Re: [PATCH v4 1/4] x86/ibt: factor out cfi and fineibt offset

Menglong Dong
2025-03-05T08:51:25Z

On Wed, Mar 5, 2025 at 4:30 PM H. Peter Anvin <hpa@zytor.com> wrote:
>
> On March 4, 2025 5:19:09 PM PST, Menglong Dong <menglong8.dong@gmail.com> wrote:
> >On Tue, Mar 4, 2025 at 10:53 PM H. Peter Anvin <hpa@zytor.com> wrote:
> >>
> >> On March 4, 2025 1:42:20 AM PST, Peter Zijlstra <peterz@infradead.org> wrote:
> >> >On Tue, Mar 04, 2025 at 03:47:45PM +0800, Menglong Dong wrote:
> >> >> We don't have to select FUNCTION_ALIGNMENT_32B, so the
> >> >> worst case is to increase ~2.2%.
> >> >>
> >> >> What do you think?
> >> >
> >> >Well, since I don't understand what you need this for at all, I'm firmly
> >> >on the side of not doing this.
> >> >
> >> >What actual problem is being solved with this meta data nonsense? Why is
> >> >it worth blowing up our I$ footprint over.
> >> >
> >> >Also note, that if you're going to be explaining this, start from
> >> >scratch, as I have absolutely 0 clues about BPF and such.
> >>
> >> I would appreciate such information as well. The idea seems dubious on the surface.
> >
> >Ok, let me explain it from the beginning. (My English is not good,
> >but I'll try to describe it as clear as possible :/)
> >
> >Many BPF program types need to depend on the BPF trampoline,
> >such as BPF_PROG_TYPE_TRACING, BPF_PROG_TYPE_EXT,
> >BPF_PROG_TYPE_LSM, etc. BPF trampoline is a bridge between
> >the kernel (or bpf) function and BPF program, and it acts just like the
> >trampoline that ftrace uses.
> >
> >Generally speaking, it is used to hook a function, just like what ftrace
> >do:
> >
> >foo:
> > endbr
> > nop5 --> call trampoline_foo
> > xxxx
> >
> >In short, the trampoline_foo can be this:
> >
> >trampoline_foo:
> > prepare a array and store the args of foo to the array
> > call fentry_bpf1
> > call fentry_bpf2
> > ......
> > call foo+4 (origin call)
> > save the return value of foo
> > call fexit_bpf1 (this bpf can get the return value of foo)
> > call fexit_bpf2
> > .......
> > return to the caller of foo
> >
> >We can see that the trampoline_foo can be only used for
> >the function foo, as different kernel function can be attached
> >different BPF programs, and have different argument count,
> >etc. Therefore, we have to create 1000 BPF trampolines if
> >we want to attach a BPF program to 1000 kernel functions.
> >
> >The creation of the BPF trampoline is expensive. According to
> >my testing, It will spend more than 1 second to create 100 bpf
> >trampoline. What's more, it consumes more memory.
> >
> >If we have the per-function metadata supporting, then we can
> >create a global BPF trampoline, like this:
> >
> >trampoline_global:
> > prepare a array and store the args of foo to the array
> > get the metadata by the ip
> > call metadata.fentry_bpf1
> > call metadata.fentry_bpf2
> > ....
> > call foo+4 (origin call)
> > save the return value of foo
> > call metadata.fexit_bpf1 (this bpf can get the return value of foo)
> > call metadata.fexit_bpf2
> > .......
> > return to the caller of foo
> >
> >(The metadata holds more information for the global trampoline than
> >I described.)
> >
> >Then, we don't need to create a trampoline for every kernel function
> >anymore.
> >
> >Another beneficiary can be ftrace. For now, all the kernel functions that
> >are enabled by dynamic ftrace will be added to a filter hash if there are
> >more than one callbacks. And hash lookup will happen when the traced
> >functions are called, which has an impact on the performance, see
> >__ftrace_ops_list_func() -> ftrace_ops_test(). With the per-function
> >metadata supporting, we can store the information that if the callback is
> >enabled on the kernel function to the metadata, which can make the performance
> >much better.
> >
> >The per-function metadata storage is a basic function, and I think there
> >may be other functions that can use it for better performance in the feature
> >too.
> >
> >(Hope that I'm describing it clearly :/)
> >
> >Thanks!
> >Menglong Dong
> >
>
> This is way too cursory. For one thing, you need to start by explaining why you are asking to put this *inline* with the code, which is something that normally would be avoided at all cost.

Hi,

Sorry that I don't understand the *inline* here, do you mean
why puting the metadata in the function padding?