FROMLIST: lib: vdso: add support for time

(cherry pick from url https://patchwork.kernel.org/patch/10053549/)

Add time() vdso support to match up with existing support in the x86's
vdso.  Currently benefitting arm and arm64 which uses the common
vgettimeofday.c implementation.  On arm provides about a ~14 fold
improvement in speed over the straight syscall, and about a ~5 fold
improvement in speed over an alternate library implementation that
relies on the vdso call to gettimeofday to fulfill the request.

We can provide __vdso_time even if we can not provide a speed
enhanced __vdso_gettimeofday.

Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Bug: 63737556
Bug: 20045882
Change-Id: I0bb3c6bafe57f9ed69350e2dd54edaae58316e8f
Signed-off-by: khusika <khusikadhamar@gmail.com>

arch/arm/kernel/vdso.c

@@ -172,6 +172,7 @@ static void __init patch_vdso(void *ehdr)
 		vdso_nullpatch_one(&einfo, "__vdso_gettimeofday");
 		vdso_nullpatch_one(&einfo, "__vdso_clock_gettime");
 		vdso_nullpatch_one(&einfo, "__vdso_clock_getres");
+		/* do not zero out __vdso_time, no cntvct_ok dependency */
 	}
 }
 

arch/arm/vdso/vdso.lds.S

@@ -83,6 +83,7 @@ VERSION
 		__vdso_clock_gettime;
 		__vdso_gettimeofday;
 		__vdso_clock_getres;
+		__vdso_time;
 	local: *;
 	};
 }

arch/arm64/kernel/vdso/compiler.h

@@ -65,5 +65,6 @@ static __always_inline notrace u64 arch_vdso_read_counter(void)
 #define __vdso_clock_gettime __kernel_clock_gettime
 #define __vdso_gettimeofday __kernel_gettimeofday
 #define __vdso_clock_getres __kernel_clock_getres
+#define __vdso_time __kernel_time
 
 #endif /* __VDSO_COMPILER_H */

arch/arm64/kernel/vdso/vdso.lds.S

@@ -88,6 +88,7 @@ VERSION
 		__kernel_gettimeofday;
 		__kernel_clock_gettime;
 		__kernel_clock_getres;
+		__kernel_time;
 	local: *;
 	};
 }

lib/vdso/vgettimeofday.c

@@ -385,3 +385,13 @@ int __vdso_clock_getres(clockid_t clock, struct timespec *res)
 
 	return 0;
 }
+
+notrace time_t __vdso_time(time_t *t)
+{
+	const struct vdso_data *vd = __get_datapage();
+	time_t result = READ_ONCE(vd->xtime_coarse_sec);
+
+	if (t)
+		*t = result;
+	return result;
+}