The only niggle is that, under the previous 7.4-x gcc-4.8.5-16
the switch in vclock_gettime.c compiled fine:

<quote><pre><code>
notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
{
	int ret = VCLOCK_NONE;
	switch (clock)
	{
	case CLOCK_REALTIME:
		ret = do_realtime(ts);
		break;
	case CLOCK_MONOTONIC:
		ret = do_monotonic(ts);
		break;
	case CLOCK_REALTIME_COARSE:
		return do_realtime_coarse(ts);
	case CLOCK_MONOTONIC_COARSE:
		return do_monotonic_coarse(ts);
        case CLOCK_MONOTONIC_RAW:
               /* if I put ANYTHING here that references
&VVAR(vsyscall_gtod_data),
                * or a call to an (inline) function that references
it, the build fails
                * because a RELOCATION references the undefined symbol
                * '__x86_indirect_thunk_rax' , when compiled with new
                * GCC 4.8.5-16 20150623 , which the previous compiler,
                * fully up-to-date as of 2018-03-13 when I updated to 20150623 ,
                * compiled without any problems.
                * So this would be enough to produce the unresolved relocation:
                {   vsyscall_gtod_data *gt __attribute__((unused)) = gtod;
                } break;
	default:
		break;
	}
	}
	if (ret == VCLOCK_NONE)
		return vdso_fallback_gettime(clock, ts);
	return 0;
}
</code></pre></code>

ONLY reducing the number of clauses in the switch from 5 to 4 actually
fixed the problem - it was nothing to do with the details of any
function being called, it is something to do with accessing the
gtod pointer in a switch with more that 4 clauses.

So it now reads:
<quote><pre><code>
notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
{
	int ret = VCLOCK_NONE;
	switch (clock)
	{case CLOCK_MONOTONIC_RAW:
		ret = do_monotonic_raw(ts);
		break;
	default:
		/* overcome gcc-4.8.5-16.el7_4.2.x86_64 BUG:
		 * if this switch contains more than 4 cases:
		 * a reference to `__x86_indirect_thunk_rax'
		 * is inserted, and final link fails with
		 * `relocation R_X86_64_PC32 against undefined symbol'.
		 */
	switch (clock) {
	case CLOCK_REALTIME:
		ret = do_realtime(ts);
		break;
	case CLOCK_MONOTONIC:
		ret = do_monotonic(ts);
		break;
	case CLOCK_REALTIME_COARSE:
		return do_realtime_coarse(ts);
	case CLOCK_MONOTONIC_COARSE:
		return do_monotonic_coarse(ts);
	default:
		break;
	}
	}
	if (ret == VCLOCK_NONE)
		return vdso_fallback_gettime(clock, ts);
	return 0;
}
</code></pre></quote>

Now this is regardless of compilation with / without -fPIC .

I would appreciate any advice as to what is going on with
gcc-4.8.5-16 (20150623) + binutils as 2.25.1-32.base.el7_4.2 here
that is causing references to __x86_indirect_thunk_rax to be
inserted in the above switch with more than 4 clauses whenever
'gtod' is referenced . The patch I sent avoids the problem, but
why does the problem arise with the new GCC version and
not the old one ?


Thanks & Regards,
Jason