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Date: Sun, 5 Nov 2017 13:30:53 +0100
From: Szabolcs Nagy <>

* Tobias Koch <> [2017-11-05 11:28:08 +0000]:
> libsigsegv defines SIGSEGV_FAULT_STACKPOINTER in fault-linux-powerpc.h as
>     #  define SIGSEGV_FAULT_STACKPOINTER  ((ucontext_t *) ucp)->uc_mcontext.regs->gpr[1]
> which doesn't match up with what's defined in musl's arch/powerpc/bits/signal.h. I took a guess and changed it to
>     #  define SIGSEGV_FAULT_STACKPOINTER  ((ucontext_t *) ucp)->uc_mcontext.gregs[1]
> to make it compile, but I don't know if that is correct. I'd appreciate any help.

in glibc uc_mcontext member of ucontext_t is an union of
pt_regs* (linux ptrace.h defines it) and mcontext_t* (same
as in musl).  but the standard requires mcontext_t there,
not an union of pointers, so the powerpc linux abi is broken.

to access gpregs in glibc you can use either of

(pt_regs*)    ucp->uc_mcontext.regs
(mcontext_t*) ucp->uc_mcontext.uc_regs

in musl it's

(mcontext_t*) ucp->uc_regs

the pointers may point to the tail of the ucontext_t struct
where musl has its ucp->uc_mcontext field with the right type,
but i don't know if that's guaranteed.

so the standard conform way (using ucp->uc_mcontext) to
access the registers is not reliable. you can try to use the
non-standard uc_regs pointer:

#ifdef __GLIBC__
#define uc_regs uc_mcontext.uc_regs
#define SIGSEGV_FAULT_STACKPOINTER ((ucontext_t *) ucp)->uc_regs->gregs[1]

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