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SubjectRe: [PATCH 3/3] perf tools: Rework prologue generation code
On Wed, Feb 23, 2022 at 02:29:56PM -0800, Andrii Nakryiko wrote:

SNIP

> > and R3 is loaded in the prologue code (first 15 instructions)
> > and it also sets 'err' (R2) with the result of the reading:
> >
> > 0: (bf) r6 = r1
> > 1: (79) r3 = *(u64 *)(r6 +96)
> > 2: (bf) r7 = r10
> > 3: (07) r7 += -8
> > 4: (7b) *(u64 *)(r10 -8) = r3
> > 5: (b7) r2 = 8
> > 6: (bf) r1 = r7
> > 7: (85) call bpf_probe_read_user#-60848
> > 8: (55) if r0 != 0x0 goto pc+2
> > 9: (61) r3 = *(u32 *)(r10 -8)
> > 10: (05) goto pc+3
> > 11: (b7) r2 = 1
> > 12: (b7) r3 = 0
> > 13: (05) goto pc+1
> > 14: (b7) r2 = 0
> > 15: (bf) r1 = r6
> >
> > 16: (b7) r1 = 100
> > 17: (6b) *(u16 *)(r10 -8) = r1
> > 18: (18) r1 = 0x6c25203a6f697270
> > 20: (7b) *(u64 *)(r10 -16) = r1
> > 21: (bf) r1 = r10
> > 22: (07) r1 += -16
> > 23: (b7) r2 = 10
> > 24: (85) call bpf_trace_printk#-54848
> > 25: (b7) r0 = 1
> > 26: (95) exit
> >
> >
> > I'm still scratching my head how to workaround this.. we do want maps
> > and all the other updates to the code, but verifier won't let it pass
> > without the prologue code
>
> ugh, perf cornered itself into supporting this crazy scheme and now

well, it just used the interface that was provided at the time

> there is no good solution. I'm still questioning the value of
> supporting this going forward. Is there an evidence that anyone is
> using this functionality at all? Is it worth it trying to carry it on
> just because we have some example that exercises this feature?

yea we discussed this again and I think we can somehow mark this
feature in perf as deprecated and remove it after some time,
because even with the workaround below it'll be pita ;-)

or people will come and scream and we will find some other solution

I already sent the rest of the changes (prog/map priv) separately
and will send some RFC for the deprecation

thanks,
jirka

>
> Anyways, one way to solve this is to add bpf_program__set_insns() that
> could be called from prog_init_fn callback (which I just realized
> hasn't landed yet, I'll send v4 today) to prepend a simple preamble
> like this:
>
> r1 = 0;
> r2 = 0;
> r3 = 0;
> f4 = 0;
> r5 = 0; /* how many input arguments we support? */
>
> This will make all input arguments initialized, libbpf will be able to
> adjust all the relocations and stuff. Once this "prototype program" is
> loaded, perf can grab final instructions and replace first X
> instructions with desired preamble.
>
> But... ugliness and horror, yeah :(
>
>
> >
> > jirka

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