Re: [PATCH v2 bpf-next 2/6] libbpf: rename static variables during linking

On Fri, Apr 23, 2021 at 4:35 PM Andrii Nakryiko
[off-list ref] wrote:

On Fri, Apr 23, 2021 at 4:06 PM Alexei Starovoitov
[off-list ref] wrote:

quoted

On Fri, Apr 23, 2021 at 2:56 PM Yonghong Song [off-list ref] wrote:

quoted

quoted

quoted

quoted

-static volatile const __u32 print_len;
-static volatile const __u32 ret1;
+volatile const __u32 print_len = 0;
+volatile const __u32 ret1 = 0;

I am little bit puzzled why bpf_iter_test_kern4.c is impacted. I think
this is not in a static link test, right? The same for a few tests below.

All the selftests are passed through a static linker, so it will
append obj_name to each static variable. So I just minimized use of
static variables to avoid too much code churn. If this variable was
static, it would have to be accessed as
skel->rodata->bpf_iter_test_kern4__print_len, for example.

Okay this should be fine. selftests/bpf specific. I just feel that
some people may get confused if they write/see a single program in
selftest and they have to use obj_varname format and thinking this
is a new standard, but actually it is due to static linking buried
in Makefile. Maybe add a note in selftests/README.rst so we
can point to people if there is confusion.

I'm not sure I understand.
Are you saying that
bpftool gen object out_file.o in_file.o
is no longer equivalent to llvm-strip ?
Since during that step static vars will get their names mangled?

Yes. Static vars and static maps. We don't allow (yet?) static
entry-point BPF programs, so those don't change.

quoted

So a good chunk of code that uses skeleton right now should either
1. don't do the linking step
or
2. adjust their code to use global vars
or
3. adjust the usage of skel.h in their corresponding user code
  to accommodate mangled static names?
Did it get it right?

Yes, you are right. But so far most cases outside of selftest that
I've seen don't use static variables (partially because they need
pesky volatile to be visible from user-space at all), global vars are
much nicer in that regard.

Right.
but wait...
why linker is mangling them at all and why they appear in the skeleton?
static vars without volatile should not be in a skeleton, since changing
them from user space might have no meaning on the bpf program.
The behavior of the bpf prog is unpredictable.
Only volatile static can theoretically be in the skeleton, but as you said
probably no one is using them yet, so we can omit them from skeleton too.