[PATCH v2] target/i386: Use assert() to sanity-check b1 in SSE decode

[Peter Maydell <peter.maydell@linaro.org>]

In the SSE decode function gen_sse(), we combine a byte
'b' and a value 'b1' which can be [0..3], and switch on them:
   b |= (b1 << 8);
   switch (b) {
   ...
   default:
   unknown_op:
       gen_unknown_opcode(env, s);
       return;
   }

In three cases inside this switch, we were then also checking for
 "if (b1 >= 2) { goto unknown_op; }".
However, this can never happen, because the 'case' values in each place
are 0x0nn or 0x1nn and the switch will have directed the b1 == (2, 3)
cases to the default already.

This check was added in commit c045af25a52e9 in 2010; the added code
was unnecessary then as well, and was apparently intended only to
ensure that we never accidentally ended up indexing off the end
of an sse_op_table with only 2 entries as a result of future bugs
in the decode logic.

Change the checks to assert() instead, and make sure they're always
immediately before the array access they are protecting.

Fixes: Coverity CID 1460207
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
v1->v2: use assert() rather than just deleting the if()s

 target/i386/tcg/translate.c | 12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

diff --git a/target/i386/tcg/translate.c b/target/i386/tcg/translate.c
index aacb605eee4..a4fee5e445d 100644
--- a/target/i386/tcg/translate.c
+++ b/target/i386/tcg/translate.c
@@ -3521,9 +3521,6 @@ static void gen_sse(CPUX86State *env, DisasContext *s, int b,
         case 0x171: /* shift xmm, im */
         case 0x172:
         case 0x173:
-            if (b1 >= 2) {
-                goto unknown_op;
-            }
             val = x86_ldub_code(env, s);
             if (is_xmm) {
                 tcg_gen_movi_tl(s->T0, val);
@@ -3542,6 +3539,7 @@ static void gen_sse(CPUX86State *env, DisasContext *s, int b,
                                 offsetof(CPUX86State, mmx_t0.MMX_L(1)));
                 op1_offset = offsetof(CPUX86State,mmx_t0);
             }
+            assert(b1 < 2);
             sse_fn_epp = sse_op_table2[((b - 1) & 3) * 8 +
                                        (((modrm >> 3)) & 7)][b1];
             if (!sse_fn_epp) {
@@ -3772,10 +3770,8 @@ static void gen_sse(CPUX86State *env, DisasContext *s, int b,
             rm = modrm & 7;
             reg = ((modrm >> 3) & 7) | REX_R(s);
             mod = (modrm >> 6) & 3;
-            if (b1 >= 2) {
-                goto unknown_op;
-            }
 
+            assert(b1 < 2);
             sse_fn_epp = sse_op_table6[b].op[b1];
             if (!sse_fn_epp) {
                 goto unknown_op;
@@ -4202,10 +4198,8 @@ static void gen_sse(CPUX86State *env, DisasContext *s, int b,
             rm = modrm & 7;
             reg = ((modrm >> 3) & 7) | REX_R(s);
             mod = (modrm >> 6) & 3;
-            if (b1 >= 2) {
-                goto unknown_op;
-            }
 
+            assert(b1 < 2);
             sse_fn_eppi = sse_op_table7[b].op[b1];
             if (!sse_fn_eppi) {
                 goto unknown_op;
-- 
2.20.1