classification
Title: Automatic Unload of Dynamic Library Cause Segfault
Type: crash Stage:
Components: ctypes Versions: Python 2.7
process
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: Chinh Nguyen
Priority: normal Keywords:

Created on 2017-04-17 14:18 by Chinh Nguyen, last changed 2017-04-17 14:18 by Chinh Nguyen.

Messages (1)
msg291798 - (view) Author: Chinh Nguyen (Chinh Nguyen) Date: 2017-04-17 14:18
I'm using ctypes to access the PAM library to change a user's password. That is, using the function pam_chauthtok. This is occurring inside a python celery worker in FreeBSD. This will work the first time, the second time generates a segfault and crashes the worker.

On attaching gdb to the worker process, I observe the following:
* Crash occurs in function login_setcryptfmt
* Setting a breakpoint there, I see the following after the first successful password change "warning: Temporarily disabling breakpoints for unloaded shared library "/lib/libcrypt.so.5"
* When there is segfault on the second password change, the location of the segfault cannot be disassemble
* It doesn't look like libcrypt is a direct dependency of libpam.

So it looks like what is happening is this:
* libcrypt is loaded (by python/system?) to invoke some password-related functions, it is then unloaded (by python/system?)
* When the same function is invoked again, somehow libcrypt does not get loaded. This results in a function call to the same function address which is now invalid.

My current work-around is to include libcrypto explicitly by binding to it though I don't use it directly. For example, libcrypt = CDLL(find_library("crypt")).

Other notes:
* This does not occur if I launch celery worker all running in the same process via the celery "green threads" module eventlet
* This only happens if the celery worker is a python child process. I don't know how celery spawns child processes.
History
Date User Action Args
2017-04-17 14:18:15Chinh Nguyencreate