[DTrace-devel] [PATCH] Do not allocate a 0-size strtab

[Kris Van Hees]

On Wed, Apr 08, 2020 at 01:35:55PM -0700, Eugene Loh wrote:
> On 04/08/2020 08:00 AM, Kris Van Hees wrote:
> 
> > If there is no stirng data to be stored in a DIFO, we were allocating
> stirng -> string
> 
> FWIW, if I look at the code, I typically see dtdo_strtab dereferenced 
> without any guards against the scenario dtdo_strtab==NULL.  I don't know 
> the code well enough to know whether to worry.  Should I?

If the memory allocation for dtdo_strtab failed, we bail by way of a fatal
error, which means nothing ever gets to use this.  So yes, it is safe.

> > a 0-size memory block.  There is no need for that - we can just assign
> > NULL to the dtdo_strtab member.
> >
> > In the case of the dt_link_stmt() code, we also happened to take the
> > dtdo_strlen value from the wrong DIFO (fdp instead of dp) causing a
> > 0-size strtab to be allocated into which we then try to write actual
> > data.  That caused memory corruption.
> >
> > Signed-off-by: Kris Van Hees <kris.van.hees at oracle.com>
> > ---
> >   libdtrace/dt_cc.c    | 14 +++++++++-----
> >   libdtrace/dt_dlibs.c | 23 +++++++++++++----------
> >   2 files changed, 22 insertions(+), 15 deletions(-)
> >
> > diff --git a/libdtrace/dt_cc.c b/libdtrace/dt_cc.c
> > index db009461..fc678692 100644
> > --- a/libdtrace/dt_cc.c
> > +++ b/libdtrace/dt_cc.c
> > @@ -2381,11 +2381,15 @@ dt_link_stmt(dtrace_hdl_t *dtp, dtrace_prog_t *pgp, dtrace_stmtdesc_t *sdp,
> >   	 * Write out the new string table.
> >   	 */
> >   	dp->dtdo_strlen = dt_strtab_size(stab);
> > -	dp->dtdo_strtab = dt_zalloc(dtp, fdp->dtdo_strlen);
> > -	if (dp->dtdo_strtab == NULL)
> > -		goto fail;
> > -	dt_strtab_write(stab, (dt_strtab_write_f *)dt_strtab_copystr,
> > -			dp->dtdo_strtab);
> > +	if (dp->dtdo_strlen > 0) {
> > +		dp->dtdo_strtab = dt_zalloc(dtp, dp->dtdo_strlen);
> > +		if (dp->dtdo_strtab == NULL)
> > +			goto fail;
> > +		dt_strtab_write(stab, (dt_strtab_write_f *)dt_strtab_copystr,
> > +				dp->dtdo_strtab);
> > +	} else
> > +		dp->dtdo_strtab = NULL;
> > +
> >   	dt_strtab_destroy(stab);
> >   
> >   	/*
> > diff --git a/libdtrace/dt_dlibs.c b/libdtrace/dt_dlibs.c
> > index 58e01221..2b8fe21d 100644
> > --- a/libdtrace/dt_dlibs.c
> > +++ b/libdtrace/dt_dlibs.c
> > @@ -554,7 +554,6 @@ done:
> >   	for (fid = fun0; fid < symc; fid++) {
> >   		dtrace_difo_t	*dp;
> >   		dof_relodesc_t	*brp;
> > -		size_t		slen;
> >   
> >   		fp = funcs[fid];
> >   		dp = fp->difo;
> > @@ -583,16 +582,20 @@ done:
> >   			rp++;
> >   		}
> >   
> > -		slen = dt_strtab_size(stab);
> > -		dp->dtdo_strtab = dt_alloc(dtp, slen);
> > -		dp->dtdo_strlen = slen;
> > -		if (dp->dtdo_strtab == NULL) {
> > -			fprintf(stderr, "Failed to alloc strtab\n");
> > -			goto out;
> > -		}
> > +		dp->dtdo_strlen = dt_strtab_size(stab);
> > +		if (dp->dtdo_strlen > 0) {
> > +			dp->dtdo_strtab = dt_alloc(dtp, dp->dtdo_strlen);
> > +			if (dp->dtdo_strtab == NULL) {
> > +				fprintf(stderr, "Failed to alloc strtab\n");
> > +				goto out;
> > +			}
> > +
> > +			dt_strtab_write(stab,
> > +					(dt_strtab_write_f *)dt_strtab_copystr,
> > +					dp->dtdo_strtab);
> > +		} else
> > +			dp->dtdo_strtab = NULL;
> >   
> > -		dt_strtab_write(stab, (dt_strtab_write_f *)dt_strtab_copystr,
> > -				dp->dtdo_strtab);
> >   		dt_strtab_destroy(stab);
> >   	}
> >   
> 
> 
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