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Subject[PATCH 18/32] x86/vdso.lds: Align !timens (host's) vdso.so entries
Date
As it has been discussed on timens RFC, adding a new conditional branch
`if (inside_time_ns)` on VDSO for all processes is undesirable.
It will add a penalty for everybody as branch predictor may mispredict
the jump. Also there are instruction cache lines wasted on cmp/jmp.

Those effects of introducing time namespace are very much unwanted
having in mind how much work have been spent on micro-optimisation
vdso code.

Addressing those problems, there are two versions of VDSO's .so:
for host tasks (without any penalty) and for processes inside of time
namespace with clk_to_ns() that subtracts offsets from host's time.

Unfortunately, to allow changing VDSO VMA on a running process,
the entry points to VDSO should have the same offsets (addresses).
That's needed as i.e. application that calls setns() may have already
resolved VDSO symbols in GOT/PLT.

Align VDSO entries for host with addresses generated from timens VDSO
(which is bigger as it has code for adding offsets).

Signed-off-by: Dmitry Safonov <dima@arista.com>
---
arch/x86/entry/vdso/vdso-layout.lds.S | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/vdso/vdso-layout.lds.S b/arch/x86/entry/vdso/vdso-layout.lds.S
index ba216527e59f..e529ee3ec9e8 100644
--- a/arch/x86/entry/vdso/vdso-layout.lds.S
+++ b/arch/x86/entry/vdso/vdso-layout.lds.S
@@ -70,7 +70,17 @@ SECTIONS
* stuff that isn't used at runtime in between.
*/

- .text : { *(.text*) } :text =0x90909090,
+ .text : {
+#if defined(CONFIG_TIME_NS) && !defined(UNALIGNED_ENTRIES)
+#ifdef BUILD_VDSO32
+# include "vdso32.entries"
+#endif
+#ifdef BUILD_VDSO64
+# include "vdso64.entries"
+#endif
+#endif
+ *(.text*)
+ } :text =0x90909090,

.altinstructions : { *(.altinstructions) } :text
.altinstr_replacement : { *(.altinstr_replacement) } :text
--
2.20.1
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