On Thu, 30 Aug 2007, Yar Tikhiy wrote:
On Wed, Aug 29, 2007 at 10:03:41AM -0400, Daniel Eischen wrote:
On Wed, 29 Aug 2007, Yar Tikhiy wrote:
On the one hand, John's approach depends on alias symbols in HEAD
so, e.g., both [EMAIL PROTECTED] (public) and [EMAIL PROTECTED]
(interim) can point to the same implementation for the "foo" interface
after STABLE with [EMAIL PROTECTED] was branched. However, the GNU build
tools don't seem to provide a straight way to that, they return an
error if multiple versions reference the same function, so more
stub code will be needed in the library, which will add to complexity
of the approach.
I think you can use -z muldefs or --allow-multiple-definition as a
linker argument to work around that. libpthread once did that (or
something like it) so it could be compatible with libpthread.so.1
which used LIBPTHREAD_1_0 instead of FBSD_1_0.
I'm afraid the ld trick won't work because the error is from as(1),
e.g.:
: $ make
: cc -O2 -fno-strict-aliasing -pipe -c foo.c
: {standard input}: Assembler messages:
: {standard input}:4: Error: multiple versions [EMAIL PROTECTED]'|[EMAIL
PROTECTED]'] for symbol `foo0'
: *** Error code 1
: $ cat foo.c
: #include <sys/cdefs.h>
:
: int
: foo0(int n)
: {
: return (n + 1);
: }
:
: __sym_compat(foo, foo0, FBSD_1.0);
: __sym_compat(foo, foo0, FBSD_1.1);
:
: int
: foo(int n)
: {
: return (n + 3);
: }
Did I do anything wrong?
Hmm, I guess it's not that simple. Take a look at the hack I had to make in
libpthread/thread/thr_private.h (line ~67). I had to create weak references
to the functions, so for your example, you'd need something like:
__weak_reference(foo0, foo0_10);
__weak_refefence(foo0, foo0_11);
__sym_compat(foo, foo0_10, FBSD_1.0);
__sym_compat(foo, foo0_11, FBSD_1.1);
I think I also needed to make the latest symbol the default, changing
the last line above to __sym_compat_default(foo, foo0_11, FBSD_1.1),
but I seem to recall earlier email from you saying this shouldn't
be needed. I _think_ I needed to for libpthread, but perhaps that
is because I had the symbols (e.g. foo) listed in both sections of
the map file:
LIBPTHREAD_1_0 {
pthread_foo;
};
FBSD_1_0 {
pthread_foo;
};
See r.19 of libpthread/pthread.map.
But this hackery isn't very pretty and is a little confusing, so
perhaps that is an argument against taking this approach? Or maybe
not, if you can come up with a set of macros that can be easily
changed to do one thing in -current and a different thing in
release brannches - you might be able to avoid changing the
compat shims when branching -current to a release or MFC'ing.
On the other hand, Symbol.map files should not contain duplicate
symbol names as it seems to violate ld(1) semantics. Therefore,
if following Daniel's way, we'll need to track which symbols moved
by more than one version since the last STABLE branch to find interim
versions. This shouldn't be too hard, but the implementation details
are somewhat different.
When someone changes the ABI, he's going to add a new set of symbols
to a Symbol.map file. At this point before he commits he should
notice that there are duplicated symbols. If he does, then he
removes them and updates ObsoleteVersions. If he doesn't, then
you have duplicate symbols ;-) Or you just have a script that
does this for you (version_gen.awk might be a good starting
point!).
For the beginning, version_gen.awk could just complain about
duplicated symbols and suggest adding them to ObsoleteVersions as
there shouldn't be any duplicates in the whole version map generated.
Is that reasonable? BTW, what is a good place for ObsoleteVersions?
src/lib/${libname}, along with Versions.def? Or next to Symbol.map?
Since duplicates might not ever occur, you would probably have an
empty ObsoleteVersions file, so I'd recommend just one of them in
src/lib/${libname}. Yes, I think having version_gen.awk complain
and abort if it encounters duplicates might be good enough for now.
Before a branch from -current, we can also manually check the
libraries to make sure there are no duplicate symbols from the last
branch (readelf -sW | awk ' {print $8}' | sort | some other magic).
--
DE
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