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[Bug 1922887] Re: STR in Thumb 32 decode problem
From: |
JIANG Muhui |
Subject: |
[Bug 1922887] Re: STR in Thumb 32 decode problem |
Date: |
Fri, 09 Apr 2021 13:43:23 -0000 |
We just test the patched version. It looks well. Now QEMU would raise
SEGILL signals, which should be the right behavior.
We are not checking should-UNDEF cases in particular. This is a case we
observed and checked manually when doing a research project with QEMU.
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https://bugs.launchpad.net/bugs/1922887
Title:
STR in Thumb 32 decode problem
Status in QEMU:
In Progress
Bug description:
Hi
It seems that QEMU does not have a proper check on the STR instruction
in Thumb32 mode.
Specifically, the machine code is 0xf84f0ddd, which is 0b1111 1000 0100 1111
0000 1101 1101 1101.
This is an STR (immediate, Thumb) instruction with a T4 encoding scheme.
The symbols is
Rn = 1111
Rt = 0000
P = 1
U = 0
W = 1
The decode ASL is below:
if P == ‘1’ && U == ‘1’ && W == ‘0’ then SEE STRT;
if Rn == ‘1101’ && P == ‘1’ && U == ‘0’ && W == ‘1’ && imm8 == ‘00000100’
then SEE PUSH;
if Rn == ‘1111’ || (P == ‘0’ && W == ‘0’) then UNDEFINED;
t = UInt(Rt); n = UInt(Rn); imm32 = ZeroExtend(imm8, 32);
index = (P == ‘1’); add = (U == ‘1’); wback = (W == ‘1’);
if t == 15 || (wback && n == t) then UNPREDICTABLE;
When Rn == 1111, it should be an undefined instruction, which should
raise SEGILL signal. However, it seems that QEMU does not check this
constraint, which should be a bug. Many thanks
Regards
Muhui
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