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
