On Wed, Jan 31, 2018 at 08:35:30AM -0800, Andy Lutomirski wrote:
> Hmm. I'm okay with this, but I'd also be okay doing nothing and
> figuring out WTF happened if an upstream kernel fails to build like
> this.
Oh sure, I'm sending it only as an FYI to show that something like this
*might* happen so that we're aware. I've taken it into our trees where
the 3.0 vdso code generates an indirect call to the thunk:
.loc 1 41 0
movq -10489696, %rax # MEM[(const struct vsyscall_gtod_data
*)-10489728B].clock.vread, MEM[(const struct vsyscall_gtod_data
*)-10489728B].clock.vread
call __x86_indirect_thunk_rax
Without the retpoline flags, the code looks like this:
170: 4c 8b 34 25 88 f0 5f mov 0xffffffffff5ff088,%r14
177: ff
178: 44 8b 2c 25 90 f0 5f mov 0xffffffffff5ff090,%r13d
17f: ff
180: ff 14 25 a0 f0 5f ff callq *0xffffffffff5ff0a0 <---
187: 4c 8b 0c 25 a8 f0 5f mov 0xffffffffff5ff0a8,%r9
18e: ff
18f: 4c 8b 04 25 b0 f0 5f mov 0xffffffffff5ff0b0,%r8
which is:
movl -10489712, %r12d # MEM[(const struct vsyscall_gtod_data
*)-10489728B].wall_time_nsec,
.LVL46:
.LBB125:
.LBB126:
.loc 1 41 0
call *-10489696 # MEM[(const struct vsyscall_gtod_data
*)-10489728B].clock.vread <---
.LVL47:
movq -10489688, %r9 # MEM[(const struct vsyscall_gtod_data
*)-10489728B].clock.cycle_last, D.23457
movq -10489680, %r8 # MEM[(const struct vsyscall_gtod_data
*)-10489728B].clock.mask, D.23457
notrace static inline long vgetns(void)
{
long v;
cycles_t (*vread)(void);
vread = gtod->clock.vread;
v = (vread() - gtod->clock.cycle_last) & gtod->clock.mask;
^^^^^^^^^^^^
so it has been converted to an absolute memory reference in that CALL -
nothing funky through a register.
--
Regards/Gruss,
Boris.
Good mailing practices for 400: avoid top-posting and trim the reply.
