1What: vDSO 2Date: July 2011 3KernelVersion: 3.0 4Contact: Andy Lutomirski <luto@kernel.org> 5Description: 6 7On some architectures, when the kernel loads any userspace program it 8maps an ELF DSO into that program's address space. This DSO is called 9the vDSO and it often contains useful and highly-optimized alternatives 10to real syscalls. 11 12These functions are called according to your platform's ABI. On many 13platforms they are called just like ordinary C function. On other platforms 14(ex: powerpc) they are called with the same convention as system calls which 15is different from ordinary C functions. Call them from a sensible context. 16(For example, if you set CS on x86 to something strange, the vDSO functions are 17within their rights to crash.) In addition, if you pass a bad 18pointer to a vDSO function, you might get SIGSEGV instead of -EFAULT. 19 20To find the DSO, parse the auxiliary vector passed to the program's 21entry point. The AT_SYSINFO_EHDR entry will point to the vDSO. 22 23The vDSO uses symbol versioning; whenever you request a symbol from the 24vDSO, specify the version you are expecting. 25 26Programs that dynamically link to glibc will use the vDSO automatically. 27Otherwise, you can use the reference parser in 28tools/testing/selftests/vDSO/parse_vdso.c. 29 30Unless otherwise noted, the set of symbols with any given version and the 31ABI of those symbols is considered stable. It may vary across architectures, 32though. 33 34Note: 35 As of this writing, this ABI documentation as been confirmed for x86_64. 36 The maintainers of the other vDSO-using architectures should confirm 37 that it is correct for their architecture. 38