Re: GDB Incorrectly Reads & Resolves Shared Library Symbols [MinGW-w64]

[Andrew Dinn <adinn@redhat.com>]

Hi William,

On 03/11/2020 23:20, William Adair wrote:
> Great feedback. It was a bit tricky to figure out how GDB was executing
> at first in main(), but I realized that backtrace_command_1 was the
> function I'm looking for in the stack.c file. The stack trace eventually
> hits print_frame_info()->print_frame() [which relies on
> find_frame_sal()] -> find_frame_funname -> lookup_minimal_symbol_by_pc()
> -> lookup_minimal_symbol_by_pc_section()
> A few immediate questions come to mind: What is the minimal symbol
> table? How can I verify that PC is pointing to the section that the
> actual symbol being executed resides in? Lastly, there's a comment: This
> code assumes that the minimal symbols are sorted by ascending address
> values. Does this imply that if the actual symbol for the PC has a lower
> address than the PC that the symbol will be wrong? (I wonder if this is
> what is happening in my case...)

Well, as I said "After that all you need to do is familiarize with the
gdb source!" ;-)

I have not looked into shared lib code much but I believe minimal
symbols are used to record details of symbols in those libs for which
gdb does not have (maybe cannot have?) full info.

The type definition is in symtab.h along with related types symbol and
general_symbol_info. Note that symbol and minimal_symbol both (publicly)
inherit common data from general_symbol_info which crops up all over the
code base. n.b. your quickest way to see what is these types are is to
use gdb:

(outer) ptype minimal_symbol
. . .
(outer) ptype symbol
. . .
(outer) ptype general_symbol_info
. . .

The stuff about address ordering may be the issue but it sounds to me
like this is a comment about how gdb expects its data to be organized.
If something is missing/out of order in gdb's data that may be to do
with the order in the shared lib or it may be because gdb has failed to
read or sort something from the lib.

gdb has a host of _debug vars you can set to trace what it is doing as
it executes a program. This command will help you find them

(outer) info var _debug

The debug vars you probably want to set for tracing symtab processing
can be enabled like this

(outer) break main
...
(outer) commands
 > set symbol_lookup_debug = 1
 > set symtab_create_debug
 > end

The above breakpoint with associated commands will set these debug vars
when gdb starts running.

I suggest you also look at the contents of the shared library outside of
gdb using objdump and/or nm. That will tell you what gdb is trying to
process and you may be able to compare it with the debug output to see
whether any symbols is getting lost or discarded.

> Thanks everyone for your time.
You are welcome. Good luck.

regards,


Andrew Dinn
-----------