Question

The third parameter of sa_sigaction is a pointer which point to a machine-dependent struct ucontext, I would like to know what I can dump from struct ucontext.

void (*sa_sigaction)(int signum, siginfo_t *info, void *ucontext)

struct ucontext {
        unsigned long     uc_flags;
        struct ucontext  *uc_link;
        stack_t           uc_stack;
        struct sigcontext uc_mcontext;
        sigset_t          uc_sigmask;   /* mask last for extensibility */
};

Particularly through uc_mcontext (if you can show me where I can know more about other data members, that’s would be great), as people usually use uc_mcontext to dump host registers like this,

ucontext->uc_mcontext.gregs[REG_EIP]

Because uc_mcontext type is struct sigcontext, I look on struct sigcontext in arch/x86/include/asm/sigcontext.h.

struct sigcontext {
        unsigned short gs, __gsh;
        unsigned short fs, __fsh;
        unsigned short es, __esh;
        unsigned short ds, __dsh;

        ... snip ...
};

Is it the right one, beacuse I don’t see gregs in struct sigcontext? Any suggestion is welcomed.

Answer 1

You are looking at the Linux kernel definition of sigcontext. You should be looking at the C library headers of struct ucontext. It is defined at /usr/include/sys/ucontext.h file

Note that it is architecture specific – e.g. the fields for x86 and PPC, for example, are totally different!