[PATCH v2 3/4] s390x/tcg: Only ignore content in r0 when specified via "b" or "x"

[David Hildenbrand <david@redhat.com>]

Using get_address() with register identifiers comming from an "r" field
is wrong: if the "r" field designates "r0", we don't read the content
and instead assume 0 - which should only be applied when the register
was specified via "b" or "x".

PoP 5-11 "Operand-Address Generation":
  "A zero in any of the B1, B2, X2, B3, or B4 fields indicates the absence
   of the corresponding address component. For the absent component, a zero
   is used in forming the intermediate sum, regardless of the contents of
   general register 0. A displacement of zero has no special significance."

This BUG became visible for CSPG as generated by LLVM-12 in the upstream
Linux kernel (v5.11-rc2), used while creating the linear mapping in
vmem_map_init(): Trying to store to address 0 results in a Low Address
Protection exception.

Debugging this was more complicated than it could have been: The program
interrupt handler in the kernel will try to crash the kernel: doing so, it
will enable DAT. As the linear mapping is not created yet (asce=0), we run
into an addressing exception while tring to walk non-existant DAT tables,
resulting in a program exception loop.

This allows for booting upstream Linux kernels compiled by clang-12. Most
of these cases seem to be broken forever.

Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 target/s390x/insn-data.def |  8 ++++----
 target/s390x/translate.c   | 15 +++++++++------
 2 files changed, 13 insertions(+), 10 deletions(-)

diff --git a/target/s390x/insn-data.def b/target/s390x/insn-data.def
index eac5136ee5..e5b6efabf3 100644
--- a/target/s390x/insn-data.def
+++ b/target/s390x/insn-data.def
@@ -1290,8 +1290,8 @@
     F(0xe313, LRAY,    RXY_a, LD,  0, a2, r1, 0, lra, 0, IF_PRIV)
     F(0xe303, LRAG,    RXY_a, Z,   0, a2, r1, 0, lra, 0, IF_PRIV)
 /* LOAD USING REAL ADDRESS */
-    E(0xb24b, LURA,    RRE,   Z,   0, 0, new, r1_32, lura, 0, MO_TEUL, IF_PRIV)
-    E(0xb905, LURAG,   RRE,   Z,   0, 0, r1, 0, lura, 0, MO_TEQ, IF_PRIV)
+    E(0xb24b, LURA,    RRE,   Z,   0, ra2, new, r1_32, lura, 0, MO_TEUL, IF_PRIV)
+    E(0xb905, LURAG,   RRE,   Z,   0, ra2, r1, 0, lura, 0, MO_TEQ, IF_PRIV)
 /* MOVE TO PRIMARY */
     F(0xda00, MVCP,    SS_d,  Z,   la1, a2, 0, 0, mvcp, 0, IF_PRIV)
 /* MOVE TO SECONDARY */
@@ -1344,8 +1344,8 @@
 /* STORE THEN OR SYSTEM MASK */
     F(0xad00, STOSM,   SI,    Z,   la1, 0, 0, 0, stnosm, 0, IF_PRIV)
 /* STORE USING REAL ADDRESS */
-    E(0xb246, STURA,   RRE,   Z,   r1_o, 0, 0, 0, stura, 0, MO_TEUL, IF_PRIV)
-    E(0xb925, STURG,   RRE,   Z,   r1_o, 0, 0, 0, stura, 0, MO_TEQ, IF_PRIV)
+    E(0xb246, STURA,   RRE,   Z,   r1_o, ra2, 0, 0, stura, 0, MO_TEUL, IF_PRIV)
+    E(0xb925, STURG,   RRE,   Z,   r1_o, ra2, 0, 0, stura, 0, MO_TEQ, IF_PRIV)
 /* TEST BLOCK */
     F(0xb22c, TB,      RRE,   Z,   0, r2_o, 0, 0, testblock, 0, IF_PRIV)
 /* TEST PROTECTION */
diff --git a/target/s390x/translate.c b/target/s390x/translate.c
index 39e33eeb67..61dd0341e4 100644
--- a/target/s390x/translate.c
+++ b/target/s390x/translate.c
@@ -3285,8 +3285,7 @@ static DisasJumpType op_lpq(DisasContext *s, DisasOps *o)
 #ifndef CONFIG_USER_ONLY
 static DisasJumpType op_lura(DisasContext *s, DisasOps *o)
 {
-    o->addr1 = get_address(s, 0, get_field(s, r2), 0);
-    tcg_gen_qemu_ld_tl(o->out, o->addr1, MMU_REAL_IDX, s->insn->data);
+    tcg_gen_qemu_ld_tl(o->out, o->in2, MMU_REAL_IDX, s->insn->data);
     return DISAS_NEXT;
 }
 #endif
@@ -4234,7 +4233,8 @@ static DisasJumpType op_ectg(DisasContext *s, DisasOps *o)
     tcg_gen_addi_i64(o->in1, regs[b1], d1);
     o->in2 = tcg_temp_new_i64();
     tcg_gen_addi_i64(o->in2, regs[b2], d2);
-    o->addr1 = get_address(s, 0, r3, 0);
+    o->addr1 = tcg_temp_new_i64();
+    gen_addi_and_wrap_i64(s, o->addr1, regs[r3], 0);
 
     /* load the third operand into r3 before modifying anything */
     tcg_gen_qemu_ld64(regs[r3], o->addr1, get_mem_index(s));
@@ -4539,8 +4539,7 @@ static DisasJumpType op_stnosm(DisasContext *s, DisasOps *o)
 
 static DisasJumpType op_stura(DisasContext *s, DisasOps *o)
 {
-    o->addr1 = get_address(s, 0, get_field(s, r2), 0);
-    tcg_gen_qemu_st_tl(o->in1, o->addr1, MMU_REAL_IDX, s->insn->data);
+    tcg_gen_qemu_st_tl(o->in1, o->in2, MMU_REAL_IDX, s->insn->data);
 
     if (s->base.tb->flags & FLAG_MASK_PER) {
         update_psw_addr(s);
@@ -5923,7 +5922,11 @@ static void in2_x2l(DisasContext *s, DisasOps *o)
 
 static void in2_ra2(DisasContext *s, DisasOps *o)
 {
-    o->in2 = get_address(s, 0, get_field(s, r2), 0);
+    int r2 = get_field(s, r2);
+
+    /* Note: *don't* treat !r2 as 0, use the reg value. */
+    o->in2 = tcg_temp_new_i64();
+    gen_addi_and_wrap_i64(s, o->in2, regs[r2], 0);
 }
 #define SPEC_in2_ra2 0
 
-- 
2.29.2