builtin: describe_command - fix incorrect path

Hi,

On 26/05/17 09:04, Youfu Zhang wrote:
> $ PATH=/extra/path:/usr/sbin:/usr/bin:/sbin:/bin \
>> sh -xc 'command -V ls; command -V ls; command -Vp ls; command -vp ls'
> + command -V ls
> ls is /bin/ls
> + command -V ls
> ls is a tracked alias for /bin/ls
> + command -Vp ls
> ls is a tracked alias for (null)
> + command -vp ls
> Segmentation fault (core dumped)
>
> describe_command should respect `path' argument. Looking up in the hash table
> may gives incorrect index in entry.u.index and finally causes incorrect output
> or SIGSEGV.

True, but only when a path is passed in. If the default path is used,
looking up in the hash table is correct, and printing tracked aliases is
intentional.

If it's desirable to drop that feature, then it should be dropped
completely, code shouldn't be left in that can no longer be used. But
it's possible to keep it working: how about this instead?

Signed-off-by: Harald van Dijk <harald@gigawatt.nl>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

src/exec.c

@@ -743,8 +743,6 @@ describe_command(out, command, path, verbose)
 	struct tblentry *cmdp;
 	const struct alias *ap;
-	path = path ?: pathval();
-
 	if (verbose) {
 		outstr(command, out);
 	}
@@ -767,8 +765,17 @@ describe_command(out, command, path, verbose)
 		goto out;
 	}
-	/* Then check if it is a tracked alias */
-	if ((cmdp = cmdlookup(command, 0)) != NULL) {
+	/* Then if the standard search path is used, check if it is
+	 * a tracked alias.
+	 */
+	if (path == NULL) {
+		path = pathval();
+		cmdp = cmdlookup(command, 0);
+	} else {
+		cmdp = NULL;
+	}
+
+	if (cmdp != NULL) {
 		entry.cmdtype = cmdp->cmdtype;
 		entry.u = cmdp->param;
 	} else {