bpf/libbpf.h   |    4 ++++
> >>  tools/lib/bpf/libbpf.map |    1 +
> >>  3 files changed, 36 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> >> index 514b1a524abb..9b8cab995580 100644
> >> --- a/tools/lib/bpf/libbpf.c
> >> +++ b/tools/lib/bpf/libbpf.c
> >> @@ -4939,8 +4939,8 @@ int bpf_program__load(struct bpf_program *prog,
> >> char *license, __u32 kern_ver)
> >>  {
> >>         int err = 0, fd, i, btf_id;
> >>
> >> -       if (prog->type == BPF_PROG_TYPE_TRACING ||
> >> -           prog->type == BPF_PROG_TYPE_EXT) {
> >> +       if ((prog->type == BPF_PROG_TYPE_TRACING ||
> >> +            prog->type == BPF_PROG_TYPE_EXT) &&
> >> !prog->attach_btf_id) {
> >>                 btf_id = libbpf_find_attach_btf_id(prog);
> >>                 if (btf_id <= 0)
> >>                         return btf_id;
> >> @@ -8132,6 +8132,35 @@ void bpf_program__bpil_offs_to_addr(struct
> >> bpf_prog_info_linear *info_linear)
> >>         }
> >>  }
> >>
> >> +int bpf_program__set_attach_target(struct bpf_program *prog,
> >> +                                  int attach_prog_fd,
> >> +                                  const char *attach_func_name)
> >> +{
> >> +       int btf_id;
> >> +
> >> +       if (!prog || attach_prog_fd < 0 || !attach_func_name)
> >> +               return -EINVAL;
> >> +
> >> +       if (attach_prog_fd)
> >> +               btf_id = libbpf_find_prog_btf_id(attach_func_name,
> >> +                                                attach_prog_fd);
> >> +       else
> >> +               btf_id =
> >> __find_vmlinux_btf_id(prog->obj->btf_vmlinux,
> >> +                                              attach_func_name,
> >> +
> >> prog->expected_attach_type);
> >> +
> >> +       if (btf_id <= 0) {
> >> +               if (!attach_prog_fd)
> >> +                       pr_warn("%s is not found in vmlinux BTF\n",
> >> +                               attach_func_name);
> >
> > libbpf_find_attach_btf_id's error reporting is misleading (it always
> > reports as if error happened with vmlinux BTF, even if attach_prog_fd
> > 0). Could you please fix that and add better error reporting here
> > for attach_prog_fd>0 case here?
> >
>
> I did not add log messages for the btf_id > 0 case as they are covered
> in the libbpf_find_prog_btf_id() function. Please let me know if this is
> not enough.

I see... libbpf_find_attach_btf_id is still wrong, so maybe let's move
this warning into __find_vmlinux_btf_id for more symmetrical (with
libbpf_find_prog_btf_id) error reporting?

>
> >> +               return btf_id;
> >> +       }
> >> +
> >> +       prog->attach_btf_id = btf_id;
> >> +       prog->attach_prog_fd = attach_prog_fd;
> >> +       return 0;
> >> +}
> >> +
> >>  int parse_cpu_mask_str(const char *s, bool **mask, int *mask_sz)
> >>  {
> >>         int err = 0, n, len, start, end = -1;
> >> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> >> index 3fe12c9d1f92..02fc58a21a7f 100644
> >> --- a/tools/lib/bpf/libbpf.h
> >> +++ b/tools/lib/bpf/libbpf.h
> >> @@ -334,6 +334,10 @@ LIBBPF_API void
> >>  bpf_program__set_expected_attach_type(struct bpf_program *prog,
> >>                                       enum bpf_attach_type type);
> >>
> >> +LIBBPF_API int
> >> +bpf_program__set_attach_target(struct bpf_program *prog, int
> >> attach_prog_fd,
> >> +                              const char *attach_func_name);
> >> +
> >>  LIBBPF_API bool bpf_program__is_socket_filter(const struct
> >> bpf_program *prog);
> >>  LIBBPF_API bool bpf_program__is_tracepoint(const struct bpf_program
> >> *prog);
> >>  LIBBPF_API bool bpf_program__is_raw_tracepoint(const struct
> >> bpf_program *prog);
> >> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> >> index b035122142bb..8aba5438a3f0 100644
> >> --- a/tools/lib/bpf/libbpf.map
> >> +++ b/tools/lib/bpf/libbpf.map
> >> @@ -230,6 +230,7 @@ LIBBPF_0.0.7 {
> >>                 bpf_program__name;
> >>                 bpf_program__is_extension;
> >>                 bpf_program__is_struct_ops;
> >> +               bpf_program__set_attach_target;
> >
> > This will have to go into LIBBPF_0.0.8 once bpf-next opens. Please
> > rebase and re-send then.
>
> Will do…
>
> >>                 bpf_program__set_extension;
> >>                 bpf_program__set_struct_ops;
> >>                 btf__align_of;
> >>
>