apr_global_mutex_lock() failing with "permission denied"

[Elison Smith <el...@gmail.com>]

I am using shared memory to share some data between my module instances in
Apache child processes, and apr_global_mutex_t  to achieve
mutually-exclusive reads and writes.

In a post_config_hook, I create the mutex using apr_global_mutex_create()
and then reopen it inside each child process by calling
apr_global_mutex_child_init() inside a child_init hook.

However, when I try to grab the lock inside a child, the operation fails
with a "permission denied". For the lock file, I  am using "/tmp/tmp" which
is a file writeable by all.

Any clues?

Re: apr_global_mutex_lock() failing with "permission denied"

[Yoichi Kawasaki <yo...@gmail.com>]

Hi Elison.

2009/3/21 Elison Smith <el...@gmail.com>:
> I am using shared memory to share some data between my module instances in
> Apache child processes, and apr_global_mutex_t  to achieve
> mutually-exclusive reads and writes.
>
> In a post_config_hook, I create the mutex using apr_global_mutex_create()
> and then reopen it inside each child process by calling
> apr_global_mutex_child_init() inside a child_init hook.
>
> However, when I try to grab the lock inside a child, the operation fails
> with a "permission denied". For the lock file, I  am using "/tmp/tmp" which
> is a file writeable by all.
>
> Any clues?
>


the platform on which your module runs is unix?

There should be such a permission problem if the parent process
starts as root while child processes as user/group that you
specify in apache configuration file.

In that case, use unixd_set_global_mutex_perms in child_init hook,
after apr_global_mutex_create just like this below:

** code snippet from mod_rewrite.c
-------------------------------------------------------------------------

#ifdef AP_NEED_SET_MUTEX_PERMS
#include "unixd.h"
#endif
...

    /* create the lockfile */
    rc = apr_global_mutex_create(&rewrite_mapr_lock_acquire, lockname,
                                 APR_LOCK_DEFAULT, p);
    if (rc != APR_SUCCESS) {
        ap_log_error(APLOG_MARK, APLOG_CRIT, rc, s,
                     "mod_rewrite: Parent could not create RewriteLock "
                     "file %s", lockname);
        return rc;
    }

#ifdef AP_NEED_SET_MUTEX_PERMS
    rc = unixd_set_global_mutex_perms(rewrite_mapr_lock_acquire);
    if (rc != APR_SUCCESS) {
        ap_log_error(APLOG_MARK, APLOG_CRIT, rc, s,
                     "mod_rewrite: Parent could not set permissions "
                     "on RewriteLock; check User and Group directives");
        return rc;
    }
#endif
-------------------------------------------------------------------------

you should also see unixd.h  (/path-to-apache/include/unixd.h)
-------------------------------------------------------------------------
/**
 * One of the functions to set mutex permissions should be called in
 * the parent process on platforms that switch identity when the
 * server is started as root.
 * If the child init logic is performed before switching identity
 * (e.g., MPM setup for an accept mutex), it should only be called
 * for SysV semaphores.  Otherwise, it is safe to call it for all
 * mutex types.
 */
AP_DECLARE(apr_status_t) unixd_set_proc_mutex_perms(apr_proc_mutex_t *pmutex);
AP_DECLARE(apr_status_t)
unixd_set_global_mutex_perms(apr_global_mutex_t *gmutex);
AP_DECLARE(apr_status_t) unixd_accept(void **accepted, ap_listen_rec
*lr, apr_pool_t *ptrans);
-------------------------------------------------------------------------

good luck!

Yoichi



-- 
-- 
Yoichi Kawasaki <yo...@gmail.com>