Access to uprobe arguments by name #1994

viktormalik · 2021-09-07T11:30:23Z

Allows to reference uprobe arguments by name, with automatic type resolution:

# bpftrace -lv 'uprobe:tests/testprogs/uprobe_test:function1'
uprobe:tests/testprogs/uprobe_test:function1
    int* n
    char c

# bpftrace -e 'uprobe:tests/testprogs/uprobe_test:function1 { printf("%d\n", *(args->n)); }'
Attaching 1 probe...
13

Arguments are accessed via the args builtin (same as with tracepoints and kfuncs). This feature relies on DWARF to obtain argument names and types, so the traced binary must be compiled with DWARF.

Currently supports only basic types (integers, chars, enums) and pointers to them. Support for C structs will follow, but it'll require more extensive changes, so I'd prefer doing it in a separate PR.

Related issue #1742 (will close it once at least all C types are fully supported).

This PR also introduces unit testing for DWARF-based features.

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

fbs · 2021-09-10T20:05:19Z

tests/data/dwarf_data.h

+// }
+
+// generated by:
+//  % cat a.c


would be nice to have cmake do this

Good idea, I tried to do so in the last commit.

Would be nice if we could do the same for BTF data in tests/data/bft_data.h. I actually have it prepared, the problem is that it requires pahole version >=1.17 which is not available in our CI environment.

tests/dwarf_common.h

fbs · 2021-09-10T20:14:16Z

Nice! Need to find the time to dive into this and test it. But it looks good

Create a new function that generates LLVM code for reading registers from context (used for "argN", "retval", and "func") builtins.

Many variables and fields related to named probe arguments are now kfunc-specific. As they will be extended to other probe types in future, we must rename them accordingly. This commit does multiple things: - rename "kfarg" to "funcarg" - rename "btf_ap_args" to "ap_args" - introduce a new typedef ProbeArgs

Postpone resolution of kfunc arguments to the point when usage of "args" or "retval" is detected in the program. This way, the arguments are resolved only if they will be really needed. In addition, this makes the FieldAnalyser code simpler and prepares it for extension with uprobe argument resolution.

Missing some of the flags may cause unexpected and hard to discover errors.

src/ast/passes/semantic_analyser.cpp

src/dwarf_parser.h

danobi · 2021-09-30T22:33:34Z

tests/CMakeLists.txt

+add_custom_target(testing_debuginfo_data
+        COMMAND
+        ${CMAKE_COMMAND}
+        -DDATA_SOURCE_DIR="${CMAKE_SOURCE_DIR}/tests/data"
+        -P ${CMAKE_SOURCE_DIR}/tests/data/generate_debuginfo_data.cmake)


I'm not sure this is correct. Will this regen the final output if the data_source.c is changed?

Instead, how about using add_custom_command() with a shell script? The output file is the generated header. The input files are the shell script and data_source.c. And then the following line add_dependencies stays the same and I think it will "just work". Should be a littler easier to grok/program too.

I'm not sure this is correct. Will this regen the final output if the data_source.c is changed?

Yes, it will. CMake will handle that.

Instead, how about using add_custom_command() with a shell script? The output file is the generated header. The input files are the shell script and data_source.c. And then the following line add_dependencies stays the same and I think it will "just work". Should be a littler easier to grok/program too.

In fact, using add_custom_command() was my original approach, but I wasn't able to make it work. I just couldn't make CMake create the correct rule for the target file.

Using a shell script instead of the CMake script is possible, but CMake allows to use configure_file() which gives quite a nice control of the contents of the generated header.

Any chance this example might help? https://github.com/libbpf/libbpf-bootstrap/blob/67a29e511cc9d0a570d4d3b9797827f3a08ccdb5/tools/cmake/FindBpfObject.cmake#L163-L170

I'm ok to merge this as is too

The example suggests to use target_sources() to create the dependency between the target (bpftrace_test) and the generated header, but unfortunately this doesn't rebuild bpftrace_test when data_source.c is changed.

It's probably possible to make it work somehow, but likely it will be more complicated than the current approach.

If a binary has DWARF available, allow to access arguments by name. Uses "args" builtin for this (the same way as kfunc/tracepoin do). Supports only arguments of integer and pointer types for now.

Uses mocking of DWARF data similar to what we use for BTF - creating a temporary ELF file from a prepared byte array. To keep it minimal, the ELF contains only the DWARF sections. Currently used for testing access to uprobe arguments by name.

Unit tests for DWARF-based features rely on mocked DWARF data that is stored in tests/data/dwarf_data.h in the form of a byte array. Instead of creating the header manually, we now generate it by CMake. This requires the `xxd` tool - add it to Docker images.

viktormalik force-pushed the uprobe-named-args branch from 9be3cfc to bce952c Compare September 7, 2021 11:31

fbs reviewed Sep 10, 2021

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tests/dwarf_common.h Outdated Show resolved Hide resolved

viktormalik force-pushed the uprobe-named-args branch from bce952c to 992982e Compare September 22, 2021 13:20

viktormalik added 4 commits September 27, 2021 12:11

Factor out codegen for register reading

c44ea53

Create a new function that generates LLVM code for reading registers from context (used for "argN", "retval", and "func") builtins.

CMake: pass BPFTRACE_FLAGS to the ast subdir

3e900e1

Missing some of the flags may cause unexpected and hard to discover errors.

viktormalik force-pushed the uprobe-named-args branch from 992982e to f658772 Compare September 29, 2021 07:54

fbs approved these changes Sep 29, 2021

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danobi reviewed Sep 30, 2021

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viktormalik force-pushed the uprobe-named-args branch 2 times, most recently from db8a495 to 2b23724 Compare October 1, 2021 05:52

viktormalik added 2 commits October 1, 2021 08:10

uprobe: access arguments by name

e92b6d2

If a binary has DWARF available, allow to access arguments by name. Uses "args" builtin for this (the same way as kfunc/tracepoin do). Supports only arguments of integer and pointer types for now.

Unit tests for DWARF-based features

7a7de62

Uses mocking of DWARF data similar to what we use for BTF - creating a temporary ELF file from a prepared byte array. To keep it minimal, the ELF contains only the DWARF sections. Currently used for testing access to uprobe arguments by name.

viktormalik force-pushed the uprobe-named-args branch from 2b23724 to a366850 Compare October 1, 2021 06:26

fbs mentioned this pull request Oct 3, 2021

0.14 release #2026

Closed

viktormalik force-pushed the uprobe-named-args branch from a366850 to a47b9b2 Compare October 4, 2021 05:51

viktormalik force-pushed the uprobe-named-args branch from a47b9b2 to ea4e7fb Compare October 4, 2021 05:55

danobi approved these changes Oct 5, 2021

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viktormalik merged commit 3f8dd63 into bpftrace:master Oct 6, 2021

viktormalik deleted the uprobe-named-args branch November 24, 2021 12:28

viktormalik mentioned this pull request Mar 28, 2023

Support parsing bitfields from BTF/DWARF #2505

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Access to uprobe arguments by name #1994

Access to uprobe arguments by name #1994

viktormalik commented Sep 7, 2021 •

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fbs Sep 10, 2021

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fbs commented Sep 10, 2021

danobi Sep 30, 2021

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viktormalik Oct 4, 2021

Access to uprobe arguments by name #1994

Access to uprobe arguments by name #1994

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viktormalik commented Sep 7, 2021 • edited Loading

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