On Mon, Mar 23, 2020 at 6:55 PM Alexander Potapenko <glider@xxxxxxxxxx> wrote:
>
> I've reduced the faulty test case to the following code:
>
> =================================
> a;
> long b;
> register unsigned long current_stack_pointer asm("rsp");
> handle_external_interrupt_irqoff() {
> asm("and $0xfffffffffffffff0, %%rsp\n\tpush $%c[ss]\n\tpush "
> "%[sp]\n\tpushf\n\tpushq $%c[cs]\n\tcall *%[thunk_target]\n"
> : [ sp ] "=&r"(b), "+r" (current_stack_pointer)
> : [ thunk_target ] "rm"(a), [ ss ] "i"(3 * 8), [ cs ] "i"(2 * 8) );
> }
> =================================
> (in fact creduce even throws away current_stack_pointer, but we
> probably want to keep it to prove the point).
>
> Clang generates the following code for it:
>
> $ clang vmx.i -O2 -c -w -o vmx.o
> $ objdump -d vmx.o
> ...
> 0000000000000000 <handle_external_interrupt_irqoff>:
> 0: 8b 05 00 00 00 00 mov 0x0(%rip),%eax # 6
> <handle_external_interrupt_irqoff+0x6>
> 6: 89 44 24 fc mov %eax,-0x4(%rsp)
> a: 48 83 e4 f0 and $0xfffffffffffffff0,%rsp
> e: 6a 18 pushq $0x18
> 10: 50 push %rax
> 11: 9c pushfq
> 12: 6a 10 pushq $0x10
> 14: ff 54 24 fc callq *-0x4(%rsp)
> 18: 48 89 05 00 00 00 00 mov %rax,0x0(%rip) # 1f
> <handle_external_interrupt_irqoff+0x1f>
> 1f: c3 retq
>
> The question is whether using current_stack_pointer as an output is
> actually a valid way to tell the compiler it should not clobber RSP.
> Intuitively it is, but explicitly adding RSP to the clobber list
> sounds a bit more bulletproof.
Ok, I am wrong: according to
https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html it's incorrect to
list RSP in the clobber list.
