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diff --git a/kernel/vdso/README.md b/kernel/vdso/README.md new file mode 100644 index 0000000..573c564 --- /dev/null +++ b/kernel/vdso/README.md @@ -0,0 +1,40 @@ +## vDSO + +The "vDSO" (virtual dynamic shared object) is a small shared library that the +kernel automatically maps into the address space of all user-space +applications. This way, user space can perform some system calls via this +interface and avoid the cost of entering kernel space. + +This is important for +system calls that the community has detected to be performed most often by all +applications. Note that the regular joe doesn't have to deal with any of this as +glibc and the likes will already take care of this. + +For much more, this is all better documented via `man vdso`. + +### This example + +No arguments are required, just: + +``` +$ make +$ ./vdso +=> vDSO base address: 0x7fab930dc000S +=> The following system calls are available via vDSO: + __vdso_gettimeofday (address: 0x7fab930dc890) + __vdso_clock_getres (address: 0x7fab930dcf70) + __vdso_time (address: 0x7fab930dcb80) + __vdso_sgx_enter_enclave (address: 0x7fab930dd530) + __vdso_getrandom (address: 0x7fab930dd020) + __vdso_clock_gettime (address: 0x7fab930dcbb0) + __vdso_getcpu (address: 0x7fab930dcfe0) +=> After running 'getcpu' via vDSO: cpu: 14 - core: 0 +``` + +That is, this program: + +1. Fetches the base address for the vDSO. +2. Parses the ELF file that is the vDSO and obtains the name of all functions + starting with `__vdso_*` (i.e. those that the kernel expects to be called). +3. As an example, it calls the `getcpu` system call by calling the parsed + address for it directly. |