Re: [PATCH] AArch64 pauth: Indicate unmasked addresses in backtrace

[Pedro Alves <palves@redhat.com>]

On 7/17/19 5:07 PM, Alan Hayward wrote:

>> I almost suggested the same, but didn't when I realized that we
>> don't always print the addresses:
>>
>> (top-gdb) bt
>> #0  gdb_main (args=0x7fffffffd3a0) at src/gdb/main.c:1186
>> #1  0x0000000000469a7e in main (argc=1, argv=0x7fffffffd4a8) at src/gdb/gdb.c:32
>>
> 
> What’s the reason for that? Surely we always know the address of a function
> in the backtrace? Can it happen in the middle of a backtrace?

"It always worked that way", at least for me.

We show an address if the PC is pointing to the middle
of a line, or we don't have debug info.  If pointing at a line
exactly, then we show no address.

 (gdb) frame
 #0  main (argc=1, argv=0x7fffffffd4a8) at src/gdb/gdb.c:29
 29        args.argc = argc;
 (gdb) si
 0x0000000000469a5f      29        args.argc = argc;
 (gdb) frame
 #0  0x0000000000469a5f in main (argc=1, argv=0x7fffffffd4a8) at src/gdb/gdb.c:29
 29        args.argc = argc;


Same logic for when displaying the frame where a program stops, when
stepping, ctrl-c, breakpoint hits, etc.

 Breakpoint 5, main (argc=1, argv=0x7fffffffd4a8) at src/gdb/gdb.c:28
               ^^^^
 28        memset (&args, 0, sizeof args);
 (gdb) p /x $pc
 $1 = 0x469a46
 (top-gdb) del
 Delete all breakpoints? (y or n) y
 (top-gdb) r
 The program being debugged has been started already.
 Start it from the beginning? (y or n) y
 Starting program: build/gdb/gdb 
 Breakpoint 6, 0x0000000000469a4a in main (argc=1, argv=0x7fffffffd4a8) at src/gdb/gdb.c:28
               ^^^^^^^^^^^^^^^^^^^^^^^^^^
 28        memset (&args, 0, sizeof args);

> 
> 
>> But if you do want to align the addresses, you could do that by
>> specifying a width for the "addr" column.
> 
>>  If "[U]" is rare, given no column
>> headers, the spaces may look a bit odd, though.
> 
> In general, it depends how a binary/library was compiled. But I’d expect a binary
> to either have it in most functions or none.
> 
> Should be easy enough to remove the extra spaces if the system doesn’t support PAC.
> 
> 
>>  Maybe you'd want to pre-compute
>> the max column width by looking at the max number of frames that fit on a
>> page, or something along those lines.
>>
> 
> hmmm... ok. I’ll see what I can do there.

If most functions have it, then I wouldn't bother trying to compute
the max column width.

But then if most functions have it, I wonder what's the point of
showing the marker, though.  :-)  Would it make sense to reverse
the logic?

Thanks,
Pedro Alves