[lustre mess] is mgc_fs_setup() reachable at all?

[Al Viro]

[nathan cc'd since he's the only person mentioned in mgc_request.c]

One general observation: drivers/taging/lustre is highly reviewer-hostile...

Found by unrelated grep:

drivers/staging/lustre/lustre/mgc/mgc_request.c:1040:		if (vallen != sizeof(struct super_block))

Huh?  The code around that line looks so:
	if (KEY_IS(KEY_SET_FS)) {
		struct super_block *sb = (struct super_block *)val;
		struct lustre_sb_info *lsi;
		if (vallen != sizeof(struct super_block))
			RETURN(-EINVAL);
		lsi = s2lsi(sb);
		rc = mgc_fs_setup(exp->exp_obd, sb, lsi->lsi_srv_mnt);
		if (rc) {
			CERROR("set_fs got %d\n", rc);
		}
		RETURN(rc);
	}
What is going on here?  We cast something to struct super_block *?
Where does it come from?  The function it's in is

int mgc_set_info_async(const struct lu_env *env, struct obd_export *exp,
		       obd_count keylen, void *key, obd_count vallen,
		       void *val, struct ptlrpc_request_set *set)

which says damn little, other than "multiplexor with no type safety".  Who
calls that?
; git grep -n -w mgc_set_info_async
drivers/staging/lustre/lustre/mgc/mgc_request.c:1001:int mgc_set_info_async(const struct lu_env *env, struct obd_export *exp,
drivers/staging/lustre/lustre/mgc/mgc_request.c:1836:	.o_set_info_async = mgc_set_info_async,
;

Umm...  OK, looks like it's only called as a method.  Unfortunately, grep for
o_set_info_async brings a lot of instances and no callers.  After a lot of
cursing, the following antisocial Fine Piece Of Code had been located:

#define OBP(dev, op)    (dev)->obd_type->typ_dt_ops->o_ ## op

along with

        rc = OBP(exp->exp_obd, set_info_async)(env, exp, keylen, key, vallen,
                                               val, set);

As far as I'm concerned, macros like that are equivalent to spitting into
reviewers' eyes.  Sometimes out of malice, sometimes just because the authors
felt like it would be a cute prank, either way it makes life really unpleasant -
when you need to guess which part of method/function name is to be searched for
to locate the call sites... it gets old very soon.
Anyway, this thing is in

static inline int obd_set_info_async(const struct lu_env *env,
				     struct obd_export *exp, obd_count keylen,
				     void *key, obd_count vallen, void *val,
				     struct ptlrpc_request_set *set)

Again, the type safety is inexistent, but let's cross the fingers and look for
callers (and hope they hadn't pulled a similar cute trick with ## here)

; git grep -n -w obd_set_info_async
drivers/staging/lustre/lustre/include/obd_class.h:508:static inline int obd_set_info_async(const struct lu_env *env,
drivers/staging/lustre/lustre/ldlm/ldlm_lockd.c:522:		rc = obd_set_info_async(req->rq_svc_thread->t_env,
drivers/staging/lustre/lustre/llite/dir.c:658:	rc = obd_set_info_async(NULL, mgc, sizeof(KEY_SET_INFO), KEY_SET_INFO,
drivers/staging/lustre/lustre/llite/llite_lib.c:558:	err = obd_set_info_async(NULL, sbi->ll_dt_exp, sizeof(KEY_CHECKSUM),
drivers/staging/lustre/lustre/llite/llite_lib.c:563:	err = obd_set_info_async(NULL, sbi->ll_dt_exp, sizeof(KEY_CACHE_SET),
drivers/staging/lustre/lustre/llite/llite_lib.c:1964:	obd_set_info_async(NULL, sbi->ll_md_exp,
drivers/staging/lustre/lustre/llite/llite_lib.c:1967:	obd_set_info_async(NULL, sbi->ll_dt_exp,
drivers/staging/lustre/lustre/llite/llite_lib.c:2032:		err = obd_set_info_async(NULL, sbi->ll_md_exp,
drivers/staging/lustre/lustre/llite/lproc_llite.c:437:		rc = obd_set_info_async(NULL, sbi->ll_dt_exp,
drivers/staging/lustre/lustre/llite/lproc_llite.c:485:	rc = obd_set_info_async(NULL, sbi->ll_dt_exp, sizeof(KEY_CHECKSUM),
drivers/staging/lustre/lustre/lmv/lmv_obd.c:281:		obd_set_info_async(NULL, tgt->ltd_exp, sizeof(KEY_INTERMDS),
drivers/staging/lustre/lustre/lmv/lmv_obd.c:2239:			err = obd_set_info_async(env, tgt->ltd_exp,
drivers/staging/lustre/lustre/lov/lov_obd.c:638:		rc = obd_set_info_async(NULL, tgt->ltd_exp,
drivers/staging/lustre/lustre/lov/lov_obd.c:2629:			err = obd_set_info_async(env, tgt->ltd_exp,
drivers/staging/lustre/lustre/lov/lov_obd.c:2633:			err = obd_set_info_async(env, tgt->ltd_exp,
drivers/staging/lustre/lustre/lov/lov_obd.c:2646:			err = obd_set_info_async(env, tgt->ltd_exp, keylen,
drivers/staging/lustre/lustre/lov/lov_obd.c:2655:			err = obd_set_info_async(env, tgt->ltd_exp,
drivers/staging/lustre/lustre/mdc/mdc_request.c:1767:		rc = obd_set_info_async(NULL, exp, strlen(KEY_CHANGELOG_CLEAR),
drivers/staging/lustre/lustre/obdclass/genops.c:624:		rc2 = obd_set_info_async(NULL, obd->obd_self_export,
drivers/staging/lustre/lustre/obdclass/obd_mount.c:277:		rc = obd_set_info_async(NULL, obd->obd_self_export,
drivers/staging/lustre/lustre/obdclass/obd_mount.c:326:		rc = obd_set_info_async(NULL, obd->obd_self_export,
drivers/staging/lustre/lustre/obdclass/obd_mount.c:425:	rc = obd_set_info_async(NULL, obd->obd_self_export,
drivers/staging/lustre/lustre/obdclass/obd_mount.c:437:	rc = obd_set_info_async(NULL, obd->obd_self_export,
;

Oh, joy... 23 call sites to sift through.  Which ones are we interested in?
The test down in the FPOS that has started it all is also reviewer-hostile -
KEY_IS(KEY_SET_FS) has no visible references to function arguments.  Oh, well...

#define KEY_IS(str) \
	(keylen >= (sizeof(str)-1) && memcmp(key, str, (sizeof(str)-1)) == 0)
#define KEY_SET_FS		"set_fs"

and KEY_SET_FS is _NOT_ mentioned anywhere outside that check.  Neither is
"set_fs" as explicit string constant.  I would've assumed the whole thing
to be dead code, but... what with the preprocessor tricks we'd already seen
there, I wouldn't bet against that thing coming from string concat.  Or from
macro stringifying set_fs (sans double quotes).  Or from any number of sick
preprocessor tricks.  Oh, well - at least we can go through that list and
drop the call sites that pass a different string constant.  A few minutes
and curses later we are down to these 5:

                        err = obd_set_info_async(env, tgt->ltd_exp,
                                                 keylen, key, vallen, val, set);
in lmv_set_info_async(),
and
			err = obd_set_info_async(env, tgt->ltd_exp,
					 keylen, key, sizeof(int),
					 &mgi->group, set);
			err = obd_set_info_async(env, tgt->ltd_exp,
					 keylen, key, vallen,
					 ((struct obd_id_info*)val)->data, set);
			err = obd_set_info_async(env, tgt->ltd_exp, keylen,
						 key, sizeof(*info->capa),
						 info->capa, set);
			err = obd_set_info_async(env, tgt->ltd_exp,
					 keylen, key, vallen, val, set);
in lov_set_info_async().

; git grep -n -w lmv_set_info_async
drivers/staging/lustre/lustre/lmv/lmv_obd.c:2212:int lmv_set_info_async(const struct lu_env *env, struct obd_export *exp,
drivers/staging/lustre/lustre/lmv/lmv_obd.c:2661:	.o_set_info_async       = lmv_set_info_async,
;

IOW, this one is another o_set_info_async instance and key/keylen/val/vallen
is passed from its caller as-is.

; git grep -n -w lov_set_info_async
drivers/staging/lustre/lustre/lov/lov_obd.c:2559:static int lov_set_info_async(const struct lu_env *env, struct obd_export *exp,
drivers/staging/lustre/lustre/lov/lov_obd.c:2850:	.o_set_info_async      = lov_set_info_async,
;

... and this one is also o_set_info_async instance.  In one of 4 call sites
we appear to pass the arguments as-is, the rest... key/keylen is passed as-is,
val/vallen isn't.  In any case, key must have originated in one of the
call sites we'd already eliminated.

So unless I've missed something in that paragon of good taste, there is no
call chain that could've lead to mgc_set_info_async() with key being "set_fs".
Incidentally, there is a couple of recursive loops in there that might or
might not lead to stack overflows.

Could somebody familiar with that thing confirm that this is, in fact, dead
code?  As the matter of fact, could maintainers please stand up and put their
names into MAINTAINERS?
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