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Re: [Qemu-devel] [Bug] [Patch] MIPS code fails at branch instruction
From: |
Thiemo Seufer |
Subject: |
Re: [Qemu-devel] [Bug] [Patch] MIPS code fails at branch instruction |
Date: |
Mon, 19 Mar 2007 21:34:45 +0000 |
User-agent: |
Mutt/1.5.13 (2006-08-11) |
Stefan Weil wrote:
> Thank you, Paul, for your explanation which clarified Thiemo's statement.
>
> I now checked how my published test code could contribute to a DoS attack.
>
> Current QEMU HEAD:
>
> * The code "hangs" as I wrote before. This is from a user's point of view.
> "Hanging" means, that the test process runs in an infinite loop using any
> CPU time it can get in the virtual machine. QEMU uses all available
> CPU time from the host CPU.
This is a bug in qemu, since it doesn't match CPU behaviour. While the
architecture spec claims UNPREDICTABLE, such a code sequence shouldn't
impede other processes on the same CPU. Throwing an RI exception should
suffice for the general case (i.e. not AR7).
> With single stepping enabled or in the debugger, the test code won't
> hang but give a random result.
>
> Patched QEMU HEAD (see appended patch file):
>
> * The code works in a well defined way. An optional message in the log file
> will show the faulty statement. It won't amount to a DoS because it
> is disabled by default.
Sorry, but I missed the "well defined". What does the jump in the branch
delay slot exactly _do_ now? Where does the PC point to when it was a
conditional branch which wasn't taken?
[snip]
> * show optional message when any branch bits in hflags are already set
> before a branch instruction is generated (so we have a branch in the
> delay slot)
Agreed on that, since it is debug output which is only written when
asked for.
Thiemo
- [Qemu-devel] [Bug] MIPS code fails at branch instruction, Stefan Weil, 2007/03/16
- Re: [Qemu-devel] [Bug] MIPS code fails at branch instruction, Thiemo Seufer, 2007/03/16
- Re: [Qemu-devel] [Bug] MIPS code fails at branch instruction, Stefan Weil, 2007/03/17
- Re: [Qemu-devel] [Bug] MIPS code fails at branch instruction, Thiemo Seufer, 2007/03/17
- Re: [Qemu-devel] [Bug] MIPS code fails at branch instruction, Stefan Weil, 2007/03/17
- Re: [Qemu-devel] [Bug] MIPS code fails at branch instruction, Paul Brook, 2007/03/17
- Re: [Qemu-devel] [Bug] [Patch] MIPS code fails at branch instruction, Stefan Weil, 2007/03/19
- Re: [Qemu-devel] [Bug] [Patch] MIPS code fails at branch instruction,
Thiemo Seufer <=
- Re: [Qemu-devel] [Bug] [Patch] MIPS code fails at branch instruction, Thiemo Seufer, 2007/03/19
- Re: [Qemu-devel] [Bug] [Patch] MIPS code fails at branch instruction, Alexander Voropay, 2007/03/20
- Re: [Qemu-devel] [Bug] [Patch] MIPS code fails at branch instruction, Thiemo Seufer, 2007/03/20
- Re: [Qemu-devel] [Bug] [Patch] MIPS code fails at branch instruction, Stefan Weil, 2007/03/20
- Re: [Qemu-devel] [Bug] [Patch] MIPS code fails at branch instruction, Thiemo Seufer, 2007/03/24
- Re: [Qemu-devel] [Bug] [Patch] MIPS code fails at branch instruction, Aurelien Jarno, 2007/03/24
- Re: [Qemu-devel] [Bug] [Patch] MIPS code fails at branch instruction, Stuart Brady, 2007/03/25
- Re: [Qemu-devel] [Bug] [Patch] MIPS code fails at branch instruction, Thiemo Seufer, 2007/03/25