On Wed, Mar 09, 2022 at 04:30:28PM -0800, Nick Desaulniers wrote:
> I observed the following error when building with
> CONFIG_LTO_CLANG_FULL=y enabled:
>
> ld.lld: error: ld-temp.o <inline asm>:7:2: symbol 'ibt_selftest_ip' is
> already defined
> ibt_selftest_ip:
> ^
>
> Seems to come from
> commit a802350ba65a ("x86/ibt: Add IBT feature, MSR and #CP handling")
>
> Commenting out the label in the inline asm, I then observed:
> vmlinux.o: warning: objtool: identify_cpu()+0x6d0: sibling call from
> callable instruction with modified stack frame
> vmlinux.o: warning: objtool: identify_cpu()+0x6e0: stack state
> mismatch: cfa1=4+64 cfa2=4+8
> These seemed to disappear when I kept CONFIG_LTO_CLANG_FULL=y but then
> disabled CONFIG_X86_KERNEL_IBT. (perhaps due to the way I hacked out
> the ibt_selftest_ip label).
Urgh.. I'm thikning this is a clang bug :/
The code in question is:
void ibt_selftest_ip(void); /* code label defined in asm below */
DEFINE_IDTENTRY_ERRORCODE(exc_control_protection)
{
/* ... */
if (unlikely(regs->ip == (unsigned long)ibt_selftest_ip)) {
regs->ax = 0;
return;
}
/* ... */
}
bool ibt_selftest(void)
{
unsigned long ret;
asm (" lea ibt_selftest_ip(%%rip), %%rax\n\t"
ANNOTATE_RETPOLINE_SAFE
" jmp *%%rax\n\t"
"ibt_selftest_ip:\n\t"
UNWIND_HINT_FUNC
ANNOTATE_NOENDBR
" nop\n\t"
: "=a" (ret) : : "memory");
return !ret;
}
There is only a single definition of that symbol, the one in the asm.
The other is a declaration, which is used in the exception handler to
compare against regs->ip.
So what this code does is trigger an explicit #CP and special case that
in the handler. For that the handler needs to know the special IP that
will trigger the failure, this is cummunicated with that symbol.
> Otherwise defconfig and CONFIG_LTO_CLANG_THIN=y both built and booted
> in a vm WITHOUT IBT support.
>
> Any idea what's the status of IBT emulation in QEMU, and if it exists,
> what's the necessary `-cpu` flag to enable it?
I have a very ugly kvm patch that goes with a very ugly qemu patch to
make it work. I would very much not recommend those getting merged.
Someone with some actual kvm/qemu foo should do one. The complicating
factor is that IA32_S_CET also contains SHSTK enable bits, so a straight
passthrough like I use relies on the guest never setting those bits or
keeping the pieces. It either needs to filter the MSR or implement the
full CET mess.
