Re: [PATCH man v4] bpf.2: various updates/follow-ups to address some fixmes

["Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Hi Daniel,

[...]
>>>>> When a jump into the new program has been performed,
>>>>> +it won't return to the old one anymore.
>>>>> +
>>>>> +If no eBPF program is found at the given index of the program array,
>>>>
>>>> I find this language a little unclear. The array does not contain eBPF
>>>> programs, but rather file descriptors. So, what does "not found" here
>>>> really mean? (I added a FIXME.)
>>>
>>> Ok, it's a bit more complicated to explain I guess. So from user space
>>> side, a lookup on that map type is not allowed. When user space adds a
>>> new file descriptor into the prog map, the kernel internally translates
>>> that to the actual program holds reference, etc. The tail call helper is
>>> mapped into a eBPF instruction, so no real helper call here. That instruction
>>> will then have a register setting as if we'd have a function call, so it
>>> has the lookup key and uses it directly to find the array slot. From
>>> there, it has access to the actual underlying program. "Not found" means
>>> conditions mentioned as earlier above.
>>
>> Okay I've changed that paragraph to now read:
>>
>>                If no eBPF program is found at the given index of the pro‐
>>                gram  array  (because the map slot doesn't contain a valid
>>                program file descriptor, the specified lookup index/key is
>>                out  of  bounds,  or the limit of 32 nested calls has been
>>                exceed) execution continues with the current eBPF program.
>>                This can be used as a fall-through for default cases.
>>
>> Okay?
> 
> Works for me, thanks!

Thanks for checking.

> I'll drop you some more patches on remaining items, at the very latest after
> LinuxCon NA & Plumbers.

Thanks. The page is already looking a lot better than the version that went
out with the last man-pages release.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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