Re: [PATCH 0/1] MR2434: include: Add memory clobber to GCC asm statement in NtCurrentTeb().

[Gabriel Ivăncescu (＠insn)]

On Mon Mar 20 21:06:13 2023 +0000, Mateusz Wajchęprzełóż wrote:
> > I don't see how can `NtCurrentTeb()` change between calls within the
>
> same function ("function" here means what is resulted after inlining)?
> Do you have a (hypothetical) example of such code?
> TEB changes within `fiberProc` function:
> #include <windows.h>
> #include <stdio.h>
> static inline __attribute__((always_inline)) void* getTEB()
> {
>    void* teb;
>    __asm__ (".byte 0x65\n\tmovq (0x30),%0" : "=r" (teb));
>    return teb;
> }
> static inline __attribute__((always_inline)) void* getTEB_volatile()
> {
>    void* teb;
>    __asm__ volatile (".byte 0x65\n\tmovq (0x30),%0" : "=r" (teb));
>    return teb;
> }
> void* testFiber;
> void* mainThrFiber;
> void* secondThrFiber;
> static void WINAPI fiberProc(void* arg);
> static DWORD WINAPI threadProc(void* arg);
> int main()
> {
>    mainThrFiber = ConvertThreadToFiber(NULL);
>    testFiber = CreateFiber(0, fiberProc, NULL);
>    printf("main TEB=%p; volatile TEB=%p\n", getTEB(), getTEB_volatile());
>    puts("main -> fiber");
>    SwitchToFiber(testFiber);
>    puts("main <- fiber");
>    HANDLE hThr = CreateThread(NULL, 0, threadProc, NULL, 0, NULL);
>    WaitForSingleObject(hThr, INFINITE);
>    return 0;
> }
> DWORD WINAPI threadProc(void* arg)
> {
>    secondThrFiber = ConvertThreadToFiber(NULL);
>    printf("thread TEB=%p; volatile TEB=%p\n", getTEB(), getTEB_volatile());
>    puts("thread -> fiber");
>    SwitchToFiber(testFiber);
>    puts("thread <- fiber");
>    return 0;
> }
> void WINAPI fiberProc(void* arg)
> {
>    printf("fiber TEB=%p; volatile TEB=%p\n", getTEB(), getTEB_volatile());
>    SwitchToFiber(mainThrFiber);
>    printf("fiber TEB=%p; volatile TEB=%p\n", getTEB(), getTEB_volatile());
>    SwitchToFiber(secondThrFiber);
> }

I see, thanks for the example with fibers, although this seems purely
theoretical as Wine code doesn't use them…