On Thu, Sep 30, 2010 at 02:50:17AM +0100, Maciej W. Rozycki wrote:
> On Wed, 29 Sep 2010, David Daney wrote:
> Not exactly. These GNU C library functions rely on the magic value of
> "1" there to recognise contexts they created themselves and which must
> therefore be handled by themselves internally (these contexts are not
> complete and only preserve the call-saved registers as specified by the
> respective MIPS ABIs, and are therefore unsafe to be passed to the
> rt_sigreturn(2) syscall). All the other values, including of course "0",
> are not treated specially and the context is passed to rt_sigreturn(2) as
> usually. This only matters in cases where e.g. setcontext(3) is used to
> exit from or return to a signal handler.
Nothing has changed in that respect; setup_sigcontext() (and its counterparts)
do _not_ use regs->regs[0]. Note
err |= __put_user(0, &sc->sc_regs[0]);
for (i = 1; i < 32; i++)
err |= __put_user(regs->regs[i], &sc->sc_regs[i]);
in there. The whole point of ->regs[0] uses (both original and modified)
is that $0 is constant 0 and thus the kernel is free to use that member
of pt_regs to indicate that syscall restart might be needed. So's libc,
for that matter (to distinguish between sigreturn and setcontext ones).
When sigframe is created we still discard the value - the fragment above
is not modified at all.
BTW, with original code regs->regs[0] *can* be 1, if you are leaving syscall
with -EINVAL. It won't reach the userland, though.
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