Messages in this thread | ![/](/images/icornerl.gif) | | Date | Sat, 5 Jul 2003 09:18:21 +1000 | From | Paul Mackerras <> | Subject | Re: [PATCH 2.5.73] Signal stack fixes #1 introduce PF_SS_ACTIVE |
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Jörn Engel writes:
> This should be the ppc specific part of the signal stack fixes. It sets the > flag, when switching to the signal stack and clears it, when switching > back. When the kernel tries to switch to the signal stack again, > without switching back, the process screwed up the signal stack, so we > kill it with a SIGSEGV.
This is madness.
There is nothing in POSIX that says that you have to exit a signal handler by returning from it (which, under Linux, ends up doing a sigreturn or rt_sigreturn system call). It is explicitly permitted to return from a RT signal handler with setcontext(), for instance. And it is at least long-standing practice to return using longjmp(). Neither setcontext nor longjmp will do a system call (yes, setcontext is a system call on sparc, but it isn't on x86 AFAIK).
So - the kernel doesn't (and can't and shouldn't need to) know about all transitions to or from a signal stack. Therefore the PF_SS_ACTIVE bit is useless since it will be wrong some of the time.
Anyway, what is the problem with taking a signal on the signal stack when you in a signal handler using the signal stack? You just keep going down the stack from where you are, which is what the code already does.
BTW, I am the PPC maintainer; Ben is the powermac maintainer.
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