2018-04-09 11:01 UTC+0200 ~ Daniel Borkmann <dan...@iogearbox.net> > On 04/06/2018 01:11 PM, Quentin Monnet wrote: >> eBPF helper functions can be called from within eBPF programs to perform >> a variety of tasks that would be otherwise hard or impossible to do with >> eBPF itself. There is a growing number of such helper functions in the >> kernel, but documentation is scarce. The main user space header file >> does contain a short commented description of most helpers, but it is >> somewhat outdated and not complete. It is more a "cheat sheet" than a >> real documentation accessible to new eBPF developers. >> >> This commit attempts to improve the situation by replacing the existing >> overview for the helpers with a more developed description. Furthermore, >> a Python script is added to generate a manual page for eBPF helpers. The >> workflow is the following, and requires the rst2man utility: >> >> $ ./scripts/bpf_helpers_doc.py \ >> --filename include/uapi/linux/bpf.h > /tmp/bpf-helpers.rst >> $ rst2man /tmp/bpf-helpers.rst > /tmp/bpf-helpers.7 >> $ man /tmp/bpf-helpers.7 >> >> The objective is to keep all documentation related to the helpers in a >> single place, and to be able to generate from here a manual page that >> could be packaged in the man-pages repository and shipped with most >> distributions . >> >> Additionally, parsing the prototypes of the helper functions could >> hopefully be reused, with a different Printer object, to generate >> header files needed in some eBPF-related projects. >> >> Regarding the description of each helper, it comprises several items: >> >> - The function prototype. >> - A description of the function and of its arguments (except for a >> couple of cases, when there are no arguments and the return value >> makes the function usage really obvious). >> - A description of return values (if not void). >> - A listing of eBPF program types (if relevant, map types) compatible >> with the helper. >> - Information about the helper being restricted to GPL programs, or not. >> - The kernel version in which the helper was introduced. >> - The commit that introduced the helper (this is mostly to have it in >> the source of the man page, as it can be used to track changes and >> update the page). >> >> For several helpers, descriptions are inspired (at times, nearly copied) >> from the commit logs introducing them in the kernel--Many thanks to >> their respective authors! They were completed as much as possible, the >> objective being to have something easily accessible even for people just >> starting with eBPF. There is probably a bit more work to do in this >> direction for some helpers. >> >> Some RST formatting is used in the descriptions (not in function >> prototypes, to keep them readable, but the Python script provided in >> order to generate the RST for the manual page does add formatting to >> prototypes, to produce something pretty) to get "bold" and "italics" in >> manual pages. Hopefully, the descriptions in bpf.h file remains >> perfectly readable. Note that the few trailing white spaces are >> intentional, removing them would break paragraphs for rst2man. >> >> The descriptions should ideally be updated each time someone adds a new >> helper, or updates the behaviour (compatibility extended to new program >> types, new socket option supported...) or the interface (new flags >> available, ...) of existing ones. >> >>  I have not contacted people from the man-pages project prior to >> sending this RFC, so I can offer no guaranty at this time that they >> would accept to take the generated man page. >> >> Cc: linux-...@vger.kernel.org >> Cc: linux-...@vger.kernel.org >> Signed-off-by: Quentin Monnet <quentin.mon...@netronome.com> > > Great work, thanks a lot for doing this! > > [...] >> + * int bpf_probe_read(void *dst, u32 size, const void *src) >> + * Description >> + * For tracing programs, safely attempt to read *size* bytes from >> + * address *src* and store the data in *dst*. >> + * Return >> + * 0 on success, or a negative error in case of failure. >> + * For >> + * *Tracing programs*. >> + * GPL only >> + * Yes >> + * Since >> + * Linux 4.1 >> + * >> + * .. commit 2541517c32be >> * >> * u64 bpf_ktime_get_ns(void) >> - * Return: current ktime >> - * >> - * int bpf_trace_printk(const char *fmt, int fmt_size, ...) >> - * Return: length of buffer written or negative error >> + * Description >> + * Return the time elapsed since system boot, in nanoseconds. >> + * Return >> + * Current *ktime*. >> + * For >> + * All program types, except >> + * **BPF_PROG_TYPE_CGROUP_DEVICE**. > > I think we should probably always just list the actual program types instead, > since when new types get added to the kernel not supporting bpf_ktime_get_ns() > helper in this example, the above statement would be misleading (potentially > more misleading than the other way around when it's not yet mentioned to be > supported).
Agreed. I realise “All program types” is really awkward here. >> + * GPL only >> + * Yes >> + * Since >> + * Linux 4.1 >> + * >> + * .. commit d9847d310ab4 >> + * [...] >> + * u32 bpf_get_smp_processor_id(void) >> + * Return >> + * The SMP (Symmetric multiprocessing) processor id. >> + * For >> + * *Networking programs*. > > Ditto plus above is actually not correct. See tracing_func_proto(): > > [...] > case BPF_FUNC_get_smp_processor_id: > return &bpf_get_smp_processor_id_proto; > [...] Thanks for the catch! I will fix on my side, even if we drop the "For" section for now. >> + * GPL only >> + * No >> + * Since >> + * Linux 4.1 >> + * >> + * .. commit c04167ce2ca0 >> + * >> + * int bpf_skb_store_bytes(struct sk_buff *skb, u32 offset, const void >> *from, u32 len, u64 flags) >> + * Description >> + * Store *len* bytes from address *from* into the packet >> + * associated to *skb*, at *offset*. *flags* are a combination of >> + * **BPF_F_RECOMPUTE_CSUM** (automatically recompute the >> + * checksum for the packet after storing the bytes) and >> + * **BPF_F_INVALIDATE_HASH** (set *skb*\ **->hash**, *skb*\ >> + * **->swhash** and *skb*\ **->l4hash** to 0). >> + * >> + * A call to this helper is susceptible to change data from the >> + * packet. Therefore, at load time, all checks on pointers >> + * previously done by the verifier are invalidated and must be >> + * performed again. >> + * Return > [...] > > Agree with Alexei that 'Description' but also 'Return' is the most useful > (the latter we can still improve a bit wrt errors). 'For' gets indeed quickly > out of hand but I do think that for BPF program developers 'Since' has good > value to quickly check when a helper could be available, but then again > this is actually tied to an individual kconfig and/or program type, and could > potentially require to add or (worst case) remove the info in a second commit > e.g. after the merge window similar with the commit sha. Probably best indeed > to stick to the signature, 'Description' and 'Return' initially. I understand that the "For" section will be the hardest to maintain up-to-date, however, I believe that--along with minimal kernel version and GPL information--it would be useful for readers (Alexei, what is your case against the "GPL only" section?). Daniel, your proposal for the "Since" information in a second time sounds good! But I do not have a solution right now for maintaining the "For" section on the long term. Anyway, I am fine with keeping just signatures, descriptions and return values for now. I will submit a new version with only those items. > We should probably also have the bpf_helpers_doc.py workflow in a separate > comment above this one such that developers don't have to look up the original > commit message, but have the required steps to render the rst/man page > available > right where they need it. Sure! I was considering adding some comments on top of the description, but haven't done it for the RFC. I will do for the next version. Thanks for the reviews! Quentin