On 03/24/2015 05:55 PM, Brian Gerst wrote:
>>> Might be nice to place a more generic description there, which
>>> registers are expected to be saved by user-space calling in here, etc.
>>
>> __kernel_vsyscall entry point has the same ABI in any 32-bit vDSO,
>> the good old int 0x80 calling convention:
>>
>> syscall# in eax,
>> params in ebx/ecx/edx/esi/edi/ebp,
>> all registers are preserved by the syscall.
>>
>> (I think we don't guarantee that all flags are preserved:
>> I have a testcase where DF gets cleared).
> 
> DF should always be clear on any function call per the C ABI.  But,
> eflags should be preserved, at least the non-privileged bits.  I'd
> like to see that testcase.

The testcase is a simplistic example of how to find and use
32-bit vDSO to perform system calls.

It also sets flags.DF before syscall, and checks whether registers
are preserved, including flags.DF.

On 32-bit kernel (on Intel CPU, where vDSO uses SYSENTER), I see this:

$ ./test32_syscall_vdso
Result:1

whereas on 64-bit it is

./test32_syscall_vdso
Result:0

"Result:1" means that DF was cleared.

See attached source.