| What: vDSO |
| Date: July 2011 |
| KernelVersion: 3.0 |
| Contact: Andy Lutomirski <luto@kernel.org> |
| Description: |
| |
| On some architectures, when the kernel loads any userspace program it |
| maps an ELF DSO into that program's address space. This DSO is called |
| the vDSO and it often contains useful and highly-optimized alternatives |
| to real syscalls. |
| |
| These functions are called according to your platform's ABI. On many |
| platforms they are called just like ordinary C function. On other platforms |
| (ex: powerpc) they are called with the same convention as system calls which |
| is different from ordinary C functions. Call them from a sensible context. |
| (For example, if you set CS on x86 to something strange, the vDSO functions are |
| within their rights to crash.) In addition, if you pass a bad |
| pointer to a vDSO function, you might get SIGSEGV instead of -EFAULT. |
| |
| To find the DSO, parse the auxiliary vector passed to the program's |
| entry point. The AT_SYSINFO_EHDR entry will point to the vDSO. |
| |
| The vDSO uses symbol versioning; whenever you request a symbol from the |
| vDSO, specify the version you are expecting. |
| |
| Programs that dynamically link to glibc will use the vDSO automatically. |
| Otherwise, you can use the reference parser in |
| tools/testing/selftests/vDSO/parse_vdso.c. |
| |
| Unless otherwise noted, the set of symbols with any given version and the |
| ABI of those symbols is considered stable. It may vary across architectures, |
| though. |
| |
| Note: |
| As of this writing, this ABI documentation as been confirmed for x86_64. |
| The maintainers of the other vDSO-using architectures should confirm |
| that it is correct for their architecture. |
