[Qemu-devel] [PATCH v1 2/2] s390x/tcg: Store only the necessary amount of doublewords for STFLE

[David Hildenbrand <david@redhat.com>]

The PoP (z14, 7-382) says:
    Doublewords to the right of the doubleword in which the
    highest-numbered facility bit is assigned for a model
    may or may not be stored.

However, stack protection in certain binaries can't deal with that.
"gzip" example code:

f1b4:       a7 08 00 03             lhi     %r0,3
f1b8:       b2 b0 f0 a0             stfle   160(%r15)
f1bc:       e3 20 f0 b2 00 90       llgc    %r2,178(%r15)
f1c2:       c0 2b 00 00 00 01       nilf    %r2,1
f1c8:       b2 4f 00 10             ear     %r1,%a0
f1cc:       b9 14 00 22             lgfr    %r2,%r2
f1d0:       eb 11 00 20 00 0d       sllg    %r1,%r1,32
f1d6:       b2 4f 00 11             ear     %r1,%a1
f1da:       d5 07 f0 b8 10 28       clc     184(8,%r15),40(%r1)
f1e0:       a7 74 00 06             jne     f1ec <file_read@@Base+0x1bc>
f1e4:       eb ef f1 30 00 04       lmg     %r14,%r15,304(%r15)
f1ea:       07 fe                   br      %r14
f1ec:       c0 e5 ff ff 9d 6e       brasl   %r14,2cc8 <__stack_chk_fail@plt>

In QEMU, we currently have:
    max_bytes = 24
the code asks for (3 + 1) doublewords == 32 bytes.

If we write 32 bytes instead of only 24, and return "2 + 1" doublewords
("one less than the number of doulewords needed to contain all of the
 facility bits"), the example code detects a stack corruption.

In my opinion, the code is wrong. However, it seems to work fine on
real machines. So let's limit storing to the minimum of the requested
and the maximum doublewords.

Cc: Stefan Liebler <stli@linux.ibm.com>
Cc: Andreas Krebbel <Andreas.Krebbel@de.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 target/s390x/misc_helper.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/target/s390x/misc_helper.c b/target/s390x/misc_helper.c
index 34476134a4..b561c5781b 100644
--- a/target/s390x/misc_helper.c
+++ b/target/s390x/misc_helper.c
@@ -678,7 +678,7 @@ uint32_t HELPER(stfle)(CPUS390XState *env, uint64_t addr)
 
     prepare_stfl();
     max_bytes = ROUND_UP(used_stfl_bytes, 8);
-    for (i = 0; i < count_bytes; ++i) {
+    for (i = 0; i < MIN(count_bytes, max_bytes); ++i) {
         cpu_stb_data_ra(env, addr + i, stfl_bytes[i], ra);
     }
 
-- 
2.20.1