Fixup generated BTF for older kernels #2934
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@xiaoqing-wei @the-prachi do you think you'd be able to test this fix on your setup? Unfortunately, we don't build static binaries for PRs but you could build it yourself by running I was able to test this on my local reproducer of #2913 but would like to have a confirmation that it fixes the issue. |
Older kernels missing [1] (most importantly the stable 5.4) do not support BTF function entries with global linkage (BTF_FUNC_GLOBAL). This causes them to reject our generated BTF for probes and prevents from running any bpftrace program. In order to fix this, the generated BTF must be patched by replacing the BTF_FUNC_GLOBAL flag by 0. This is normally done by libbpf [2] but only when program loading via bpf_object is used. Since we're still using direct loading using bpf_btf_load and bpf_prog_load, we need to mimic libbpf's behavior and do the patching manually. Note that we cannot fix this directly in codegen b/c LLVM would optimize all our probes away. The BTF fixup itself is pretty simple but required to copy a portion of code from libbpf internals. The reason is that libbpf doesn't export functions for iterating BTF in a non-const way. This also requires to feature-detect support for BTF_FUNC_GLOBAL, so we extend BPFfeature with BTF feature detection. [1] torvalds/linux@51c39bb1d5d1 [2] torvalds/linux@2d3eb67f64ec
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I was able to reproduce the issue on 5.4.266 kernel and confirm that this fix works. So, I'll go ahead and merge this as it's blocking me on other work. |
I saw there's a new tagged build and it's working for me. https://github.com/iovisor/bpftrace/releases/download/v0.20.0/binary_tools_man-bundle.tar.xz |
Older kernels missing torvalds/linux@51c39bb1d5d1 (most importantly the stable 5.4) do not support BTF function entries with global linkage (
BTF_FUNC_GLOBAL). This causes them to reject our generated BTF for probes and prevents from running any bpftrace program.In order to fix this, the generated BTF must be patched by replacing the
BTF_FUNC_GLOBALflag by 0. This is normally done by libbpf (torvalds/linux@2d3eb67f64ec) but only when program loading via bpf_object is used. Since we're still using direct loading usingbpf_btf_loadandbpf_prog_load, we need to mimic libbpf's behavior and do the patching manually. Note that we cannot fix this directly in codegen b/c LLVM would optimize all our probes away.The BTF fixup itself is pretty simple but required to copy a portion of code from libbpf internals. The reason is that libbpf doesn't export functions for iterating BTF in a non-const way.
This also requires to feature-detect support for
BTF_FUNC_GLOBAL, so we extendBPFfeaturewith BTF feature detection.Fixes #2913.
Checklist
man/adoc/bpftrace.adocand if needed indocs/reference_guide.mdCHANGELOG.md