vdso gettimeofday with 64 bit kernel & application compiled for 32 bit

Question

is vdso supported for a 32 bit application which is running on a 64 bit kernel with glibc version 2.15.? If yes, how do I make it work for 32 bit application running on 64 bit kernel.? Cause even though dlopen on "linux-vdso.so.1" is success, dlsym on "__vdso_gettimeofday" fails.

On the same system I able to do a dlopen on "linux-vdso.so.1" & dlsym on "__vdso_gettimeofday" from a application compiled for 64 bit.

Answer 1

On my 64-bit Linux 4.4.15, the 32-bit vdso has these symbols:

readelf -Ws vdso32

Symbol table '.dynsym' contains 9 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 00000ce0     9 FUNC    GLOBAL DEFAULT   12 __kernel_sigreturn@@LINUX_2.5
     2: 00000d00    13 FUNC    GLOBAL DEFAULT   12 __kernel_vsyscall@@LINUX_2.5
     3: 00000ad0   438 FUNC    GLOBAL DEFAULT   12 __vdso_gettimeofday@@LINUX_2.6
     4: 00000c90    42 FUNC    GLOBAL DEFAULT   12 __vdso_time@@LINUX_2.6
     5: 00000770   853 FUNC    GLOBAL DEFAULT   12 __vdso_clock_gettime@@LINUX_2.6
     6: 00000cf0     8 FUNC    GLOBAL DEFAULT   12 __kernel_rt_sigreturn@@LINUX_2.5
     7: 00000000     0 OBJECT  GLOBAL DEFAULT  ABS LINUX_2.5
     8: 00000000     0 OBJECT  GLOBAL DEFAULT  ABS LINUX_2.6

This suggests that the __vdso_gettimeofday you are looking for has been added in kernel 2.6, and that your kernel version is older.