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+ All what you never wanted to know about sigaction(),
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+ struct sigaction, and sigset_t.
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+
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+
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+Before vda started messing with sigset_t, struct sigaction
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+and sigaction() functions, things looked this way:
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+
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+
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+ Structures
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+
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+MIPS:
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+
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+Ignoring bogus "#if defined(__mips__) ..." block in
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+libc/sysdeps/linux/common/bits/kernel_sigaction.h
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+and using
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+libc/sysdeps/linux/mips/bits/kernel_sigaction.h
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+as an authoritative source:
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+
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+HAVE_SA_RESTORER is #defined
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+struct old_kernel_sigaction {
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+ unsigned sa_flags;
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+ sighandler_t k_sa_handler;
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+ unsigned long sa_mask;
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+ unsigned pad0[3]; /* reserved, keep size constant */
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+ /* Abi says here follows reserved int[2] */
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+ void (*sa_restorer)(void);
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+#if (_MIPS_SZPTR < 64)
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+ /* For 32 bit code we have to pad struct sigaction to get
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+ * constant size for the ABI */
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+ int pad1[1]; /* reserved */
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+#endif
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+};
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+struct kernel_sigaction {
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+ unsigned int sa_flags;
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+ sighandler_t k_sa_handler;
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+ kernel_sigset_t sa_mask;
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+ void (*sa_restorer)(void);
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+ int s_resv[1]; /* reserved */
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+};
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+struct sigaction {
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+ unsigned sa_flags;
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+ sighandler_t sa_handler;
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+ sigset_t sa_mask;
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+ /* The ABI says here are two unused ints following. */
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+ /* Restore handler. */
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+ void (*sa_restorer)(void);
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+#if _MIPS_SZPTR < 64
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+ int sa_resv[1];
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+#endif
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+};
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+
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+IA64:
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+
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+Has no old_sigaction. What a relief.
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+
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+struct kernel_sigaction {
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+ sighandler_t k_sa_handler;
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+ unsigned long sa_flags;
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+ sigset_t sa_mask;
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+};
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+struct sigaction {
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+ sighandler_t sa_handler;
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+ unsigned long sa_flags;
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+ sigset_t sa_mask;
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+};
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+
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+Alpha:
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+
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+struct old_kernel_sigaction {
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+ sighandler_t k_sa_handler;
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+ unsigned long sa_mask;
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+ unsigned sa_flags;
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+};
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+struct kernel_sigaction {
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+ sighandler_t k_sa_handler;
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+ unsigned sa_flags;
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+ sigset_t sa_mask;
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+};
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+struct sigaction {
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+ sighandler_t sa_handler;
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+ sigset_t sa_mask;
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+ unsigned sa_flags;
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+};
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+
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+HPPA:
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+
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+struct kernel_sigaction {
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+ sighandler_t k_sa_handler;
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+ unsigned long sa_flags;
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+ sigset_t sa_mask;
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+};
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+struct sigaction {
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+ sighandler_t sa_handler;
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+ unsigned long sa_flags;
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+ sigset_t sa_mask;
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+};
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+
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+The rest, kernel side:
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+
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+HAVE_SA_RESTORER #defined
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+struct old_kernel_sigaction {
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+ sighandler_t k_sa_handler;
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+ unsigned long sa_mask;
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+ unsigned long sa_flags;
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+ void (*sa_restorer)(void);
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+};
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+struct kernel_sigaction {
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+ sighandler_t k_sa_handler;
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+ unsigned long sa_flags;
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+ void (*sa_restorer)(void);
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+ sigset_t sa_mask;
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+};
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+
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+On userspace side, Sparc has special struct sigaction:
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+
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+struct sigaction {
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+ sighandler_t sa_handler;
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+ sigset_t sa_mask;
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+ unsigned long sa_flags;
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+ void (*sa_restorer)(void); /* Not used by Linux/Sparc */
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+};
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+
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+And finally the rest has:
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+
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+struct sigaction {
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+ sighandler_t sa_handler;
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+ sigset_t sa_mask;
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+ int sa_flags;
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+ void (*sa_restorer)(void);
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+};
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+
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+Userspace sigset_t was uniformly defined as vector of longs
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+big enough to hold 1024 (!) bits - carried over from glibc.
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+Since the only arch whose struct kernel_sigaction contains sa_mask
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+not as a last member is MIPS, MIPS has special kernel_sigset_t,
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+which is an array of longs long enough for 128 bits.
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+Other arches still used userspace sigset_t in struct kernel_sigaction,
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+but it did not really matter because overlong kernel_sigaction
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+does not hurt in sigaction() [explained below].
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+On kernel side, all arches define _NSIG to 65 (meaning
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+there are 64 signals, 1..64) except MIPS, which define it to 129.
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+
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+
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+ Functions
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+
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+sigaction() [libc function] usually has two kernel_sigaction's
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+on stack and copy (userspace) struct sigaction members into
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+first one, executes syscall, then pulls out the result from
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+second one. This accomodates differences in layouts of structs.
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+
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+The only typically present quirk is what to do with sa_restorer.
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+
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+ libc/sysdeps/linux/arm/sigaction.c
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+
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+if HAVE_SA_RESTORER and (sa_flags & SA_RESTORER) is not set,
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+sets sa_restorer to
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+(flags & SA_SIGINFO) ? __default_rt_sa_restorer : __default_sa_restorer,
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+and sets SA_RESTORER,
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+otherwise passes it as-is. Which is kinda strange, because AFAICS
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+HAVE_SA_RESTORER is *not* defined for ARM.
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+
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+ libc/sysdeps/linux/i386/sigaction.c
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+
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+Forcibly sets SA_RESTORER and sa_restorer:
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+kact.sa_flags = act->sa_flags | SA_RESTORER;
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+kact.sa_restorer = ((act->sa_flags & SA_SIGINFO) ? &restore_rt : &restore);
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+
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+ libc/sysdeps/linux/x86_64/sigaction.c
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+
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+Forcibly sets SA_RESTORER and sa_restorer:
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+kact.sa_flags = act->sa_flags | SA_RESTORER;
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+kact.sa_restorer = &restore_rt;
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+
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+ libc/sysdeps/linux/mips/sigaction.c
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+
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+# ifdef HAVE_SA_RESTORER
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+# if _MIPS_SIM == _ABIO32
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+ kact.sa_restorer = act->sa_restorer;
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+# else
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+ kact.sa_restorer = &restore_rt;
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+# endif
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+# endif
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+No confusion here, HAVE_SA_RESTORER is #defined for MIPS
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+
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+ libc/sysdeps/linux/avr32/sigaction.c
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+
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+if (kact.sa_flags & SA_RESTORER) {
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+ kact.sa_restorer = act->sa_restorer;
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+} else {
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+ kact.sa_restorer = __default_rt_sa_restorer;
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+ kact.sa_flags |= SA_RESTORER;
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+}
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+Does not check HAVE_SA_RESTORER, but avr32 falls
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+in "completely ordinary" category on both kernel and
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+userspace sides, and those have it defined.
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+
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+ libc/sysdeps/linux/xtensa/sigaction.c
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+
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+if (kact.sa_flags & SA_RESTORER) {
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+ kact.sa_restorer = act->sa_restorer;
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+} else {
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+ kact.sa_restorer = __default_sa_restorer;
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+ kact.sa_flags |= SA_RESTORER;
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+}
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+Thus, similar to avr32.
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+
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+ libc/signal/sigaction.c (i.e. the all other arches)
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+
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+# ifdef HAVE_SA_RESTORER
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+ kact.sa_restorer = act->sa_restorer;
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+# endif
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+Plain translation, just sa_restorer copy is protected
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+by HAVE_SA_RESTORER #define check. Looks like here
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+HAVE_SA_RESTORER will be undef'ed only for IA64,
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+Alpha an HPPA.
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+
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+
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+ Proposed overhaul past 0.9.30
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+
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+Since we can define libc-side structures at will:
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+make sigset_t and struct sigaction identical on kernel side and libc side
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+within each arch. If arches do not need special handling of sa_restorer,
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+then sigaction() can directly use passed struct sigaction as-is.
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+Otherwise, a copy is still needed, although sigaction() might have
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+just one struct kernel_sigaction on stack and use it both for passing
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+data to kernel and for receiving it back. Might save a few bytes.
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+
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+To this effect:
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+
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+* Make sigset_t size match kernel side on all arches.
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+ This is easy since all arches have 64 signals and only MIPS has 128.
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+
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+* Modify libc/sysdeps/linux/$ARCH/bits/sigaction.h
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+ so that its struct sigaction matches kernel's. If sa_restorer
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+ field is present in libc but is missing in kernel_sigaction,
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+ add it at the bottom in order to not mess up kernel_sigaction layout.
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+
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+* Modify libc/sysdeps/linux/$ARCH/sigaction.c
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+ to implement the logic above. In "common" pseudo-arch
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+ (libc/signal/sigaction.c file),
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+ we would not even need to do any copying, as described above.
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+
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+* Document discovered arch quirks while debugging this mess.
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+
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+* Deal with old_kernel_sigaction later.
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