Re: Invalid ARM instruction for clang-compiled Android code

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

On Fri, 15 Nov 2019 at 05:03, Michael Goffioul
<michael.goffioul@gmail.com> wrote:
> When running QEMU user mode on some code compiled by clang (dynamic linker from AOSP-10), the emulator chokes on this instruction:
>
>    9aa92:       e8c0 2277       strexd  r7, r2, r2, [r0]

I think that ought to be a valid insn...

> From debugging, I determined that op_strex() calls unallocated_encoding(), which I think leads to the SIGILL signal generated.
>
> I run the emulator without specifying the ARM cpu type, I think it then defaults to "any", which should support all instructions, if I'm not mistaken.
>
> Is this instruction really invalid? Or am I doing something wrong?

Which version of QEMU are you using? (Looking at the code I
suspect we still have this bug in master, but it's always
useful to specify what version you're using in a bug report.)

Richard, I think we're tripping over the check you added
in commit af2882289951e. Specifically:

+    /* We UNDEF for these UNPREDICTABLE cases.  */
+    if (a->rd == 15 || a->rn == 15 || a->rt == 15
+        || a->rd == a->rn || a->rd == a->rt
+        || (s->thumb && (a->rd == 13 || a->rt == 13))
+        || (mop == MO_64
+            && (a->rt2 == 15
+                || a->rd == a->rt2 || a->rt == a->rt2
+                || (s->thumb && a->rt2 == 13)))) {
+        unallocated_encoding(s);
+        return true;
+    }

in the mop == MO_64 subclause we check for
 a->rt == a->rt2
so we will UNDEF for rt == rt2, as in this example. But the
pseudocode in the spec doesn't say that rt == rt2 is
an UNPREDICTABLE case. (It is an UNDPREDICTABLE
case for LDREXD, but STREXD lets you write the same
register twice if you want to.) Or am I misreading this?

thanks
-- PMM