[PATCH v2] accel/tcg/user-exec: Fix read-modify-write of code on s390 hosts

[Ilya Leoshkevich]

x86_64 dotnet/runtime uses cmpxchg for code patching. When running it
under s390x qemu-linux user, cpu_signal_handler() does not recognize
this as a write and does not restore PAGE_WRITE cleared by
tb_page_add(), incorrectly forwarding the signal to the guest code.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---

v1: https://lists.nongnu.org/archive/html/qemu-devel/2021-08/msg00464.html
v1 -> v2: Fix comment style, fix CSST detection (Richard).

 accel/tcg/user-exec.c | 48 ++++++++++++++++++++++++++++++++++++-------
 1 file changed, 41 insertions(+), 7 deletions(-)

diff --git a/accel/tcg/user-exec.c b/accel/tcg/user-exec.c
index 90d1a2d327..8fed542622 100644
--- a/accel/tcg/user-exec.c
+++ b/accel/tcg/user-exec.c
@@ -680,18 +680,26 @@ int cpu_signal_handler(int host_signum, void *pinfo,
 
     pc = uc->uc_mcontext.psw.addr;
 
-    /* ??? On linux, the non-rt signal handler has 4 (!) arguments instead
-       of the normal 2 arguments.  The 3rd argument contains the "int_code"
-       from the hardware which does in fact contain the is_write value.
-       The rt signal handler, as far as I can tell, does not give this value
-       at all.  Not that we could get to it from here even if it were.  */
-    /* ??? This is not even close to complete, since it ignores all
-       of the read-modify-write instructions.  */
+    /*
+     * ??? On linux, the non-rt signal handler has 4 (!) arguments instead
+     * of the normal 2 arguments.  The 4th argument contains the "Translation-
+     * Exception Identification for DAT Exceptions" from the hardware (aka
+     * "int_parm_long"), which does in fact contain the is_write value.
+     * The rt signal handler, as far as I can tell, does not give this value
+     * at all.  Not that we could get to it from here even if it were.
+     * So fall back to parsing instructions.  Treat read-modify-write ones as
+     * writes, which is not fully correct, but for tracking self-modifying code
+     * this is better than treating them as reads.  Checking si_addr page flags
+     * might be a viable improvement, albeit a racy one.
+     */
+    /* ??? This is not even close to complete.  */
     pinsn = (uint16_t *)pc;
     switch (pinsn[0] >> 8) {
     case 0x50: /* ST */
     case 0x42: /* STC */
     case 0x40: /* STH */
+    case 0xba: /* CS */
+    case 0xbb: /* CDS */
         is_write = 1;
         break;
     case 0xc4: /* RIL format insns */
@@ -702,6 +710,12 @@ int cpu_signal_handler(int host_signum, void *pinfo,
             is_write = 1;
         }
         break;
+    case 0xc8: /* SSF format insns */
+        switch (pinsn[0] & 0xf) {
+        case 0x2: /* CSST */
+            is_write = 1;
+        }
+        break;
     case 0xe3: /* RXY format insns */
         switch (pinsn[2] & 0xff) {
         case 0x50: /* STY */
@@ -715,7 +729,27 @@ int cpu_signal_handler(int host_signum, void *pinfo,
             is_write = 1;
         }
         break;
+    case 0xeb: /* RSY format insns */
+        switch (pinsn[2] & 0xff) {
+        case 0x14: /* CSY */
+        case 0x30: /* CSG */
+        case 0x31: /* CDSY */
+        case 0x3e: /* CDSG */
+        case 0xe4: /* LANG */
+        case 0xe6: /* LAOG */
+        case 0xe7: /* LAXG */
+        case 0xe8: /* LAAG */
+        case 0xea: /* LAALG */
+        case 0xf4: /* LAN */
+        case 0xf6: /* LAO */
+        case 0xf7: /* LAX */
+        case 0xfa: /* LAAL */
+        case 0xf8: /* LAA */
+            is_write = 1;
+        }
+        break;
     }
+
     return handle_cpu_signal(pc, info, is_write, &uc->uc_sigmask);
 }
 
-- 
2.31.1