doc: bpf_get_current_task helper

[pchaigno]

Add an entry for bpf_get_current_task in the reference guide.

[goldshtn]

It would be nice to specify what you can get from the task_struct. Also, wasn’t there some issue with field randomization in recent kernel versions?

[yonghong-song]

wasn’t there some issue with field randomization in recent kernel versions?

4.13 has issues and you will need to put the below before your header files:

#define randomized_struct_fields_start  struct {
#define randomized_struct_fields_end    };

Other kernel versions should be okay.

[pchaigno]

It would be nice to specify what you can get from the task_struct.

@goldshtn I've added a sentence with a few examples. If you have other more relevant examples, please tell me!

4.13 has issues and you will need to put the below before your header files:

Since the randomized_struct_fields_xxx directives are already defined in linux/compiler.h, which itself is included by linux/sched.h, do we really need to make a note about it? It looks like a very specific corner case (?)

[yonghong-song]

At linux:include/linux directory, for 4.13,

-bash-4.2$ egrep -r randomized_struct_fields_start                                                                                   
compiler-gcc.h:#define randomized_struct_fields_start   struct {
compiler.h:#ifndef randomized_struct_fields_start
compiler.h:# define randomized_struct_fields_start
sched.h:        randomized_struct_fields_start
-bash-4.2$

4.15

-bash-4.2$ egrep -r randomized_struct_fields_start
compiler-clang.h:#define randomized_struct_fields_start struct {
compiler-gcc.h:#define randomized_struct_fields_start   struct {
sched.h:        randomized_struct_fields_start
compiler_types.h:#ifndef randomized_struct_fields_start
compiler_types.h:# define randomized_struct_fields_start
-bash-4.2$ 

for clang, randomized_struct_fields_start is not defined in 4.13, it is defined in 4.14 stable and 4.15. So people use task_struct with 4.13 in their bpf programs may have issues. It would be good to make a note of this, so we can refer to it when people have issues.

[pchaigno]

I see. I though it was only necessary when the kernel headers and the kernel used differed because I couldn't reproduce with any bcc tool under 4.13. I'll add a note.

[pchaigno]

Done.

[yonghong-song]

LGTM. Thanks!