"BTF" is a debug information format used by LLVM's BPF backend.
The format is much smaller in scope than DWARF, the following info is
available:
- full set of C types used in the binary file;
- types for global values;
- line number / line source code information .
BTF information is embedded in ELF as .BTF and .BTF.ext sections.
Detailed format description could be found as a part of Linux Source
tree, e.g. here: [1].
This commit modifies `llvm-objdump` utility to use line number
information provided by BTF if DWARF information is not available.
E.g., the goal is to make the following to print source code lines,
interleaved with disassembly:
$ clang --target=bpf -g test.c -o test.o
$ llvm-strip --strip-debug test.o
$ llvm-objdump -Sd test.o
test.o: file format elf64-bpf
Disassembly of section .text:
<foo>:
; void foo(void) {
r1 = 0x1
; consume(1);
call -0x1
r1 = 0x2
; consume(2);
call -0x1
; }
exit
A common production use case for BPF programs is to:
- compile separate object files using clang with `-g -c` flags;
- link these files as a final "static" binary using bpftool linker ([2]).
The bpftool linker discards most of the DWARF sections
(line information sections as well) but merges .BTF and .BTF.ext sections.
Hence, having `llvm-objdump` capable to print source code using .BTF.ext
is valuable.
The commit consists of the following modifications:
- llvm/lib/DebugInfo/BTF aka `DebugInfoBTF` component is added to host
the code needed to process BTF (with assumption that BTF support
would be added to some other tools as well, e.g. `llvm-readelf`):
- `DebugInfoBTF` provides `llvm::BTFParser` class, that loads information
from `.BTF` and `.BTF.ext` sections of a given `object::ObjectFile`
instance and allows to query this information.
Currently only line number information is loaded.
- `DebugInfoBTF` also provides `llvm::BTFContext` class, which is an
implementation of `DIContext` interface, used by `llvm-objdump` to
query information about line numbers corresponding to specific
instructions.
- Structure `DILineInfo` is modified with field `LineSource`.
`DIContext` interface uses `DILineInfo` structure to communicate
line number and source code information.
Specifically, `DILineInfo::Source` field encodes full file source code,
if available. BTF only stores source code for selected lines of the
file, not a complete source file. Moreover, stored lines are not
guaranteed to be sorted in a specific order.
To avoid reconstruction of a file source code from a set of
available lines, this commit adds `LineSource` field instead.
- `Symbolize` class is modified to use `BTFContext` instead of
`DWARFContext` when DWARF sections are not available but BTF
sections are present in the object file.
(`Symbolize` is instantiated by `llvm-objdump`).
- Integration and unit tests.
Note, that DWARF has a notion of "instruction sequence".
DWARF implementation of `DIContext::getLineInfoForAddress()` provides
inexact responses if exact address information is not available but
address falls within "instruction sequence" with some known line
information (see `DWARFDebugLine::LineTable::findRowInSeq()`).
BTF does not provide instruction sequence groupings, thus
`getLineInfoForAddress()` queries only return exact matches.
This does not seem to be a big issue in practice, but output
of the `llvm-objdump -Sd` might differ slightly when BTF
is used instead of DWARF.
[1] https://www.kernel.org/doc/html/latest/bpf/btf.html
[2] https://github.com/libbpf/bpftool
Depends on https://reviews.llvm.org/D149501
Reviewed By: MaskRay, yonghong-song, nickdesaulniers, #debug-info
Differential Revision: https://reviews.llvm.org/D149058