llvm dev - May 2019 - How to rewrite custom function in dfsan?

Hi,

I am trying to rewrite the custom function __dfsw_read in
compiler-rt/lib/dfsan/dfsan_custom.cc, I wrote the function  __dfsw_read
like this

dfsan_lib.c
```
#include <stdio.h>
#include <sanitizer/dfsan_interface.h>
__attribute__((visibility("default"))) ssize_t
__dfsw_read (int __fd, void *__buf, size_t __nbytes,
            dfsan_label __fd_label, dfsan_label __buf_label, dfsan_label
__nbytes_label,
            dfsan_label *ret_label)
{
    ssize_t ret = read(__fd, __buf, __nbytes);

    if(ret != -1)
    {
        assign_taint_labels_exf(__buf, fd, ret, __nbytes);
        fd += ret;
    }

    *ret_label = 0;
    return ret;
}
```
with following ABI List:

dfsan_abilist.txt
```
fun:read=uninstrumented
fun:read=custom
```

Then, I pack the lib with
```
clang -Wall -fPIC -fsanitize-blacklist=./dfsan_abilist.txt -o dfsan_lib.o
dfsan_lib.c
ar -rc dfsan_lib.o dfsan_lib.a
```

And then compile with my test program test.c
```
clang -fsanitize=dataflow -fsanitize-blacklist=./dfsan_abilist.txt -fPIC
test.c -ldfsan_lib
```

Then error occurs which says that "__dfsw_read" has been defined in
compiler-rt/lib/dfsan/dfsan_custom.cc.

Is there something I am missing or doing wrong in my steps?

Thanks!
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