Support casting int <-> int array #2686

viktormalik · 2023-07-19T12:34:22Z

This PR adds support for conversions between integers and integer arrays. The main purpose is to allow conversion from/to byte arrays.

A good use-case for this are scripts working with IPv4 addresses which the kernel sometimes stores as 32-bit integers (e.g. in struct sock_common) and sometimes as 4-byte arrays (e.g. in TCP tracepoints). This feature, in combination with array comparison added in #2387, allows to compare such representations:

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)
    {
        ...
    }
}

Similarly, it is now possible to compare the int IPv4 addresses with absolute addresses obtained from the pton builtin:

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

Both the cast and the destination type must have the same size. When casting to an array, it is possible to omit the size which will be determined automatically from the size of the cast value.

This also does some refactoring of type parsing. See individual commit messages for more implementation details.

Resolves #2634.

Checklist

Language changes are updated in man/adoc/bpftrace.adoc and if needed in docs/reference_guide.md
User-visible and non-trivial changes updated in CHANGELOG.md
The new behaviour is covered by tests

viktormalik · 2023-07-19T13:03:08Z

src/ast/passes/semantic_analyser.cpp

+  if ((cast.type.IsIntTy() && !rhs.IsIntTy() && !rhs.IsPtrTy() &&
+       !rhs.IsCtxAccess() && !(rhs.IsArrayTy())) ||
+      // casting from/to int arrays must respect the size
+      (cast.type.IsArrayTy() &&
+       (!rhs.IsIntTy() || cast.type.GetSize() != rhs.GetSize())) ||
+      (rhs.IsArrayTy() &&
+       (!cast.type.IsIntTy() || cast.type.GetSize() != rhs.GetSize())))


I realize that this is a mess and wanted to refactor it but then found out that the cast checks will require some improvement, so I'd rather post it as a separate PR.

danobi · 2023-07-24T16:54:57Z

src/lexer.l

src/ast/passes/semantic_analyser.cpp

danobi · 2023-07-24T17:15:51Z

src/ast/passes/semantic_analyser.cpp

neat use of FLA semantics.

tests/semantic_analyser.cpp

src/ast/passes/codegen_llvm.cpp

docs/reference_guide.md

viktormalik · 2023-07-25T07:22:16Z

Thanks for the review @danobi! I addressed all the comments and added them as new commits to simplify reviewing. Will fixup the commits afterwards.

viktormalik · 2023-08-03T14:35:59Z

This is quite a big feature and could use another review.

@fbs, @ajor

ajor

Really nice - the code's a lot simpler than I'd expected!

tests/runtime/array

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).

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

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).

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).

Add examples of casts to both the Reference guide and manpage.

viktormalik requested review from ajor, danobi and fbs as code owners July 19, 2023 12:34

viktormalik force-pushed the bytearray-cast branch from 3c85d26 to 6142314 Compare July 19, 2023 12:36

github-advanced-security bot found potential problems Jul 19, 2023

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danobi reviewed Jul 24, 2023

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danobi approved these changes Jul 25, 2023

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viktormalik force-pushed the bytearray-cast branch from 9e4d84d to ca66281 Compare July 25, 2023 12:59

viktormalik force-pushed the bytearray-cast branch from ca66281 to 0d05f44 Compare August 3, 2023 14:35

ajor approved these changes Aug 18, 2023

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tests/runtime/array Outdated Show resolved Hide resolved

viktormalik added 6 commits August 23, 2023 08:38

Parser: refactor simple types

95f6b22

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).

Casting to arrays: auto-determine number of elems

2e5186e

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).

Document casting ints to int arrays

5fe8184

Add examples of casts to both the Reference guide and manpage.

Update CHANGELOG (int <-> int array casting)

388e7e3

viktormalik force-pushed the bytearray-cast branch from 0d05f44 to 388e7e3 Compare August 23, 2023 06:38

ajor merged commit f676cc8 into bpftrace:master Aug 25, 2023
21 checks passed

viktormalik deleted the bytearray-cast branch August 31, 2023 11:12

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Support casting int <-> int array #2686

Support casting int <-> int array #2686

viktormalik commented Jul 19, 2023

viktormalik Jul 19, 2023

danobi Jul 24, 2023

danobi Jul 24, 2023

viktormalik commented Jul 25, 2023

viktormalik commented Aug 3, 2023

ajor left a comment

Support casting int <-> int array #2686

Support casting int <-> int array #2686

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viktormalik commented Jul 19, 2023

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