C path matching issue

C language code
```
#include
#include
#include
#include

char buf[32];

int main(int argc, char* argv[], char* envp[]){
if(argc<2){
printf("./fd [输入一个参数]\n");
return 0;
}

int fd = atoi( argv[1] ) - 0x1234;
int len = 0;

len = read(fd, buf, 32);

if(!strcmp("LETMEWIN\n", buf)){
printf("恭喜你！挑战成功！\n");
exit(0);
}

printf("程序结束\n");
return 0;
}
```

This is my rule.
```
/**
* @kind path-problem
* @problem.severity warning
* @id getenv-to-gethostbyname
*/
import cpp
import semmle.code.cpp.dataflow.new.DataFlow
import semmle.code.cpp.dataflow.ExternalFlow

module ReadConfig implements DataFlow::ConfigSig{
predicate isSource(DataFlow::Node source) {
exists(Parameter p, ArrayExpr ae |
p.getFunction().getName() = "main" and
ae.getArrayBase() = p.getAnAccess() and
source.asExpr() = ae
)
}

predicate isSink(DataFlow::Node sink) {
exists(FunctionCall fc |
fc.getTarget().getName() = "read" and
sink.asExpr() = fc.getArgument(0)
)
}

}

module ReadConfigFlow = DataFlow::Global;
import ReadConfigFlow::PathGraph

from ReadConfigFlow::PathNode source, ReadConfigFlow::PathNode sink
where ReadConfigFlow::flowPath(source, sink)
select sink.getNode(), source, sink, "read",
source, "input"
```

The source can match argv[1].

The sink can also match the file descriptor (fd) of the read function.

However, the rule as a whole fails to match the path.

Please help me.

Start with the supplied C example and the ReadConfig DataFlow configuration, comparing the argv[1] source with the read call's file-descriptor argument. Trace how the fd assignment and read argument are represented in the PathGraph, then determine why the individual matches do not produce a complete path. Done means documenting or reproducing the cause and identifying the expected matching behavior.

Scritto dal modello di indicizzazione a partire dal testo della issue.