Don't reference out-of-bounds array elements in brin_minmax_multi.c

The primary fix here is to fix has_matching_range() so it does not
reference ranges->values[-1] when nranges == 0.  Similar problems existed
in AssertCheckRanges() too.  It does not look like any of these problems
could lead to a crash as the array in question is at the end of the Ranges
struct, and values[-1] is memory that belongs to other fields in the
struct.  However, let's get rid of these rather unsafe coding practices.

In passing, I (David) adjusted some comments to try to make it more clear
what some of the fields are for in the Ranges struct.  I had to study the
code to find out what nsorted was for as I couldn't tell from the
comments.

Author: Ranier Vilela
Discussion: https://postgr.es/m/CAEudQAqJQzPitufX-jR=YUbJafpCDAKUnwgdbX_MzSc93wuvdw@mail.gmail.com
Backpatch-through: 14, where multi-range brin was added.

diff --git a/src/backend/access/brin/brin_minmax_multi.c b/src/backend/access/brin/brin_minmax_multi.c
index 4415669719f1ad332e1f6ad8f30fbc3b958b0b90..dbad7764316d09141a63ba7b5b20759f53ba1cfb 100644 (file)
--- a/src/backend/access/brin/brin_minmax_multi.c
+++ b/src/backend/access/brin/brin_minmax_multi.c
@@ -142,19 +142,23 @@ typedef struct MinMaxMultiOptions
  * The Ranges struct stores the boundary values in a single array, but we
  * treat regular and single-point ranges differently to save space. For
  * regular ranges (with different boundary values) we have to store both
- * values, while for "single-point ranges" we only need to save one value.
+ * the lower and upper bound of the range, while for "single-point ranges"
+ * we only need to store a single value.
  *
  * The 'values' array stores boundary values for regular ranges first (there
  * are 2*nranges values to store), and then the nvalues boundary values for
  * single-point ranges. That is, we have (2*nranges + nvalues) boundary
  * values in the array.
  *
- * +---------------------------------+-------------------------------+
- * | ranges (sorted pairs of values) | sorted values (single points) |
- * +---------------------------------+-------------------------------+
+ * +-------------------------+----------------------------------+
+ * | ranges (2 * nranges of) | single point values (nvalues of) |
+ * +-------------------------+----------------------------------+
  *
  * This allows us to quickly add new values, and store outliers without
- * making the other ranges very wide.
+ * having to widen any of the existing range values.
+ *
+ * 'nsorted' denotes how many of 'nvalues' in the values[] array are sorted.
+ * When nsorted == nvalues, all single point values are sorted.
  *
  * We never store more than maxvalues values (as set by values_per_range
  * reloption). If needed we merge some of the ranges.
@@ -173,10 +177,10 @@ typedef struct Ranges
    FmgrInfo   *cmp;
 
    /* (2*nranges + nvalues) <= maxvalues */
-   int         nranges;        /* number of ranges in the array (stored) */
-   int         nsorted;        /* number of sorted values (ranges + points) */
-   int         nvalues;        /* number of values in the data array (all) */
-   int         maxvalues;      /* maximum number of values (reloption) */
+   int         nranges;        /* number of ranges in the values[] array */
+   int         nsorted;        /* number of nvalues which are sorted */
+   int         nvalues;        /* number of point values in values[] array */
+   int         maxvalues;      /* number of elements in the values[] array */
 
    /*
     * We simply add the values into a large buffer, without any expensive
@@ -318,102 +322,99 @@ AssertCheckRanges(Ranges *ranges, FmgrInfo *cmpFn, Oid colloid)
     * Check that none of the values are not covered by ranges (both sorted
     * and unsorted)
     */
-   for (i = 0; i < ranges->nvalues; i++)
+   if (ranges->nranges > 0)
    {
-       Datum       compar;
-       int         start,
-                   end;
-       Datum       minvalue,
-                   maxvalue;
-
-       Datum       value = ranges->values[2 * ranges->nranges + i];
-
-       if (ranges->nranges == 0)
-           break;
-
-       minvalue = ranges->values[0];
-       maxvalue = ranges->values[2 * ranges->nranges - 1];
-
-       /*
-        * Is the value smaller than the minval? If yes, we'll recurse to the
-        * left side of range array.
-        */
-       compar = FunctionCall2Coll(cmpFn, colloid, value, minvalue);
-
-       /* smaller than the smallest value in the first range */
-       if (DatumGetBool(compar))
-           continue;
-
-       /*
-        * Is the value greater than the maxval? If yes, we'll recurse to the
-        * right side of range array.
-        */
-       compar = FunctionCall2Coll(cmpFn, colloid, maxvalue, value);
-
-       /* larger than the largest value in the last range */
-       if (DatumGetBool(compar))
-           continue;
-
-       start = 0;              /* first range */
-       end = ranges->nranges - 1;  /* last range */
-       while (true)
+       for (i = 0; i < ranges->nvalues; i++)
        {
-           int         midpoint = (start + end) / 2;
-
-           /* this means we ran out of ranges in the last step */
-           if (start > end)
-               break;
+           Datum       compar;
+           int         start,
+                       end;
+           Datum       minvalue = ranges->values[0];
+           Datum       maxvalue = ranges->values[2 * ranges->nranges - 1];
+           Datum       value = ranges->values[2 * ranges->nranges + i];
 
-           /* copy the min/max values from the ranges */
-           minvalue = ranges->values[2 * midpoint];
-           maxvalue = ranges->values[2 * midpoint + 1];
+           compar = FunctionCall2Coll(cmpFn, colloid, value, minvalue);
 
            /*
-            * Is the value smaller than the minval? If yes, we'll recurse to
-            * the left side of range array.
+            * If the value is smaller than the lower bound in the first range
+            * then it cannot possibly be in any of the ranges.
             */
-           compar = FunctionCall2Coll(cmpFn, colloid, value, minvalue);
-
-           /* smaller than the smallest value in this range */
            if (DatumGetBool(compar))
-           {
-               end = (midpoint - 1);
                continue;
-           }
 
-           /*
-            * Is the value greater than the minval? If yes, we'll recurse to
-            * the right side of range array.
-            */
            compar = FunctionCall2Coll(cmpFn, colloid, maxvalue, value);
 
-           /* larger than the largest value in this range */
+           /*
+            * Likewise, if the value is larger than the upper bound of the
+            * final range, then it cannot possibly be inside any of the
+            * ranges.
+            */
            if (DatumGetBool(compar))
-           {
-               start = (midpoint + 1);
                continue;
-           }
 
-           /* hey, we found a matching range */
-           Assert(false);
+           /* bsearch the ranges to see if 'value' fits within any of them */
+           start = 0;          /* first range */
+           end = ranges->nranges - 1;  /* last range */
+           while (true)
+           {
+               int         midpoint = (start + end) / 2;
+
+               /* this means we ran out of ranges in the last step */
+               if (start > end)
+                   break;
+
+               /* copy the min/max values from the ranges */
+               minvalue = ranges->values[2 * midpoint];
+               maxvalue = ranges->values[2 * midpoint + 1];
+
+               /*
+                * Is the value smaller than the minval? If yes, we'll recurse
+                * to the left side of range array.
+                */
+               compar = FunctionCall2Coll(cmpFn, colloid, value, minvalue);
+
+               /* smaller than the smallest value in this range */
+               if (DatumGetBool(compar))
+               {
+                   end = (midpoint - 1);
+                   continue;
+               }
+
+               /*
+                * Is the value greater than the minval? If yes, we'll recurse
+                * to the right side of range array.
+                */
+               compar = FunctionCall2Coll(cmpFn, colloid, maxvalue, value);
+
+               /* larger than the largest value in this range */
+               if (DatumGetBool(compar))
+               {
+                   start = (midpoint + 1);
+                   continue;
+               }
+
+               /* hey, we found a matching range */
+               Assert(false);
+           }
        }
    }
 
-   /* and values in the unsorted part must not be in sorted part */
-   for (i = ranges->nsorted; i < ranges->nvalues; i++)
+   /* and values in the unsorted part must not be in the sorted part */
+   if (ranges->nsorted > 0)
    {
        compare_context cxt;
-       Datum       value = ranges->values[2 * ranges->nranges + i];
-
-       if (ranges->nsorted == 0)
-           break;
 
        cxt.colloid = ranges->colloid;
        cxt.cmpFn = ranges->cmp;
 
-       Assert(bsearch_arg(&value, &ranges->values[2 * ranges->nranges],
-                          ranges->nsorted, sizeof(Datum),
-                          compare_values, (void *) &cxt) == NULL);
+       for (i = ranges->nsorted; i < ranges->nvalues; i++)
+       {
+           Datum       value = ranges->values[2 * ranges->nranges + i];
+
+           Assert(bsearch_arg(&value, &ranges->values[2 * ranges->nranges],
+                              ranges->nsorted, sizeof(Datum),
+                              compare_values, (void *) &cxt) == NULL);
+       }
    }
 #endif
 }
@@ -923,8 +924,8 @@ has_matching_range(BrinDesc *bdesc, Oid colloid, Ranges *ranges,
 {
    Datum       compar;
 
-   Datum       minvalue = ranges->values[0];
-   Datum       maxvalue = ranges->values[2 * ranges->nranges - 1];
+   Datum       minvalue;
+   Datum       maxvalue;
 
    FmgrInfo   *cmpLessFn;
    FmgrInfo   *cmpGreaterFn;
@@ -936,6 +937,9 @@ has_matching_range(BrinDesc *bdesc, Oid colloid, Ranges *ranges,
    if (ranges->nranges == 0)
        return false;
 
+   minvalue = ranges->values[0];
+   maxvalue = ranges->values[2 * ranges->nranges - 1];
+
    /*
     * Otherwise, need to compare the new value with boundaries of all the
     * ranges. First check if it's less than the absolute minimum, which is