On Mon, 3 Mar 2014 12:45:55 -0500 Chuck Lever <chuck.lever@xxxxxxxxxx> wrote: > > On Mar 2, 2014, at 10:21 PM, NeilBrown <neilb@xxxxxxx> wrote: > > > On Thu, 27 Feb 2014 08:57:56 -0800 Chuck Lever <chuck.lever@xxxxxxxxxx> wrote: > > > >> > >> On Feb 26, 2014, at 2:58 PM, NeilBrown <neilb@xxxxxxx> wrote: > >> > >>> On Wed, 26 Feb 2014 08:02:42 -0800 Chuck Lever <chuck.lever@xxxxxxxxxx> wrote: > >>> > >>>> > >>>> On Feb 26, 2014, at 6:39 AM, Simo Sorce <simo@xxxxxxxxxx> wrote: > >>>> > >>>>> On Wed, 2014-02-26 at 16:16 +1100, NeilBrown wrote: > >>>>>> See $SUBJ > >>>>>> > >>>>>> Shared libraries are usually versioned so you can release a new version with > >>>>>> an incompatible API and gradually transition to it. > >>>>>> > >>>>>> A rpc.mountd dlopens libnfsjunct.so with no version it is effectively > >>>>>> prohibited from ever changing the API in an incompatible way. > >>>>>> > >>>>>> Both Fedora and openSUSE get upset about packaging a libFOO.so in a non > >>>>>> "-devel" package and so trip over this library which clearly needs to be > >>>>>> installed even if you aren't doing 'devel'opment. > >>>>> > >>>>> Keep in mind this rule is there only for real shared libraries that are > >>>>> loaded by the the system loader. > >>>>> > >>>>> however it is waived for 'modules' that are opened dynamically but are > >>>>> private to the application. > >>>>> > >>>>>> I would like to change mountd as per the patch below to use the ".0" file. > >>>>>> I believe this will not break any installation as the ".so" is installed as a > >>>>>> symlink to the ".0" (or maybe ".0.0.0"). > >>>>>> > >>>>>> Would this be acceptable? > >>>>> > >>>>> It looks to me like this is an internal module for mountd that is not > >>>>> for use by other apps (which is why it is not versioned and can be > >>>>> changed at will as it is deployed at the same time mountd is ? > >>>> > >>>> The plug-in API is versioned internally, but maybe I got that wrong, and should remove the API version field in favor of having consumers load via a specific .so number. > >>> > >>> The problem I see with using the internal versioning is that if the version > >>> is wrong, mountd fails to provide the required service. > >>> So while I don't object to storing the version and performing the test, we > >>> should design work-flows so that the test can only fail if there is a serious > >>> configuration error, not just during a software upgrade. > >>> > >>>> > >>>>> Or am I wrong here ? > >>>>> > >>>>> If I am not wrong I would be against this change personally and would > >>>>> rather move the .so file in a private library dir (if it is not already > >>>>> there) to make it clear it is a private module. > >>>> > >>>> rpc.mountd is the only user currently, but it’s not necessarily private to mountd. A generic storage manager tool might use it to resolve NFS and FedFS referrals for display, for example. We could add plug-in API functions for creating and removing referrals to enable generic tools to perform these operations. > >>> > >>> This is the answer I was looking for to the question I asked earlier - thanks. > >>> (So this is not an 'intimate library' to use Simo's term - it is truly a > >>> shared library). > >>> > >>> If, one day, an incompatible ABI change was needed then we could have an > >>> rpc.mountd installed (or still running) which requires one ABI, and a > >>> generic storage manager tool which requires the other. > >>> So we really need them to be stored in two different files. > >>> e.g. libnfsjunct.so.0 and libnfsjunct.so.1 > >> > >> I was hoping this would never happen. One plug-in library should be able to serve mountd or any other tool that might need to play with junctions. > > > > Certainly that is the hope. I think everyone who writes a shared library > > hopes they will get it right first time, and that if a change is ever needed > > then all users can be upgraded simultaneously. > > > > $ ls -l /lib64/lib*.so.1 | grep -c '^-' > > 4 > > $ ls -l /lib64/lib*.so.1.* | grep -c '^-' > > 17 > > $ ls -l /lib64/lib*.so.[2-9]* | grep -c '^-' > > 20 > > > > That seems to happen often, but not always. That is why we have shared > > library versioning. > > > >> > >> Only a crazy developer like me would ever need to have more than one library version at a time, and even then, it’s pretty simple to build what I need and reinstall, rather than having more than one installed at a time. > >> > >>> To put it another way... libnfsjunct really is a shared library. > >>> The *only* reason that rpc.mountd treats it differently to other shared > >>> libraries is so that it can fail gracefully if the library isn't available > >>> (thus removing hard dependencies) - a difference that I am very comfortable > >>> with. > >>> In every other way it should be treated like a shared library > >>> - it should live in the standard /lib64 or whatever > >>> - each application determines at compile-time what version it needs and finds > >>> it by appending the version number to the base file name > >>> - the "libfoo.so" file should live in the "-devel" package along with the > >>> include file(s) > >>> > >>> > >>> So rather than dlopening "libnfsjunct.so.0" rpc.mountd should probably > >>> use a library name provided by the include file > >> > >> I’m dense, I still don’t see why this makes a difference. I’ll admit that linker fu is something I’ve left to others, so don’t be afraid to spell it out slowly for me. > > > > I'll try (might make sure I understand it too). > > The following is based in part on section 3.1.1 of > > http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html > > > > A shared library (like a cat) has three different names. > > > > 1/ The file name. This is normally /$LIBDIR/libFOO.so.maj.min.release > > (e.g. /usr/lib/libnfsjunct.so.0.0.0), though it can be almost whatever you > > like. It is used by installers to install the library, and by ldconfig. > > ldconfig only wants it to start "lib" or "ld-" and to have ".so" somewhere > > in the name. > > > > 2/ The "soname". This is /$LIBDIR/libFOO.so.maj (i.e. only major number). > > ldconfig will create a symlink from this name to the "most recent" library > > found with that SONAME (a field in the shared library: > > objdump -x $LIBRARY | grep SONAME > > ). > > An application which needs to be linked will contain the "soname" of each > > library that it wants to use. "ldd" lists these and the matching filename > > for each. ld.so effective calls "dlopen" on each "soname". > > > > 3/ The "linker name". This is the name that is used when you compile code. > > You typically specify "-lFOO" and the linker interprets that at > > "$LIBPATH/libFOO.so" and finds a shared library. It extracts the SONAME > > from this library and stores that in that generated binary. > > Naturally the library version found at the "linker name" must match the > > include files describing data structures etc in the library. > > > > To follow this pattern as closely as possible, and yet allow rpc.mountd to > > use dlopen() to load the library: > > - the "soname" should be passed to dlopen(). (That is what ld.so does) > > - that name should be determined from the compile-time environment. (that is > > what 'ld' does). > > > > i.e. we should pass "libnfsjunct.so.0" to dlopen() (if the current > > fedfs-utils provides the compile-time environment). We could determine that > > string with a little script which runs > > > > objdump -x /lib64/libnfsjunct.so | sed -n -e 's/^ *SONAME *//p' > > > > or we could simply keep it in the include file (which must be in-sync with > > the .so). > > > > Doing this > > 1/ ensures that we have the full flexibility of shared libraries should we > > ever need that. > > 2/ makes the nfsjunct library look just like any other shared library and so > > avoids confusion for package checkers. > > > > Does that clarify at all? > > Thank you Neil, it’s coming into focus for me. > > We had some conversation about this at Connectathon last week. It seems like a better design would look like: > > o A separate directory under /usr/lib{64} where fedfs-utils would install its plug-ins While I don't object to this I wonder if it is worth the effort. Using a subdirectory would require rpc.mountd to know exactly what the full path was, and so would need to know if "64" was needed etc. dlopen("fedfs-plugin/libfoo.so.1") will not follow the standard search path (no search happens at all if a '/' is present), and dlopen("libfoo.so.1") does not search subdirectories. When I look in /lib64 on my machine I find, for example libnss_compat-2.18.so libnss_files.so.2 libnss_mdns6_minimal.so.2 libnss_compat.so.2 libnss_hesiod-2.18.so libnss_mdns_minimal.so.2 libnss_db-2.18.so libnss_hesiod.so.2 libnss_nis-2.18.so libnss_db.so.2 libnss_mdns.so.2 libnss_nis.so.2 libnss_dns-2.18.so libnss_mdns4.so.2 libnss_nisplus-2.18.so libnss_dns.so.2 libnss_mdns4_minimal.so.2 libnss_nisplus.so.2 libnss_files-2.18.so libnss_mdns6.so.2 which are all 'nss' plugins which a dlopen()ed by nsswitch. There is also libdevmapper-event-*.so* which are plugins loaded as needed by dmeventd. So there is clear precedent for pluggins living directly in /lib64 (or similar). There are a few directories in /lib64: 32/. ast/. device-mapper/. engines/. ksh/. multipath/. security/. Of these only 32, ast, and ksh (which is a symlink to ast) contain files with "soname" names. /usr/lib contains a few more directories with soname files: sane sasl2 qtcreator being the largest. So there is also some precedent for putting plugins in sub-directories. I had a look at the code for sane, and it duplicates the searching of LD_LIBRARY_PATH (if set) from ld.so, and requires 'configure' to work out the correct libdir, to which it appends "/sane". It looks like a lot of complexity that I would rather avoid myself.... > > o Plug-in consumers would dlopen() via the plug-in library's soname to guarantee ABI compatibility > > o The API version field would be deprecated > > o We didn’t discuss how consumers discover the plug-in soname, but if the API is defined in the header and the soname has to match, maybe that’s the way to go > > I don’t think any of these changes would alter the “loose-ness” of current coupling between rpc.mountd and the plug-in library (to address Steve’s concern), but they would make a better guarantee that mountd was loading the correct plug-in library version. All the rest I completely agree with. > > I’m not sure exactly how to get from point A to point B. Probably fedfs-utils would have to package the plug-in library in the old and new places until all distributed versions of mountd was changed to find the plug-ins in the right place. That would have to be the case to allow nfs-utils downgrades for a particular distribution. > One option would before the next fedfs release to update the major version of libnfsjunct to '1' and to discard the API version field and include the soname in the include file. Then nfs-utils can determine at build time whether .0 or .1 is present (probably via some #define in the include file) and load the appropriate one. Then distros can install the .0 shared library where it is expected, and the .1 shared library where that is expected. There would be no need to install the same version at two different locations. (Distros are already, presumably, quite capable of install multiple versions of shared libraries). It would require having two source packages for fedfs, so maybe it would end up a bit awkward ... not sure. Thanks, NeilBrown > > > > > > > Thanks, > > NeilBrown > > > > > >>> > >>> diff --git a/utils/mountd/cache.c b/utils/mountd/cache.c > >>> index ca35de28847a..1a8c20492869 100644 > >>> --- a/utils/mountd/cache.c > >>> +++ b/utils/mountd/cache.c > >>> @@ -1139,7 +1139,11 @@ static struct exportent *lookup_junction(char *dom, const char *pathname, > >>> struct link_map *map; > >>> void *handle; > >>> > >>> - handle = dlopen("libnfsjunct.so", RTLD_NOW); > >>> +#ifdef JP_LIB_NAME > >>> + handle = dlopen(JP_LIB_NAME, RTLD_NOW); > >>> +#else > >>> + handle = dlopen("libnfsjunct.so.0", RTLD_NOW); > >>> +#endif > >>> if (handle == NULL) { > >>> xlog(D_GENERAL, "%s: dlopen: %s", __func__, dlerror()); > >>> return NULL; > >> > >> -- > >> Chuck Lever > >> chuck[dot]lever[at]oracle[dot]com > >> > >> > > > > -- > Chuck Lever > chuck[dot]lever[at]oracle[dot]com > >
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