Re: [patch 3/3, nios2] fixes for new implementation of signal handler trampolines

Yao Qi
2015-04-28T11:56:13Z

Sandra Loosemore <sandra@codesourcery.com> writes:

> Earlier versions of the nios2 kernel used to allocate code for signal
> handler trampolines on the stack, but when the port was accepted
> upstream it was changed to instead put the trampoline at a fixed
> address in low memory (0x1044).
>
> Moving the code off the stack changed the layout of the stack frame,
> so the first part of this fix involves updating the offset to the
> register save area. This is not an exported interface from the
> kernel; I noticed e.g. the existing aarch64 gdb support includes a
> huge block of comments explaining the kernel's signal handler stack
> frame layout but ultimately also relies on using magic numbers to
> access the register save area. I used a somewhat smaller block of
> comments for nios2 but I think now it is clear where the magic numbers
> come from and what kernel code this corresponds to.

We can make this magic number less magic by documenting how it is
calculated. We did something similar in
tic6x-linux-tdep.c:tic6x_linux_rt_sigreturn_init,

/* The base of struct sigcontext is computed by examining the definition of
struct rt_sigframe in linux kernel source arch/c6x/kernel/signal.c. */
CORE_ADDR base = (sp + TIC6X_SP_RT_SIGFRAME
/* Pointer type *pinfo and *puc in struct rt_sigframe. */
+ 4 + 4
+ TIC6X_SIGINFO_SIZE
+ 4 + 4 /* uc_flags and *uc_link in struct ucontext. */
+ TIC6X_STACK_T_SIZE);

>
> The second problem is that the trampoline is not writable by user
> processes so GDB cannot set software breakpoints there. I've tried to
> deal with that in the single-step hook by having it effectively step
> over the trampoline by setting the breakpoint on its return address,
> but for operations like "finish" or "advance" that use the stack
> unwinder to get the location to set the breakpoint, it seems like
> there is nothing to do but kfail the tests.

Could you address this in a separated patch?

--
Yao (齐尧)