gdb step into function in executable compiled with -z now

Sorry, the linker option should obviously read "-z now", not "-r now"

On 10/02/2017 08:48 PM, Manfred wrote:
> Hi all,
> 
> I would like some explanation about the following behaviour, that I 
> can't understand from the documentation:
> 
> Using gcc and gdb, I am experiencing that with an executable linked with 
> -z now I can't step into functions defined in shared libraries.
> Just removing -r now from the linker options allows gdb to step in as 
> expected
> Here is some sample code (gcc 7.2.1 and gdb 8.0.1 on x86_64):
> 
> // lib/foo.h
> void foo(int);
> 
> // main.c
> #include <stdio.h>
> #include "lib/foo.h"
> 
> int main()
> {
>    const int arg = 13;
>    printf("calling foo(%d)...\n", arg);
>    foo(arg);
>    puts("back into main()");
> }
> 
> // lib/foo.c
> #include <stdio.h>
> #include "foo.h"
> 
> void foo(int val)
> {
>    printf("running foo(%d)\n", val);
> }
> 
> building the above with -z now:
> 
> $ make
> cc -g -Wall -fPIC   -c -o main.o main.c
> make -C ./lib
> make[1]: Entering directory './lib'
> cc -g -Wall -fPIC   -c -o foo.o foo.c
> gcc -shared -o libfoo.so foo.o -Wl,-h,libfoo.so.0 -Wl,-z,relro,-z,now 
> -Wl,--export-dynamic -Wl,--as-needed -fstack-protector -Wl,-rpath,`pwd` 
> -Wl,-Bdynamic
> ln -s libfoo.so libfoo.so.0
> make[1]: Leaving directory './lib'
> gcc -o foo main.o -pie -Wl,-z,relro -Wl,-z,now -lpthread 
> -Wl,-no-undefined -Wl,--export-dynamic -Wl,--as-needed -fstack-protector 
> -Wl,-rpath,`pwd`/lib -L`pwd`/lib -Wl,-Bdynamic -lfoo
> 
> and gdb:
> 
> $ gdb foo
> GNU gdb (GDB) Fedora 8.0.1-26.fc26
> Copyright (C) 2017 Free Software Foundation, Inc.
> .....
> Reading symbols from foo...done.
> (gdb) b main
> Breakpoint 1 at 0x8c2: file main.c, line 7.
> (gdb) run
> Starting program: ./foo
> [Thread debugging using libthread_db enabled]
> Using host libthread_db library "/lib64/libthread_db.so.1".
> 
> Breakpoint 1, main () at main.c:7
> 7      const int arg = 13;
> (gdb) next
> 8      printf("calling foo(%d)...\n", arg);
> (gdb)
> calling foo(13)...
> 9      foo(arg);
> (gdb) step                            <== this doesn't work
> running foo(13)
> 10      puts("back into main()");
> (gdb) cont
> Continuing.
> back into main()
> [Inferior 1 (process 7113) exited normally]
> (gdb)
> 
> linking the executable without -z now:
> 
> $ make
> cc -g -Wall -fPIC   -c -o main.o main.c
> make -C ./lib
> make[1]: Entering directory './lib'
> cc -g -Wall -fPIC   -c -o foo.o foo.c
> gcc -shared -o libfoo.so foo.o -Wl,-h,libfoo.so.0 -Wl,-z,relro,-z,now 
> -Wl,--export-dynamic -Wl,--as-needed -fstack-protector -Wl,-rpath,`pwd` 
> -Wl,-Bdynamic
> ln -s libfoo.so libfoo.so.0
> make[1]: Leaving directory './lib'
> *** -r now *removed* in the following ***
> gcc -o foo main.o -pie -Wl,-z,relro  -lpthread -Wl,-no-undefined 
> -Wl,--export-dynamic -Wl,--as-needed -fstack-protector 
> -Wl,-rpath,`pwd`/lib -L`pwd`/lib -Wl,-Bdynamic -lfoo
> 
> and gdb:
> 
> $ gdb foo
> GNU gdb (GDB) Fedora 8.0.1-26.fc26
> ......
> Reading symbols from foo...done.
> (gdb) b main
> Breakpoint 1 at 0x8e2: file main.c, line 7.
> (gdb) run
> Starting program: ./foo
> [Thread debugging using libthread_db enabled]
> Using host libthread_db library "/lib64/libthread_db.so.1".
> 
> Breakpoint 1, main () at main.c:7
> 7      const int arg = 13;
> (gdb) next
> 8      printf("calling foo(%d)...\n", arg);
> (gdb)
> calling foo(13)...
> 9      foo(arg);
> (gdb) step                            <== this does work now
> foo (val=13) at foo.c:7
> 7      printf("running foo(%d)\n", val);
> (gdb) cont
> Continuing.
> running foo(13)
> back into main()
> [Inferior 1 (process 7200) exited normally]
> (gdb)
> 
> stepi does step into, but it is less friendly.
> 
> Thanks in advance,
> Manfred