linuxgdbcore-file

My gdb generate different core file when I attach to a process(64-bit and 32-bit)

I have a file 'manager_server.ex'

$ file manager_server.ex
manager_server.ex: ELF 32-bit LSB  executable, Intel 80386, version 1 (GNU/Linux), dynamically linked (uses shared libs), for GNU/Linux 2.6.24, BuildID[sha1]=709713ebf61f867e84d1d2818baea02b562c7ee7, not stripped

I ran this file and then deleted it. Now I try generate a core file by gdb(generate-core-file),but I get different core file(64-bit and 32-bit). This is a correct core file：

$ sudo gdb manager_server.ex -p 28108
(gdb) generate-core-file
(gdb) quit
$ file core.28108
core.28108: ELF 32-bit LSB  core file Intel 80386, version 1 (SYSV), SVR4-style, from '/home/ubuntu/sstx10006/manager_server'

those are not:

$ sudo gdb
(gdb) set archi i386
(gdb) attach 28108
(gdb) generate-core-file
(gdb) quit
$ file core.28108
core.28108: ELF 64-bit LSB  core file x86-64, version 1 (SYSV), SVR4-style, from '/sstx10006/manager_server'

$ sudo gdb
(gdb) set archi i386:x86-64
(gdb) attach 28108
(gdb) generate-core-file
(gdb) quit
$ file core.28108
core.28108: ELF 64-bit LSB  core file x86-64, version 1 (SYSV), SVR4-style, from '/home/ubuntu/sstx10006/manager_server'

$ sudo gdb
(gdb) attach 28108
(gdb) generate-core-file
(gdb) quit
$ file core.28108
core.28108: ELF 64-bit LSB  core file x86-64, version 1 (SYSV), SVR4-style, from '/sstx10006/manager_server'

I want to know what make them different. I asked ChatGPT, but the explanation it provided was incorrect.

Solution

  • I want to know what make them different.

    It looks that when the executable is not deleted, both gdb -p $pid and (gdb) attach $pid read the /proc/$pid/exe symlink and use that executable. But when the file is deleted, GDB fails to load the manager_server.ex (deleted).

    1. This looks like a bug in GDB: it could use /proc/$pid/exe directly instead of dereferencing it.
    2. GDB warns about "not understanding what's going on":
    (gdb) attach 2655529
Attaching to process 2655529
No executable file now.
warning: Could not load vsyscall page because no executable was specified
    1. The following works correctly:
    gdb /proc/2655529/exe
attach 2655529
(gdb) gcore