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Re: [qemu-s390x] [PATCH v1 2/2] s390x/tcg: Store only the necessary amou
Re: [qemu-s390x] [PATCH v1 2/2] s390x/tcg: Store only the necessary amount of doublewords for STFLE
Mon, 3 Jun 2019 12:45:54 +0200
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On 03.06.19 12:38, Stefan Liebler wrote:
> On 5/31/19 4:56 PM, David Hildenbrand wrote:
>> 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 <address@hidden>
>> 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.
> Hi David,
> Thanks for catching this. I've reported the "gzip" example to Ilya and
> indeed, r0 is setup too large. He will fix it in gzip.
> You've mentioned, that this is detected in certain binaries.
> Can you please share those occurrences.
thanks for your reply.
I didn't track all occurrences, it *could* be that it was only gzip in
the background making other processes fail.
For example, the systemd "vitual console setup" unit failed, too, which
was fixed by this change.
I also remember, seeing segfaults in rpmbuild, for example. They only
"changed" with this fix - I m still chasing different errors. :)
You mentioned "He will fix it in gzip", so I assume this is a gzip issue
and not a gcc/glibc/whatever toolchain issue?
David / dhildenb