TF-flag management on x86 for tracing individual function calls

trap.h

// clobbers %rax ; this could cause issues with variadic functions
#define trace_wrap(Ret,Func,...)                                        \
    ({                                                                  \
        long long temp;                                                 \
        void flag_wrap();                                               \
        /* Write the function pointer into the red zone on the stack    \
         * and hope that the function we call doesn't use up that much  \
         * stack for its arguments */                                   \
        asm volatile("movq %0,-120(%%rsp)" : : "a"(Func));              \
        flag_wrap(__VA_ARGS__);                                         \
        /* This ends up being `mov %rax, %rax` but is necessary to      \
         * tell GCC that we need to use the %rax that flag_wrap         \
         * just produced (indirectly). */                               \
        asm volatile("mov %%rax, %0" : "=a"(temp));                     \
        *(Ret*)&temp;                                                   \
    })                                                                  \
    //

wrap.S

.globl flag_wrap
#ifdef __ELF__
.type flag_wrap,@function
#endif
.balign 16
flag_wrap:
    pushf
    orw     $0x0100, (%rsp)
    popf

    callq   *-112(%rsp)

    pushf
    andw    $0xfeff, (%rsp)
    popf

    ret
#ifdef __ELF__
.size flag_wrap, . - WRAP_NAME
#endif