Hello.
At Mon, 26 Feb 2018 23:50:41 +0200, Ants Aasma <ants.aa...@eesti.ee> wrote in
<CA+CSw_uD6dZdg9BbOLCx+oY0rufsRSp5V5W8teonCB=OK=7...@mail.gmail.com>
> On Mon, Feb 26, 2018 at 11:28 PM, Andres Freund <and...@anarazel.de> wrote:
> > So RTLD_LOCAL is out of the question, but I think we can get a good bit
> > of the benefit by either specifying -Wl,-Bsymbolic at shlib build time,
> > or RTLD_DEEPBIND at dlopen() time. Either leads to the opened shared
> > library effectively being put at the beginning of the search path,
> > therefore avoiding the issue that an earlier loaded shared library or
> > symbols from the main binary can accidentally overwrite things in the
> > shared library itself. Which incidentally also makes loading a bit
> > faster.
>
> I think this would also fix oracle_fdw crashing when postgres is
> compiled with --with-ldap. At least RTLD_DEEPBIND helped. [1]
>
> [1]
> https://www.postgresql.org/message-id/CA%2BCSw_tPDYgnzCYW0S4oU0mTUoUhZ9pc7MRBPXVD-3Zbiwni9w%40mail.gmail.com
I saw several cases of the misbinding specifically among
extensions using different versions of the same library or a part
of which was just transplanted from another extension. Now I'm
seeing a case nearby again and found this thread.
Some pl-libraries (plpython does but plperl doesn't for me)
mandates to export their symbols for external modules. So
RTLD_GLOBAL is needed for such extensions but not needed in most
cases. We need a means to instruct whether a module wants to
expose symbols or not.
The extension control file doesn't seem the place. The most
appropriate way seems to be another magic data.
With the attached patch, external modules are loaded RTLD_LOCAL
by default. PG_MODULE_EXPORT_SYMBOL in a module source files let
it loaded RTLD_GLOBAL. I suppose this is the change with the
least impact on existing modules because I believe most of them
don't need to export symbols.
I think this works for all platforms that have dlopen or
shl_load. Windows doesn't the equivalent but anyway we should
explicitly declare using dllexport.
Any opinions or thoughts?
regards.
--
Kyotaro Horiguchi
NTT Open Source Software Center
>From 114692655060bf74d01e0f5452e89f3bd332dfa1 Mon Sep 17 00:00:00 2001
From: Kyotaro Horiguchi <horiguchi.kyot...@lab.ntt.co.jp>
Date: Thu, 24 Jan 2019 13:35:19 +0900
Subject: [PATCH] Add control on wheter symbols in external module is exported
We load exteral modlues with RTLD_GLOBAL, which is problematic when
two or more modules share the same symbol. On the other hand some
modules mandates to export its symbols. This patch enables modules
to decide whether to export them or not. They is defaultly hidden.
plpython module is modified along with as it needs to export symbols.
---
doc/src/sgml/xfunc.sgml | 8 ++++++++
src/backend/utils/fmgr/dfmgr.c | 13 ++++++++++++-
src/include/fmgr.h | 8 ++++++++
src/pl/plpython/plpy_main.c | 1 +
4 files changed, 29 insertions(+), 1 deletion(-)
diff --git a/doc/src/sgml/xfunc.sgml b/doc/src/sgml/xfunc.sgml
index e18272c33a..9a65e76eda 100644
--- a/doc/src/sgml/xfunc.sgml
+++ b/doc/src/sgml/xfunc.sgml
@@ -1875,6 +1875,14 @@ PG_MODULE_MAGIC;
(Presently, unloads are disabled and will never occur, but this may
change in the future.)
</para>
+ <para>
+ The dynamic loader loads a file as its symbols are hidden from external
+ modules by default. A file is loaded exporting the symbols by writhing
+ this in one of the module source files.
+<programlisting>
+PG_MODULE_EXPORT_SYMBOL;
+</programlisting>
+ </para>
</sect2>
diff --git a/src/backend/utils/fmgr/dfmgr.c b/src/backend/utils/fmgr/dfmgr.c
index 456297a531..98ac1cc438 100644
--- a/src/backend/utils/fmgr/dfmgr.c
+++ b/src/backend/utils/fmgr/dfmgr.c
@@ -236,7 +236,7 @@ internal_load_library(const char *libname)
#endif
file_scanner->next = NULL;
- file_scanner->handle = dlopen(file_scanner->filename, RTLD_NOW | RTLD_GLOBAL);
+ file_scanner->handle = dlopen(file_scanner->filename, RTLD_NOW | RTLD_LOCAL);
if (file_scanner->handle == NULL)
{
load_error = dlerror();
@@ -281,6 +281,17 @@ internal_load_library(const char *libname)
errhint("Extension libraries are required to use the PG_MODULE_MAGIC macro.")));
}
+ /* Check if the module wants to export symbols */
+ if (dlsym(file_scanner->handle, PG_MODULE_EXPORT_SYMBOL_NAME_STRING))
+ {
+ elog(DEBUG3,
+ "loaded dynamic-link library \"%s\" with RTLD_GLOBAL", libname);
+ /* want to export, reload it the way */
+ dlclose(file_scanner->handle);
+ file_scanner->handle = dlopen(file_scanner->filename,
+ RTLD_NOW | RTLD_GLOBAL);
+ }
+
/*
* If the library has a _PG_init() function, call it.
*/
diff --git a/src/include/fmgr.h b/src/include/fmgr.h
index ead17f0e44..c072850646 100644
--- a/src/include/fmgr.h
+++ b/src/include/fmgr.h
@@ -456,6 +456,14 @@ PG_MAGIC_FUNCTION_NAME(void) \
} \
extern int no_such_variable
+#define PG_MODULE_EXPORT_SYMBOL_NAME Pg_module_exrpot_symbols
+#define PG_MODULE_EXPORT_SYMBOL_NAME_STRING "Pg_module_exrpot_symbols"
+
+#define PG_MODULE_EXPORT_SYMBOL \
+extern PGDLLEXPORT void PG_MODULE_EXPORT_SYMBOL_NAME(void); \
+extern PGDLLEXPORT void PG_MODULE_EXPORT_SYMBOL_NAME(void){}
+
+
/*-------------------------------------------------------------------------
* Support routines and macros for callers of fmgr-compatible functions
diff --git a/src/pl/plpython/plpy_main.c b/src/pl/plpython/plpy_main.c
index 6a66eba176..9165b5f455 100644
--- a/src/pl/plpython/plpy_main.c
+++ b/src/pl/plpython/plpy_main.c
@@ -42,6 +42,7 @@
extern void _PG_init(void);
PG_MODULE_MAGIC;
+PG_MODULE_EXPORT_SYMBOL;
PG_FUNCTION_INFO_V1(plpython_validator);
PG_FUNCTION_INFO_V1(plpython_call_handler);
--
2.16.3
