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| -rw-r--r-- | docs/sigaction.txt | 245 |
1 files changed, 245 insertions, 0 deletions
diff --git a/docs/sigaction.txt b/docs/sigaction.txt new file mode 100644 index 000000000..b380e725b --- /dev/null +++ b/docs/sigaction.txt @@ -0,0 +1,245 @@ + All what you never wanted to know about sigaction(), + struct sigaction, and sigset_t. + + +Before vda started messing with sigset_t, struct sigaction +and sigaction() functions, things looked this way: + + + Structures + +MIPS: + +Ignoring bogus "#if defined(__mips__) ..." block in +libc/sysdeps/linux/common/bits/kernel_sigaction.h +and using +libc/sysdeps/linux/mips/bits/kernel_sigaction.h +as an authoritative source: + +HAVE_SA_RESTORER is #defined +struct old_kernel_sigaction { + unsigned sa_flags; + sighandler_t k_sa_handler; + unsigned long sa_mask; + unsigned pad0[3]; /* reserved, keep size constant */ + /* Abi says here follows reserved int[2] */ + void (*sa_restorer)(void); +#if (_MIPS_SZPTR < 64) + /* For 32 bit code we have to pad struct sigaction to get + * constant size for the ABI */ + int pad1[1]; /* reserved */ +#endif +}; +struct kernel_sigaction { + unsigned int sa_flags; + sighandler_t k_sa_handler; + kernel_sigset_t sa_mask; + void (*sa_restorer)(void); + int s_resv[1]; /* reserved */ +}; +struct sigaction { + unsigned sa_flags; + sighandler_t sa_handler; + sigset_t sa_mask; + /* The ABI says here are two unused ints following. */ + /* Restore handler. */ + void (*sa_restorer)(void); +#if _MIPS_SZPTR < 64 + int sa_resv[1]; +#endif +}; + +IA64: + +Has no old_sigaction. What a relief. + +struct kernel_sigaction { + sighandler_t k_sa_handler; + unsigned long sa_flags; + sigset_t sa_mask; +}; +struct sigaction { + sighandler_t sa_handler; + unsigned long sa_flags; + sigset_t sa_mask; +}; + +Alpha: + +struct old_kernel_sigaction { + sighandler_t k_sa_handler; + unsigned long sa_mask; + unsigned sa_flags; +}; +struct kernel_sigaction { + sighandler_t k_sa_handler; + unsigned sa_flags; + sigset_t sa_mask; +}; +struct sigaction { + sighandler_t sa_handler; + sigset_t sa_mask; + unsigned sa_flags; +}; + +HPPA: + +struct kernel_sigaction { + sighandler_t k_sa_handler; + unsigned long sa_flags; + sigset_t sa_mask; +}; +struct sigaction { + sighandler_t sa_handler; + unsigned long sa_flags; + sigset_t sa_mask; +}; + +The rest, kernel side: + +HAVE_SA_RESTORER #defined +struct old_kernel_sigaction { + sighandler_t k_sa_handler; + unsigned long sa_mask; + unsigned long sa_flags; + void (*sa_restorer)(void); +}; +struct kernel_sigaction { + sighandler_t k_sa_handler; + unsigned long sa_flags; + void (*sa_restorer)(void); + sigset_t sa_mask; +}; + +On userspace side, Sparc has special struct sigaction: + +struct sigaction { + sighandler_t sa_handler; + sigset_t sa_mask; + unsigned long sa_flags; + void (*sa_restorer)(void); /* Not used by Linux/Sparc */ +}; + +And finally the rest has: + +struct sigaction { + sighandler_t sa_handler; + sigset_t sa_mask; + int sa_flags; + void (*sa_restorer)(void); +}; + +Userspace sigset_t was uniformly defined as vector of longs +big enough to hold 1024 (!) bits - carried over from glibc. +Since the only arch whose struct kernel_sigaction contains sa_mask +not as a last member is MIPS, MIPS has special kernel_sigset_t, +which is an array of longs long enough for 128 bits. +Other arches still used userspace sigset_t in struct kernel_sigaction, +but it did not really matter because overlong kernel_sigaction +does not hurt in sigaction() [explained below]. +On kernel side, all arches define _NSIG to 65 (meaning +there are 64 signals, 1..64) except MIPS, which define it to 129. + + + Functions + +sigaction() [libc function] usually has two kernel_sigaction's +on stack and copy (userspace) struct sigaction members into +first one, executes syscall, then pulls out the result from +second one. This accomodates differences in layouts of structs. + +The only typically present quirk is what to do with sa_restorer. + + libc/sysdeps/linux/arm/sigaction.c + +if HAVE_SA_RESTORER and (sa_flags & SA_RESTORER) is not set, +sets sa_restorer to +(flags & SA_SIGINFO) ? __default_rt_sa_restorer : __default_sa_restorer, +and sets SA_RESTORER, +otherwise passes it as-is. Which is kinda strange, because AFAICS +HAVE_SA_RESTORER is *not* defined for ARM. + + libc/sysdeps/linux/i386/sigaction.c + +Forcibly sets SA_RESTORER and sa_restorer: +kact.sa_flags = act->sa_flags | SA_RESTORER; +kact.sa_restorer = ((act->sa_flags & SA_SIGINFO) ? &restore_rt : &restore); + + libc/sysdeps/linux/x86_64/sigaction.c + +Forcibly sets SA_RESTORER and sa_restorer: +kact.sa_flags = act->sa_flags | SA_RESTORER; +kact.sa_restorer = &restore_rt; + + libc/sysdeps/linux/mips/sigaction.c + +# ifdef HAVE_SA_RESTORER +# if _MIPS_SIM == _ABIO32 + kact.sa_restorer = act->sa_restorer; +# else + kact.sa_restorer = &restore_rt; +# endif +# endif +No confusion here, HAVE_SA_RESTORER is #defined for MIPS + + libc/sysdeps/linux/avr32/sigaction.c + +if (kact.sa_flags & SA_RESTORER) { + kact.sa_restorer = act->sa_restorer; +} else { + kact.sa_restorer = __default_rt_sa_restorer; + kact.sa_flags |= SA_RESTORER; +} +Does not check HAVE_SA_RESTORER, but avr32 falls +in "completely ordinary" category on both kernel and +userspace sides, and those have it defined. + + libc/sysdeps/linux/xtensa/sigaction.c + +if (kact.sa_flags & SA_RESTORER) { + kact.sa_restorer = act->sa_restorer; +} else { + kact.sa_restorer = __default_sa_restorer; + kact.sa_flags |= SA_RESTORER; +} +Thus, similar to avr32. + + libc/signal/sigaction.c (i.e. the all other arches) + +# ifdef HAVE_SA_RESTORER + kact.sa_restorer = act->sa_restorer; +# endif +Plain translation, just sa_restorer copy is protected +by HAVE_SA_RESTORER #define check. Looks like here +HAVE_SA_RESTORER will be undef'ed only for IA64, +Alpha an HPPA. + + + Proposed overhaul past 0.9.30 + +Since we can define libc-side structures at will: +make sigset_t and struct sigaction identical on kernel side and libc side +within each arch. If arches do not need special handling of sa_restorer, +then sigaction() can directly use passed struct sigaction as-is. +Otherwise, a copy is still needed, although sigaction() might have +just one struct kernel_sigaction on stack and use it both for passing +data to kernel and for receiving it back. Might save a few bytes. + +To this effect: + +* Make sigset_t size match kernel side on all arches. + This is easy since all arches have 64 signals and only MIPS has 128. + +* Modify libc/sysdeps/linux/$ARCH/bits/sigaction.h + so that its struct sigaction matches kernel's. If sa_restorer + field is present in libc but is missing in kernel_sigaction, + add it at the bottom in order to not mess up kernel_sigaction layout. + +* Modify libc/sysdeps/linux/$ARCH/sigaction.c + to implement the logic above. In "common" pseudo-arch + (libc/signal/sigaction.c file), + we would not even need to do any copying, as described above. + +* Document discovered arch quirks while debugging this mess. + +* Deal with old_kernel_sigaction later. |