[Vm-dev] FFI Struct Argument Pass By Value Fails on Mac 64 bit

[Ben Coman]

On 22 November 2017 at 21:59, Ben Coman <btc at openinworld.com> wrote:

>
>
> On 22 November 2017 at 13:38, Todd Blanchard <tblanchard at mac.com> wrote:
>
>>
>> I've been trying to track this down for a couple weeks now.
>>
>> I have concluded that structs passed by value to functions on the 64 bit
>> VM are not properly populated.  The struct's memory is all zero'd.
>>
>> I found this while trying to work with LibClang and found that functions
>> that fetched code locations from code ranges always returned invalid zero'd
>> locations.  After spending some time with lldb I have traced the problem
>> into the native code and found that the argument is not correct.
>>
>> I've carved out the wee bit of clang to reproduce this in a tiny library.
>>
>> The gist of it is below and the entire file is included.  Basically the
>> struct passed to the function clang_getRangeStart is zero'd memory
>> regardless of the data I send from the image side.
>>
>> The build command I used on sierra is clang -shared -undefined
>> dynamic_lookup -o microclang.dylib microclang.c
>>
>
> On Ubuntu 16.04 I used...
> $ clang -shared -fPIC -o libmicroclang.so microclang.c
>
> $ clang test.c -L. -l microclang
>    test.c:6:53: error: no member named 'begin_int_data' in
> 'CXSourceLocation'
>    if(clang_getRangeStart(clang_getArbitraryRange()).begin_int_data == 0)
>
> I presume you meant...
>     if(clang_getRangeStart(clang_getArbitraryRange()).int_data == 0)
> so correcting and continuing...
>
> $ clang test.c -L. -l microclang
> $ LD_LIBRARY_PATH=. ./a.out
> That failed
>
> So I'm not sure how to proceed.
> I was expecting that would work while Pharo failed.
>
> Now interestingly...
> $ clang test.c microlang.c
> $ ./a.out
> That worked
>
>
> So it seems a similar problem exists outside our FFI.
>
> cheers -ben
>
> P.S. I refactored you code to extract a header file (attached)
>

The issue is still beyond my ken, but I've made some progress towards
isolating/understanding the issue.
Attached zip exploded here for easy reference...


___microlang.h___
typedef unsigned  uintptr_t;

typedef struct {
  const void *ptr_data[2];
} CXSourceRange_;

CXSourceRange_ clang_getArbitraryRange_();
int clang_getRangeEnd_(CXSourceRange_ range);



___microclang.c___
#include "microclang.h"
const char* libraryString = "library_pointer";

CXSourceRange_ clang_getArbitraryRange_()
{       CXSourceRange_ range = {0};
        range.ptr_data[0] = (void*)libraryString;
        return range;
}

int clang_getRangeEnd_(CXSourceRange_ range)
{       // Special decoding for CXSourceLocations for CXLoadedDiagnostics.
        if ((uintptr_t)range.ptr_data[0] & 0x1)
        {       return 0;       }
        else
        {       return 1;       }
}




___test.c___
#include <stdio.h>
#include "microclang.h"
const char* localString =  "local_pointer";

void test( CXSourceRange_ range, char *note )
{       int result = clang_getRangeEnd_(range);
        if(result == 0)
        {       printf("That failed (%s)\n", note); }
        else
        {       printf("That worked (%s)\n", note); }
}

int main()
{       CXSourceRange_ range1 = clang_getArbitraryRange_();
        test(range1, "library string");

        CXSourceRange_ range2 = {0};
        range2.ptr_data[0] = (void*)localString;
        test(range2, "local string");
}



___Makefile___
default: clean static shared

clean:
        rm -f *so *App
        @echo

shared:
        clang -g -o libmicroclang.so -shared -fPIC microclang.c
        clang -g -o sharedApp test.c -L. -lmicroclang
        LD_LIBRARY_PATH=. ./sharedApp
        @echo

static:
        clang -g -o staticApp test.c microclang.c
        ./staticApp
        @echo



Now running...
$ make > report

gives...
___report___
rm -f *so *App

clang -g -o staticApp test.c microclang.c
./staticApp
That worked (library string)
That worked (local string)

clang -g -o libmicroclang.so -shared -fPIC microclang.c
clang -g -o sharedApp test.c -L. -lmicroclang
LD_LIBRARY_PATH=. ./sharedApp
That failed (library string)
That worked (local string)


cheers -ben
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