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Added kernel recompile guidance for libbpf CO-RE #2876

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merged 3 commits into from
Apr 19, 2020
Merged

Added kernel recompile guidance for libbpf CO-RE #2876

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teroz Apr 17, 2020
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18 changes: 17 additions & 1 deletion libbpf-tools/README.md
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Expand Up @@ -46,7 +46,7 @@ For more reproducible builds, vmlinux.h header file is pre-generated and
checked in along the other sources. This is done to avoid dependency on
specific user/build server's kernel configuration, because vmlinux.h
generation depends on having a kernel with BTF type information built-in
(which is enabled by `CONFIG_DEBUG_INFO_BTF=y` Kconfig option).
(which is enabled by `CONFIG_DEBUG_INFO_BTF=y` Kconfig option See below).

vmlinux.h is generated from upstream Linux version at particular minor
version tag. E.g., `vmlinux_505.h` is generated from v5.5 tag. Exact set of
Expand All @@ -73,3 +73,19 @@ Given bpftool package can't yet be expected to be available widely across many
distributions, bpftool binary is checked in into BCC repository in bin/
subdirectory. Once bpftool package is more widely available, this can be
changed in favor of using pre-packaged version of bpftool.


Re-compiling your Kernel with CONFIG_DEBUG_INFO_BTF=y
-----------------------------------------------------
libbpf probes to see if your sys fs exports the file `/sys/kernel/btf/vmlinux` (from Kernel 5.5+) or if you have the ELF version in your system [`code`](https://github.com/libbpf/libbpf/blob/master/src/btf.c)
Please note the ELF file could exist without the BTF info in it. Your Kconfig should contain the options below

1. Compile options
```code
CONFIG_DEBUG_INFO_BTF=y
CONFIG_DEBUG_INFO=y
```
2. Also, make sure that you have pahole 1.13 (or preferably 1.16+) during the
kernel build (it comes from dwarves package). Without it, BTF won't be
generated, and on older kernels you'd get only warning, but still would
build kernel successfully