gdb does not stop at printf for ppc

[Sachin Monga]

Hi Stewards


Summary:

There is a test case in GDB where printf is getting converted into 
__printfieee128 when program is compiled with "--no-builtin" flag.
The program is working fine on a machine with glibc 2.34, but not on 
another machine where glibc 2.40 is installed.


Test program:
$ cat test.c
#include <stdio.h>

int
main ()
{
   printf ("Hello World!\n");
   return 0;
}

GDB operation:
1. Load the binary
2. Break main
3. Run
4. Once program stops at main, then Break printf
5. Continue

Expected ouput: GDB should stop at printf function

Actual Output on Machine1(with glibc 2.34) : GDB stop at printf symbol

Actual Output on Machine2(with glibc 2.40) : GDB does NOT stop at printf 
symbol


Analysis:
Glibc debug package with source is required. Once it is installed.
Then program is working fine with glibc 2.34, but not where glibc 2.40 
is installed.


1. Install debug info package for glibc
sudo dnf debuginfo-install glibc

Machine1: glibc packages
$ rpm -q glibc glibc-debuginfo glibc-debugsource
glibc-2.34-168.el9_6.23.ppc64le
glibc-debuginfo-2.34-168.el9_6.23.ppc64le
glibc-debugsource-2.34-168.el9_6.23.ppc64le



Machine2: glibc 2.40 packages
$ rpm -q glibc glibc-debuginfo glibc-debugsource
glibc-2.40-28.fc41.ppc64le
glibc-debuginfo-2.40-28.fc41.ppc64le
glibc-debugsource-2.40-28.fc41.ppc64le
~/GDB/latest_gdb/print$


I execute additional program to check behaviour in both the machine

$] cat test_print_map.c
#include <stdio.h>

int main(void) {
     printf("Hello World!\n");

     // Print address of printf and its ieee128 variant
     printf("Address of printf:        %p\n", (void*)printf);
#ifdef __GLIBC__
     extern int __printfieee128(const char *, ...);
     printf("Address of __printfieee128: %p\n", (void*)__printfieee128);
#endif

     return 0;
}

Compilation
gcc -O0 -g -o test_print_map test_print_map.c --no-builtin


Machine 2 (with glibc 2.40)
$ ./test_printf_map
Hello World!
Address of printf:        0x7fff806870c0  <---- Same address
Address of __printfieee128: 0x7fff806870c0        <------ Same address
Abhay@ltcd97-lp3:~/GDB/latest_gdb/print$


Machine1 (with glibc 2.34)
$ ./test_printf_map
Hello World!
Address of printf:        0x20002a066980  <-------- Different address
Address of __printfieee128: 0x20002a082e80     <-------- Different address
[abhay@kubota print]$


Based on above output, my assumption are (may be it is not 100% true):
Here, the compiler + linker decide how to bind the symbol:

In glibc 2.40 :
printf is just a strong alias to __printfieee128, so both resolve to the 
same address at link time.

In glibc 2.34 :
printf lives at a different symbol table entry than __printfieee128.
May be two different definitions or pointing to IO_printf(). I am not sure.


So the query is why gdb is not able to stop at printf symbol where glibc 
2.40 is installed ? Is this a bug in glibc or I am missing something ?


Regards:
Sachin.