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Don't use bgw_main even to specify in-core bgworker entrypoints.

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On EXEC_BACKEND builds, this can fail if ASLR is in use.

Backpatch to 9.5.  On master, completely remove the bgw_main field
completely, since there is no situation in which it is safe for an
EXEC_BACKEND build.  On 9.6 and 9.5, leave the field intact to avoid
breaking things for third-party code that doesn't care about working
under EXEC_BACKEND.  Prior to 9.5, there are no in-core bgworker
entrypoints.

Petr Jelinek, reviewed by me.

Discussion: http://postgr.es/m/09d8ad33-4287-a09b-a77f-77f8761adb5e@2ndquadrant.com
This commit is contained in:
Robert Haas
2017-03-31 20:35:51 -04:00
parent c281cd5fe1
commit 2113ac4cbb
8 changed files with 75 additions and 66 deletions
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36
doc/src/sgml/bgworker.sgml
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@ -54,9 +54,8 @@ typedef struct BackgroundWorker
int bgw_flags;
BgWorkerStartTime bgw_start_time;
int bgw_restart_time; /* in seconds, or BGW_NEVER_RESTART */
bgworker_main_type bgw_main;
char bgw_library_name[BGW_MAXLEN]; /* only if bgw_main is NULL */
char bgw_function_name[BGW_MAXLEN]; /* only if bgw_main is NULL */
char bgw_library_name[BGW_MAXLEN];
char bgw_function_name[BGW_MAXLEN];
Datum bgw_main_arg;
char bgw_extra[BGW_EXTRALEN];
int bgw_notify_pid;
@ -130,27 +129,13 @@ typedef struct BackgroundWorker
process in case of a crash.
</para>
<para>
<structfield>bgw_main</structfield> is a pointer to the function to run when
the process is started. This field can only safely be used to launch
functions within the core server, because shared libraries may be loaded
at different starting addresses in different backend processes. This will
happen on all platforms when the library is loaded using any mechanism
other than <xref linkend="guc-shared-preload-libraries">. Even when that
mechanism is used, address space layout variations will still occur on
Windows, and when <literal>EXEC_BACKEND</> is used. Therefore, most users
of this API should set this field to NULL. If it is non-NULL, it takes
precedence over <structfield>bgw_library_name</> and
<structfield>bgw_function_name</>.
</para>
<para>
<structfield>bgw_library_name</structfield> is the name of a library in
which the initial entry point for the background worker should be sought.
The named library will be dynamically loaded by the worker process and
<structfield>bgw_function_name</structfield> will be used to identify the
function to be called. If loading a function from the core code,
<structfield>bgw_main</> should be set instead.
function to be called. If loading a function from the core code, this must
be set to "postgres".
</para>
<para>
@ -161,13 +146,10 @@ typedef struct BackgroundWorker
<para>
<structfield>bgw_main_arg</structfield> is the <type>Datum</> argument
to the background worker main function. Regardless of whether that
function is specified via <structfield>bgw_main</> or via the combination
of <function>bgw_library_name</> and <function>bgw_function_name</>,
this main function should take a single argument of type <type>Datum</>
and return <type>void</>. <structfield>bgw_main_arg</structfield> will be
passed as the argument. In addition, the global variable
<literal>MyBgworkerEntry</literal>
to the background worker main function. This main function should take a
single argument of type <type>Datum</> and return <type>void</>.
<structfield>bgw_main_arg</structfield> will be passed as the argument.
In addition, the global variable <literal>MyBgworkerEntry</literal>
points to a copy of the <structname>BackgroundWorker</structname> structure
passed at registration time; the worker may find it helpful to examine
this structure.
@ -215,7 +197,7 @@ typedef struct BackgroundWorker
<para>
Signals are initially blocked when control reaches the
<structfield>bgw_main</> function, and must be unblocked by it; this is to
background worker's main function, and must be unblocked by it; this is to
allow the process to customize its signal handlers, if necessary.
Signals can be unblocked in the new process by calling
<function>BackgroundWorkerUnblockSignals</> and blocked by calling