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

[Kris Van Hees]

On Wed, Apr 08, 2020 at 01:53:58PM -0700, Eugene Loh wrote:
> On 04/08/2020 01:40 PM, Kris Van Hees wrote:
> 
> > 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.
> 
> But what I mean is that if the table has 0 size, we don't even try to 
> allocate the table... we just set a NULL pointer, which is a different 
> situation from an allocation failing and indicating an error.  Again, I 
> just don't know if this means there can be a problem.  Maybe nothing to 
> worry about:  if a problem turns up, it'll make itself known!

Ah ok.  Sorry, you confused me because you wrote "If the memory allocation for
dtdo_strtab failed" so I focused on the allocation failure case.

If we have no strings, dtdo_strlen will be 0, dtdo_strtab will be NULL, and
that is safe because when there are no strings, there is nothing in the DIFO
that will reference dtdo_strtab.  If anything referenced a string it would be
an entry to either the variable table or one of the relocation tables.  And
in that case those strings would have been added, and thus dtdo_strlen would
not be zero.

> >>> 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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> 
> 
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