Re: Incorrect symbol name displayed in backtrace

[William Tambe <tambewilliam@gmail.com>]

On Thu, Feb 6, 2020 at 10:32 PM Simon Marchi <simark@simark.ca> wrote:
>
> On 2020-02-05 11:36 p.m., William Tambe wrote:
> > Below is an example of backtrace where I find that GDB printed a
> > symbol name that is not found at the address shown in the backtrace.
> > In the example below, GDB says that  __kprobes_text_start() is at
> > 0x0039f000, however when I used p (void *)0x0039f000, it prints the
> > correct symbol name at 0x0039f000.
> >
> > (gdb) bt
> > #0  0x05000100 in _start ()
> > #1  0x0039f000 in __kprobes_text_start ()
> > Backtrace stopped: frame did not save the PC
> > (gdb) p (void *)0x0039f000
> > $6 = (void *) 0x39f000 <__tramp_exit>
> >
> >
> > Any idea what is the difference in the way symbol names are printed in
> > the backtrace and by the command "p" ?
>
> The only reason I could imagine is that for the backtrace, GDB looks for
> the closest code symbol, whereas with the print command, it looks for
> any kind of symbol.  And perhaps that in this case, __kprobes_text_start
> is a code symbol whereas __tramp_exit is a data symbol.
>
> Note that when GDB says:
>
> #1  0x0039f000 in __kprobes_text_start ()
>
> It doesn't mean that __kprobes_text_start == 0x0039f000, it's just that for
> all it knows, the current address (0x0039f000) is in the function
> __kprobes_text_start.  What does "print __kprobes_text_start" say?

(gdb) p __kprobes_text_start
$22 = 0x39eda8 <__kprobes_text_start> ""

__kprobes_text_start is not a function; it is declared as follow:
extern char __kprobes_text_start[]

>
> It's hard to tell without seeing the actual binary, so this is just a guess.

Is it possible to have pointers in the code where symbol names are
printed for the backtrace and where they are printed for the command
"p" ?

>
> Simon