Messages in this thread | | | Subject | Re: [PATCH bpf-next 1/2] bpf: Add a bpf_kallsyms_lookup helper | From | Yonghong Song <> | Date | Sun, 13 Dec 2020 22:47:01 -0800 |
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On 12/11/20 6:40 AM, Florent Revest wrote: > On Wed, Dec 2, 2020 at 10:18 PM Alexei Starovoitov > <alexei.starovoitov@gmail.com> wrote: >> I still think that adopting printk/vsnprintf for this instead of >> reinventing the wheel >> is more flexible and easier to maintain long term. >> Almost the same layout can be done with vsnprintf >> with exception of \0 char. >> More meaningful names, etc. >> See Documentation/core-api/printk-formats.rst > > I agree this would be nice. I finally got a bit of time to experiment > with this and I noticed a few things: > > First of all, because helpers only have 5 arguments, if we use two for > the output buffer and its size and two for the format string and its > size, we are only left with one argument for a modifier. This is still > enough for our usecase (where we'd only use "%ps" for example) but it > does not strictly-speaking allow for the same layout that Andrii > proposed.
See helper bpf_seq_printf. It packs all arguments for format string and puts them into an array. bpf_seq_printf will unpack them as it parsed through the format string. So it should be doable to have more than "%ps" in format string.
> >> If we force fmt to come from readonly map then bpf_trace_printk()-like >> run-time check of fmt string can be moved into load time check >> and performance won't suffer. > > Regarding this bit, I have the impression that this would not be > possible, but maybe I'm missing something ? :) > > The iteration that bpf_trace_printk does over the format string > argument is not only used for validation. It is also used to remember > what extra operations need to be done based on the modifier types. For > example, it remembers whether an arg should be interpreted as 32bits or > 64bits. In the case of string printing, it also remembers whether it is > a kernel-space or user-space pointer so that bpf_trace_copy_string can > be called with the right arg. If we were to run the iteration over the format > string in the verifier, how would you recommend that we > "remember" the modifier type until the helper gets called ? >
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