What does *%gs:0x10 do in assembler?

Question

What does following syntax do in GASM?

*%gs:0x10

I know that call *%gs:0x10 call will __kernel_vsyscall, but i don't know what *%register:value do.

It's NASM equivalent looks like this: call DWORD PTR gs:0x10

Answer 1

It's a near absolute indirect (FF /2) call to the target of the pointer in gs:0x10.
Note that gs is a selector register, not a general purpose register (see Protected mode).
The instruction read the DWORD at the offset 0x10 (relative to segment gs) and makes a call to its value.
A direct call would have another effect entirely, possibly involving call gates.

---

gs:0x10 is where libc copies the address of __kernel_vsyscall during its initialization.

The AT&T syntax for the control transfer instructions is

Branch addressing using registers or memory operands must be prefixed by a '*'. To specify a "far" control tranfers, a 'l' must be prefixed, as in ljmp, lcall, etc. For example,

GAS syntax          NASM syntax
==========          ===========

jmp *100            jmp  near [100]
call *100           call near [100]
jmp *%eax           jmp  near eax
jmp *%ecx           call near ecx
jmp *(%eax)         jmp  near [eax]
call *(%ebx)        call near [ebx]
ljmp *100           jmp  far  [100]
lcall *100          call far  [100]
ljmp *(%eax)        jmp  far  [eax]
lcal *(%ebx)        call far  [ebx]
ret                 retn
lret                retf
lret $0x100         retf 0x100

Segment-offset pointers are specified using the following format:

jmp $segment, $offset