target/mips: Avoid shift by negative number in page_table_walk_refill()

Coverity points out that in page_table_walk_refill() we can
shift by a negative number, which is undefined behaviour
(CID 1452918, 1452920, 1452922).  We already catch the
negative directory_shift and leaf_shift as being a "bail
out early" case, but not until we've already used them to
calculated some offset values.

The shifts can be negative only if ptew > 1, so make the
bail-out-early check look directly at that, and only
calculate the shift amounts and the offsets based on them
after we have done that check. This allows
us to simplify the expressions used to calculate the
shift amounts, use an unsigned type, and avoids the
undefined behaviour.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
[PMD: Check for ptew > 1, use unsigned type]
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20230717213504.24777-3-philmd@linaro.org>

target/mips/tcg/sysemu/tlb_helper.c

@@ -624,7 +624,7 @@ static uint64_t get_tlb_entry_layout(CPUMIPSState *env, uint64_t entry,
 static int walk_directory(CPUMIPSState *env, uint64_t *vaddr,
         int directory_index, bool *huge_page, bool *hgpg_directory_hit,
         uint64_t *pw_entrylo0, uint64_t *pw_entrylo1,
-        int directory_shift, int leaf_shift)
+        unsigned directory_shift, unsigned leaf_shift)
 {
     int dph = (env->CP0_PWCtl >> CP0PC_DPH) & 0x1;
     int psn = (env->CP0_PWCtl >> CP0PC_PSN) & 0x3F;
@@ -730,21 +730,11 @@ static bool page_table_walk_refill(CPUMIPSState *env, vaddr address,
 
     /* Other HTW configs */
     int hugepg = (env->CP0_PWCtl >> CP0PC_HUGEPG) & 0x1;
-
-    /* HTW Shift values (depend on entry size) */
-    int directory_shift = (ptew > 1) ? -1 :
-            (hugepg && (ptew == 1)) ? native_shift + 1 : native_shift;
-    int leaf_shift = (ptew > 1) ? -1 :
-            (ptew == 1) ? native_shift + 1 : native_shift;
+    unsigned directory_shift, leaf_shift;
 
     /* Offsets into tables */
-    int goffset = gindex << directory_shift;
-    int uoffset = uindex << directory_shift;
-    int moffset = mindex << directory_shift;
-    int ptoffset0 = (ptindex >> 1) << (leaf_shift + 1);
-    int ptoffset1 = ptoffset0 | (1 << (leaf_shift));
-
-    uint32_t leafentry_size = 1 << (leaf_shift + 3);
+    unsigned goffset, uoffset, moffset, ptoffset0, ptoffset1;
+    uint32_t leafentry_size;
 
     /* Starting address - Page Table Base */
     uint64_t vaddr = env->CP0_PWBase;
@@ -766,10 +756,22 @@ static bool page_table_walk_refill(CPUMIPSState *env, vaddr address,
         /* no structure to walk */
         return false;
     }
-    if ((directory_shift == -1) || (leaf_shift == -1)) {
+    if (ptew > 1) {
         return false;
     }
 
+    /* HTW Shift values (depend on entry size) */
+    directory_shift = (hugepg && (ptew == 1)) ? native_shift + 1 : native_shift;
+    leaf_shift = (ptew == 1) ? native_shift + 1 : native_shift;
+
+    goffset = gindex << directory_shift;
+    uoffset = uindex << directory_shift;
+    moffset = mindex << directory_shift;
+    ptoffset0 = (ptindex >> 1) << (leaf_shift + 1);
+    ptoffset1 = ptoffset0 | (1 << (leaf_shift));
+
+    leafentry_size = 1 << (leaf_shift + 3);
+
     /* Global Directory */
     if (gdw > 0) {
         vaddr |= goffset;