Re: extending ucontext (Re: [PATCH v26 25/30] x86/cet/shstk: Handle signals for shadow stack)

Florian Weimer
2021-04-30T06:45:54Z

* Andy Lutomirski:

> The kernel has:
>
> struct rt_sigframe {
> char __user *pretcode;
> struct ucontext uc;
> struct siginfo info;
> /* fp state follows here */
> };
>
> This is roughly the actual signal frame. But userspace does not have
> this struct declared, and user code does not know the sizes of the
> fields. So it's accessed in a nonsensical way. The signal handler
> function is passed a pointer to the whole sigframe implicitly in RSP,
> a pointer to &frame->info in RSI, anda pointer to &frame->uc in RDX.
> User code can *find* the fp state by following a pointer from
> mcontext, which is, in turn, found via uc:
>
> struct ucontext {
> unsigned long uc_flags;
> struct ucontext *uc_link;
> stack_t uc_stack;
> struct sigcontext uc_mcontext; <-- fp pointer is in here
> sigset_t uc_sigmask; /* mask last for extensibility */
> };

I must say that I haven't reviewed this in detail, but for historic
reasons, glibc userspace has a differently-sized sigset_t. So the
kernel ucontext (used in signals) and user ucontext (used for
swapcontext et al.) are already fully decoupled?

Thanks,
Florian