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Dmitrii Dolgov says: ==================== Relax tracing prog recursive attach rules Currently, it's not allowed to attach an fentry/fexit prog to another fentry/fexit. At the same time it's not uncommon to see a tracing program with lots of logic in use, and the attachment limitation prevents usage of fentry/fexit for performance analysis (e.g. with "bpftool prog profile" command) in this case. An example could be falcosecurity libs project that uses tp_btf tracing programs for offloading certain part of logic into tail-called programs, but the use-case is still generic enough -- a tracing program could be complicated and heavy enough to warrant its profiling, yet frustratingly it's not possible to do so use best tooling for that. Following the corresponding discussion [1], the reason for that is to avoid tracing progs call cycles without introducing more complex solutions. But currently it seems impossible to load and attach tracing programs in a way that will form such a cycle. Replace "no same type" requirement with verification that no more than one level of attachment nesting is allowed. In this way only one fentry/fexit program could be attached to another fentry/fexit to cover profiling use case, and still no cycle could be formed. The series contains a test for recursive attachment, as well as a fix + test for an issue in re-attachment branch of bpf_tracing_prog_attach. When preparing the test for the main change set, I've stumbled upon the possibility to construct a sequence of events when attach_btf would be NULL while computing a trampoline key. It doesn't look like this issue is triggered by the main change, because the reproduces doesn't actually need to have an fentry attachment chain. [1]: https://lore.kernel.org/bpf/20191108064039.2041889-16-ast@kernel.org/ ==================== Link: https://lore.kernel.org/r/20240103190559.14750-1-9erthalion6@gmail.com Signed-off-by: Alexei Starovoitov <ast@kernel.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.