[llvm-dev] Broken relocation for generating offsets?

Thu Mar 22 23:51:51 PDT 2018

In my case I wouldn't exceed the 2GB module address range...
As I understood - but I'm still a cub with the LLVM and everything - the 
"OFFSET" thingy should calculate the offset from the instruction to the 
address of the symbol. But LLVM creates a jump to that symbol.

From:   Reid Kleckner <rnk at google.com>
To:     bjoern.gaier at horiba.com, Lang Hames <lhames at gmail.com>
Cc:     llvm-dev <llvm-dev at lists.llvm.org>
Date:   22.03.2018 18:43
Subject:        Re: [llvm-dev] Broken relocation for generating offsets?

I wouldn't be surprised if JITing COFF files on Windows doesn't work so 
well, since the object file format assumes most symbols are dllimport or 
within the local 2GB module address range.

I'm not familiar with the current JIT state of the art, though.

On Thu, Mar 22, 2018 at 1:45 AM via llvm-dev <llvm-dev at lists.llvm.org> 
wrote:
Hello, 

I append another clue I found out: The problem is definitely not caused by 
"__ImageBase" the problem comes with the "OFFSET". I generated another 
object file which crashed. The commonality: 
mov         edx, DWORD PTR ?normalPlanschbecken@@3HA ; normalPlanschbecken 

lea         rcx, OFFSET FLAT
:??_C at _0CC@LCMJAIPO at Reading?5?$CCnormalPlanschbecken?$CC?5?$CFi@ 
jmp         printf

From:        via llvm-dev <llvm-dev at lists.llvm.org> 
To:        llvm-dev at lists.llvm.org 
Date:        06.03.2018 10:45 
Subject:        [llvm-dev] Broken relocation for generating offsets? 
Sent by:        "llvm-dev" <llvm-dev-bounces at lists.llvm.org> 

Hello LLVM-Mailing-List, 

I discovered a strange behavior when dealing with object files generated 
by the compiler of Visual Studio 2015. 

When jitting bc files I also add object files to look up functions. These 
object files are coming from visual studio. When using a switch case 
instruction that compiler often generates code based of __ImageBase. I 
show you a short snippet of the assembly output. 
mov         eax, DWORD PTR ?myInt@@3HA ; myInt 
lea         rdi, OFFSET FLAT:__ImageBase 
xor         ebx, ebx

Then these offset is used to jump to some labels like "$LL4 at execute:". 

When the object file gets added to the jitting process this offset 
generation seems to be broken. Executing the code coming from the object 
file will lead to a crash. The crash address will always be exactly the 
address I used to overload __ImageBase with. So it seems that the address 
relocation is wrong with generating offsets? 

Kind regards 
Björn 

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LLVM Developers mailing list
llvm-dev at lists.llvm.org
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Als GmbH eingetragen im Handelsregister Bad Homburg v.d.H. HRB 9816, 
USt.ID-Nr. DE 114 165 789
Geschäftsführer: Dr. Hiroshi Nakamura, Dr. Robert Plank, Markus Bode, 
Heiko Lampert, Hiroshi Kawamura, Takashi Nagano, Takeshi Fukushima.

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