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authorQais Yousef <qais.yousef@arm.com>2021-01-19 15:22:36 +0300
committerAlexei Starovoitov <ast@kernel.org>2021-01-21 01:13:59 +0300
commit6939f4ef16d48f2093f337162cfc041d0e30ed25 (patch)
tree6ecd96bd4b33e542db6e479a76d26f013bc07fd2 /Documentation/bpf/bpf_design_QA.rst
parent86e6b4e993cf0c4dbe4c0ebfe052c89b9f9a2ade (diff)
downloadlinux-6939f4ef16d48f2093f337162cfc041d0e30ed25.tar.xz
trace: bpf: Allow bpf to attach to bare tracepoints
Some subsystems only have bare tracepoints (a tracepoint with no associated trace event) to avoid the problem of trace events being an ABI that can't be changed. >From bpf presepective, bare tracepoints are what it calls RAW_TRACEPOINT(). Since bpf assumed there's 1:1 mapping, it relied on hooking to DEFINE_EVENT() macro to create bpf mapping of the tracepoints. Since bare tracepoints use DECLARE_TRACE() to create the tracepoint, bpf had no knowledge about their existence. By teaching bpf_probe.h to parse DECLARE_TRACE() in a similar fashion to DEFINE_EVENT(), bpf can find and attach to the new raw tracepoints. Enabling that comes with the contract that changes to raw tracepoints don't constitute a regression if they break existing bpf programs. We need the ability to continue to morph and modify these raw tracepoints without worrying about any ABI. Update Documentation/bpf/bpf_design_QA.rst to document this contract. Signed-off-by: Qais Yousef <qais.yousef@arm.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Acked-by: Yonghong Song <yhs@fb.com> Link: https://lore.kernel.org/bpf/20210119122237.2426878-2-qais.yousef@arm.com
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diff --git a/Documentation/bpf/bpf_design_QA.rst b/Documentation/bpf/bpf_design_QA.rst
index 2df7b067ab93..0e15f9b05c9d 100644
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@@ -208,6 +208,12 @@ data structures and compile with kernel internal headers. Both of these
kernel internals are subject to change and can break with newer kernels
such that the program needs to be adapted accordingly.
+Q: Are tracepoints part of the stable ABI?
+------------------------------------------
+A: NO. Tracepoints are tied to internal implementation details hence they are
+subject to change and can break with newer kernels. BPF programs need to change
+accordingly when this happens.
+
Q: How much stack space a BPF program uses?
-------------------------------------------
A: Currently all program types are limited to 512 bytes of stack