[Bug tapsets/26843] New: print_ubacktrace_fileline is broken on master

["agentzh at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=26843

            Bug ID: 26843
           Summary: print_ubacktrace_fileline is broken on master
           Product: systemtap
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: tapsets
          Assignee: systemtap at sourceware dot org
          Reporter: agentzh at gmail dot com
  Target Milestone: ---

Seems like `print_ubacktrace_fileline` is broken on Ubuntu 20 x86_64 (kernel
Linux ubuntu20-pkg 5.4.0-24-generic #28-Ubuntu SMP Thu Apr 9 22:16:42 UTC 2020
x86_64 x86_64 x86_64 GNU/Linux) It can be reproduced with the following minimal
example using the latest systemtap git master (commit 9102da049a):

First prepare the target C program:

```C
int main(void) {
    return 0;
}
```

Compile it to generate executable file `./a.out`:

```
gcc -g a.c
```

Prepare the following simple .stp file:

```
function foo() {
    print_ubacktrace_fileline();
}

probe process.function("main") {
    foo();
    exit();
}
```

And run both:

```
$ stap a.stp -c ./a.out
WARNING: Kernel function symbol table missing [man warning::symbols]
 0x55cfc28cd131 : main+0x8/0x17 [/home/agentzh/git/systemtap-plus/a.out]
 0x7f59d5d7d0b3 [/usr/lib/x86_64-linux-gnu/libc-2.31.so+0x270b3/0x1f2000]
WARNING: Missing unwind data for a module, rerun with 'stap -d
/usr/lib/x86_64-linux-gnu/libc-2.31.so'
```

No file name or line numbers in the output. (The warnings can be safely ignored
since they are not relevant here).

For comparison on CentOS 6, for example, it works fine using the same version
of stap:

```
$ stap a.stp -c ./a.out
 0x4004b6 : main+0x4/0xe at /home/agentzh/git/systemtap/a.c:2
[/home/agentzh/git/systemtap/a.out]
 0x7fe4d3140d20 [/lib64/libc-2.12.so+0x1ed20/0x394000]
WARNING: Missing unwind data for a module, rerun with 'stap -d
/lib64/libc-2.12.so'
```

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