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Support casting int <-> int array #2686

Merged
merged 6 commits into from
Aug 25, 2023
Merged

Support casting int <-> int array #2686

merged 6 commits into from
Aug 25, 2023
+381 −27

Commits on Aug 23, 2023

  1. Parser: refactor simple types 

    Split simple types into integer types and builtin types. This is useful
as it will allow special treatment of integer types and their usage in
constructions where builtin types don't make sense (e.g. arrays).
    @viktormalik
    viktormalik committed Aug 23, 2023
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  2. Support casting integers to integer arrays 

    The syntax is the same as for other casts:

    $array = (int8[8])42;

For now, it is only possible to cast integers to integer arrays. Both
the cast value and the target array must have the same size.

The integer being cast is copied to the BPF stack and the pointer to it
is then reinterpreted as the target array pointer.

This, in combination with recently added array comparison, allows to
compare values which are stored under different types in kernel.
E.g., IP addresses are represented with 32-bit int in "struct sock" [1]
but with byte array in TCP tracepoints [2]. This feature allows to
compare them:

    kfunc:tcp_connect
    {
        @ = args->sk->__sk_common.skc_daddr;
    }
    tracepoint:tcp:tcp_rcv_space_adjust
    {
        // map values are always 64-bit so first cast to uint32
        // and then to the target array
        if ((uint8[4])(uint32)@ == args->daddr)
        {
            ...
        }
    }

[1] https://elixir.bootlin.com/linux/v6.4/source/include/net/sock.h#L167
[2] https://elixir.bootlin.com/linux/v6.4/source/include/trace/events/tcp.h#L64
    @viktormalik
    viktormalik committed Aug 23, 2023
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  3. Casting to arrays: auto-determine number of elems 

    Support omitting the array size when casting to arrays as it can be
computed automatically in semantic analyser.

E.g. for `(int8[])42`, the array will have 8 elements (since ints are by
default 64-bit).
    @viktormalik
    viktormalik committed Aug 23, 2023
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  4. Support casting integer arrays to integers 

    It is only possible to cast the entire array to an integer of the same
size. The main purpose is to allow conversion of byte arrays such as
those returned by the 'pton' builtin:

    kfunc:tcp_connect
    {
        if (args->sk->__sk_common.skc_daddr == (uint32)pton("127.0.0.1"))
            ...
    }

Since arrays are internally stored as pointers, the codegen for this
just reinterprets the pointer to the expected integer size and loads the
data as an integer (by a direct load or by an additional proberead).
    @viktormalik
    viktormalik committed Aug 23, 2023
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  5. Document casting ints to int arrays 

    Add examples of casts to both the Reference guide and manpage.
    @viktormalik
    viktormalik committed Aug 23, 2023
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  6. Update CHANGELOG (int <-> int array casting)

    @viktormalik
    viktormalik committed Aug 23, 2023
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