Add InitGlobalVariable to support setting global variables in bpf code

[mozillazg]

closes #27

[grantseltzer]

I think this approach isn't quite correct. When I've used global variables like this in pure C libbpf, the variables are stored as key/value pairs in a map just called .rodata. They way you show your approach working in the selftest creates a new map for each variable with one entry in each map. Screenshot below is testing with a libbpf C native approach:

[mozillazg]

I think this approach isn't quite correct. When I've used global variables like this in pure C libbpf, the variables are stored as key/value pairs in a map just called .rodata. The way you show your approach working in the selftest creates a new map for each variable with one entry in each map.

@grantseltzer We can't init multiple global variables like the pure C way with skel->rodata->foobar due to can't update the internet map with a global variable name, so I use multiple internal maps to workaround. Do you know any better way to support multiple global variables?

[grantseltzer]

Is it possible to get a handle on the '.rodata' map and iterate over the keys or just set the values directly with this set initial value API function?

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On Tue, Oct 11, 2022 at 9:15 PM Huang Huang ***@***.***> wrote: I think this approach isn't quite correct. When I've used global variables like this in pure C libbpf, the variables are stored as key/value pairs in a map just called .rodata. The way you show your approach working in the selftest creates a new map for each variable with one entry in each map. @grantseltzer <https://github.com/grantseltzer> We can't init multiple global variables like the pure C way with skel->rodata->foobar due to can't update the internet map with a global variable name, so I use multiple internal maps to workaround. Do you know any better way to support multiple global variables? — Reply to this email directly, view it on GitHub <#258 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ACS2XBICYWFZ52REBC2WGDDWCYGLRANCNFSM6AAAAAAQ6LZCB4> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[grantseltzer]

@mozillazg I've opened mozillazg#2 with a POC of what I have in mind. It works with the selftest as is, but it fails if you change one of the data types to not match the others (bpf_map__set_initial_value returns errno 22 (invalid arg))

Any thoughts?

[mozillazg]

Is it possible to get a handle on the '.rodata' map and iterate over the
keys

We can't iterate the map before load bpf object. The '.rodata' map only has one element and the value don't have a well-known struct

I've opened mozillazg#2 with a POC of what I have in mind. It works with the selftest as is, but it fails if you change one of the data types to not match the others (bpf_map__set_initial_value returns errno 22 (invalid arg)). Any thoughts?

It looks like we can't use normal way to update the map value. Is it possible to update the memory of value?

# parse elf info
elf_info = parse_object("main.bpf.o")

# get global variable info
offset, size = get_global_variable_info(elf_info, "foobar")    # how ??

# get address of map value via bpf_map__initial_value
map_value_pointer = bpf_map__initial_value(...)

# update value of global variable via update memory 
update_global_variable(map_value_pointer, offset, &new_foo_value, size)  # how ??

# maybe need to put it back?
bpf_map__set_initial_value(...)

[grantseltzer]

The error in my approach appears to be an issue with struct memory alignment. If the types add up to a perfect alignment of 8 bytes it works, otherwise it gives the "invalid argument" error.

[mozillazg]

The error in my approach appears to be an issue with struct memory alignment. If the types add up to a perfect alignment of 8 bytes it works, otherwise it gives the "invalid argument" error.

@grantseltzer 👌

It looks like we can't use normal way to update the map value. Is it possible to update the memory of value?

# parse elf info
elf_info = parse_object("main.bpf.o")

# get global variable info
offset, size = get_global_variable_info(elf_info, "foobar")    # how ??

# get address of map value via bpf_map__initial_value
map_value_pointer = bpf_map__initial_value(...)

# update value of global variable via update memory 
update_global_variable(map_value_pointer, offset, &new_foo_value, size)  # how ??

# maybe need to put it back?
bpf_map__set_initial_value(...)

A POC of this way: mozillazg#3

[mozillazg]

@grantseltzer Which solution would you prefer?

[grantseltzer]

@mozillazg Your solution in mozillazg#3 works great! Nicely done, really enjoyed collaborating with you on this!

Only thing is I kindly ask you remove the print statements and get the selftest working (remove time.Sleep). Also my suggestion for the GoDoc comment about InitGlobalVariable to something like:

// InitGlobalVariable sets global variables (defined in .data or .rodata)
// in bpf code. It must be called before the BPF object is loaded.

[mozillazg]

@grantseltzer updated codes to use solution like mozillazg#3 . please review it again.

[grantseltzer]

Looks great, well done and thank you very much @mozillazg!