[DTrace-devel] [PATCH v3 1/2] Add support for the print() action

[Kris Van Hees]

On Wed, Nov 22, 2023 at 12:47:51PM +0000, Nick Alcock via DTrace-devel wrote:
> On 22 Nov 2023, Alan Maguire via DTrace-devel verbalised:

...

> > +	if (dt_regset_xalloc_args(drp) == -1)
> > +		longjmp(yypcb->pcb_jmpbuf, EDT_NOREG);
> > +	emit(dlp, BPF_MOV_REG(BPF_REG_3, addr->dn_reg));
> > +	dt_regset_free(drp, addr->dn_reg);
> > +	emit(dlp, BPF_MOV_REG(BPF_REG_1, BPF_REG_9));
> > +	emit(dlp, BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, data_off));
> > +	emit(dlp, BPF_MOV_IMM(BPF_REG_2, size));
> > +	dt_regset_xalloc(drp, BPF_REG_0);
> > +	emit(dlp, BPF_CALL_HELPER(BPF_FUNC_probe_read));
> > +	dt_regset_free_args(drp);
> > +	dt_regset_free(drp, BPF_REG_0);
> > +}
> 
> So this (obviously, sensibly) does no pointer chasing, so this is for
> printing a single contiguous block of bytes which happens to be a
> complex aggregate structure? Sounds sensible.

Interestingly, the Solaris version provides some pointer chasing, for strings.
That might be something we could add in a 2nd phase (or now if Alan is up for
it).  The approach they described in email exchanges involved placing the
strings after the value data block, in the order they appear.  Since the CTF
data of the entity being dumped is known at the time of compilation that is
definitely quite doable.  The question is whether it is desirable (or should
be under some optional argument control (a constant as a flag) or something,
because it could inflate the data being dumped quite a bit.