在哪些条件下进入共享库应该在 gdb 中起作用? [英] Conditions under which stepping into shared library should work in gdb?

问题描述

There are many questions related to specific errors why stepping into a shared library with gdb isn't working. None of them provide a systematic answer on how to confirm where the the cause is. This questions is about the ways to diagnose the setup.

Setup example

main.c

#include <stdio.h>
#include "myshared.h"

int main(void)
{
    int a = 3;
    print_from_lib();
    return 0;
}

myshared.h

void print_from_lib();

myshared.c

#include <stdio.h>

void print_from_lib()
{
    printf("Printed from shared library
");
}

Place all the files in the same directory.

export LIBRARY_PATH=$PWD:$LIBRARY_PATH
export LD_LIBRARY_PATH=$PWD:$LD_LIBRARY_PATH
gcc -ggdb -c -Wall -Werror -fpic myshared.c -o myshared-ggdb.o
gcc -ggdb -shared -o libmyshared-ggdb.so myshared-ggdb.o
gcc -ggdb main.c -lmyshared-ggdb -o app-ggdb

Getting the error

$ gdb ./app-ggdb 
GNU gdb (Ubuntu 7.12.50.20170314-0ubuntu1) 7.12.50.20170314-git
...### GDB STARTING TEXT
Reading symbols from app-ggdb...done.
(gdb) break 7
Breakpoint 1 at 0x78f: file main.c, line 7.
(gdb) run
Starting program: /home/user/share-lib-example/app-ggdb 

Breakpoint 1, main () at main.c:7
7       print_from_lib();
(gdb) s
Printed from shared library
8       return 0;

gdb is not stepping inside of the function

Necessary but not sufficient checks

Debug symbols in the binaries

$ objdump --syms libmyshared-ggdb.so | grep debug
0000000000000000 l    d  .debug_aranges 0000000000000000              .debug_aranges
0000000000000000 l    d  .debug_info    0000000000000000              .debug_info
0000000000000000 l    d  .debug_abbrev  0000000000000000              .debug_abbrev
0000000000000000 l    d  .debug_line    0000000000000000              .debug_line
0000000000000000 l    d  .debug_str     0000000000000000              .debug_str

Symbols recognized by gdb

$ gdb ./app-ggdb
...### GDB STARTING TEXT
Reading symbols from app-ggdb...done.
(gdb) break 7
Breakpoint 1 at 0x78f: file main.c, line 7.
(gdb) run
Starting program: /home/user/share-lib-example/app-ggdb 

Breakpoint 1, main () at main.c:7
7       print_from_lib();
(gdb)(gdb) info sharedlibrary
From                To                  Syms Read   Shared Object Library
0x00007ffff7dd7aa0  0x00007ffff7df55c0  Yes         /lib64/ld-linux-x86-64.so.2
0x00007ffff7bd5580  0x00007ffff7bd5693  Yes         /home/user/share-lib-example/libmyshared-ggdb.so
0x00007ffff782d9c0  0x00007ffff797ed43  Yes         /lib/x86_64-linux-gnu/libc.so.6

Confirm .gdbinit isn't the cause

~/.gdbinit contains commands automatically executed upon starting gdb. ref.

Running gdb with the -nx flags can exclude .gdbinit as the source of the problem.

Question

Am looking for suggestions to complete the list of Necessary but not sufficient checks.

Current issue [Update from Mark Plotnick]

This step bug is reproducible on Ubuntu 17.04 amd64 with both a 64- and 32-bit executable and library.

The bug isn't reproducible on Ubuntu 17.04 i386. (gcc 6.3.0-12ubuntu2, gdb 7.12.50 and 8.0, no .gdbinit.).

Possibly relevant: gcc on 17.04 amd64 has been built (by Canonical) to generate pie executables by default.

Question

Can flags with which gcc was build with interfere with debugging? How can you identify if your gcc is the cause?

解决方案

Your problem is self-imposed: don't do this: set step-mode on, and step will work as you expect.

From the GDB manual:

set step-mode
set step-mode on
The set step-mode on command causes the step command to stop at the first
instruction of a function which contains no debug line information
rather than stepping over it.

This is useful in cases where you may be interested in inspecting the machine
instructions of a function which has no symbolic info and do not want
GDB to automatically skip over this function.

You are interested in the opposite of the above -- you want to step into the print_from_lib function and avoid stopping inside the PLT jump stub and the dynamic loader's symbol resolution function.

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