Is there a way to create a vDSO with the latest kernel?

Question

I'm trying to do a vDSO using latest kernel source code. I was following this tutorial https://www.linuxjournal.com/content/creating-vdso-colonels-other-chicken?page=0,0 However I didn't find some functions like update_vsyscall() and vdso directory in linux-4.20.13/arch/x86/vdso. My question is: is there a way to make a virtual syscall, like gettimeofday(), using the new kernel codes?

Answer 1

vdso directory in

arch/x86/entry/vdso

https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/arch/x86/entry/vdso?h=v4.20.13

For example, add a function directly to vclock_gettime.c, rebuild the kernel, and reboot

diff --git a/arch/x86/entry/vdso/vclock_gettime.c b/arch/x86/entry/vdso/vclock_gettime.c
index 007b3fe9d727..a49bef48a9dc 100644
--- a/arch/x86/entry/vdso/vclock_gettime.c
+++ b/arch/x86/entry/vdso/vclock_gettime.c
@@ -238,3 +238,8 @@ notrace time_t __vdso_time(time_t *t)
 }
 time_t time(time_t *t)
    __attribute__((weak, alias("__vdso_time")));
+
+notrace int __vdso_add(int x, int y)
+{
+   return x + y;
+}
diff --git a/arch/x86/entry/vdso/vdso.lds.S b/arch/x86/entry/vdso/vdso.lds.S
index d3a2dce4cfa9..4b976c119845 100644
--- a/arch/x86/entry/vdso/vdso.lds.S
+++ b/arch/x86/entry/vdso/vdso.lds.S
@@ -25,6 +25,7 @@ VERSION {
            __vdso_getcpu;
            time;
            __vdso_time;
+           __vdso_add;
    local: *;
    };
 }

Write a test case in user mode

gcc -otest test.c vdso64.so

#include <stdio.h>

extern int __vdso_add(int x, int y);

int main()
{
    printf("vdso_add: %d\n", __vdso_add(1, 3));
    return 0;
}

Answer 2

Please see example in linux kernel /src/Documentation/vDSO/vdso_test.c. not sure how the main() above will work without call vdso_sym() for example, in vdso_test.c to use gettimeofday:

gtod_t gtod = (gtod_t)vdso_sym("LINUX_2.6", "__vdso_gettimeofday");