xfarray: warn against sorting arrays with identical elements

LOLLM complains about a potential underflow here if xfarray_qsort_push
is called with lo==0.  However, this isn't possible in most cases
because filesystem metadata records cannot be identical.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Assisted-by: LOLLM # finding obvious bugs
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

fs/xfs/scrub/xfarray.c

@@ -682,6 +682,18 @@ xfarray_qsort_push(
 		return -EFSCORRUPTED;
 	}
 
+	/*
+	 * Avoid the integer underflow below in (lo - 1).  This shouldn't
+	 * be possible because the pivot is the median of nine distinct
+	 * filesystem metadata records, so at least four records will be less
+	 * than the pivot, which means the pivot will not be in the low end of
+	 * the range by the time we get here.
+	 */
+	if (lo == 0) {
+		ASSERT(lo != 0);
+		return -EFSCORRUPTED;
+	}
+
 	si->max_stack_used = max_t(uint8_t, si->max_stack_used,
 					    si->stack_depth + 2);