Re: BUG #15367: Crash in pg_fe_scram_free when using foreign tables

Andrew Gierth <andrew@tao11.riddles.org.uk> writes:
> "Tom" == Tom Lane <tgl@sss.pgh.pa.us> writes:
>  Tom> Could it be that somehow, when libpq is loaded into the backend
>  Tom> address space as it is here, libpq winds up calling the backend's
>  Tom> copy of pg_saslprep rather than its own?

> Yes, that is a thing that happens with ELF-style shared libs when you
> dynamically load shared libraries that weren't linked with -Bsymbolic.

If true, that's potentially pretty disastrous, because *every* src/port
or src/common function that's used by libpq would be similarly subject
to replacement by its backend version, and every behavioral difference
between the frontend and backend coding would be a source of trouble.

However, the info I can find about this suggests that it's only relevant
for exported functions, not ones that are hidden by means of a
version-script.  Since libpq doesn't export pg_saslprep, this shouldn't
be happening?

But wait, this problem is being reported on OpenBSD.  I wonder whether
their linker behaves anything like Linux's ... hm, it looks like it
*is* the same linker.

I tried setting up a scram-auth-based postgres_fdw connection on
my RHEL6 box and forcing it through the questionable code path
by issuing a dummy ALTER USER MAPPING command from another session.
It worked fine, so the problem doesn't seem to be present on Linux.

Repeating the same thing on a nearby OpenBSD 6.0 image ... kaboom!
It fails exactly as Jeremy describes.  Furthermore, I can confirm
that the attached patch fixes it.

I wonder which other platforms we have that behave like this?
And how come we've not seen symptoms before?  It's hard to believe
that src/common/saslprep.c is the only thing libpq imports
that has FRONTEND-vs-not-FRONTEND behavior differences.

            regards, tom lane

diff --git a/src/Makefile.shlib b/src/Makefile.shlib
index 95b82a6..1555be3 100644
--- a/src/Makefile.shlib
+++ b/src/Makefile.shlib
@@ -149,6 +149,11 @@ ifeq ($(PORTNAME), openbsd)
     ifdef soname
       LINK.shared    += -Wl,-x,-soname,$(soname)
     endif
+    BUILD.exports    = ( echo '{ global:'; $(AWK) '/^[^\#]/ {printf "%s;\n",$$1}' $<; echo ' local: *; };' ) >$@
+    exports_file    = $(SHLIB_EXPORTS:%.txt=%.list)
+    ifneq (,$(exports_file))
+      LINK.shared    += -Wl,--version-script=$(exports_file)
+    endif
     SHLIB_LINK        += -lc
   else
     LINK.shared        = $(LD) -x -Bshareable -Bforcearchive