forked from Minki/linux
83bd01024b
cf http://lkml.org/lkml/2007/10/3/41 To summarize: on Linux, SA_ONSTACK decides whether you are already on the signal stack based on the value of the SP at the time of a signal. If you are not already inside the range, you are not "on the signal stack" and so the new signal handler frame starts over at the base of the signal stack. sigaltstack (and sigstack before it) was invented in BSD. There, the SA_ONSTACK behavior has always been different. It uses a kernel state flag to decide, rather than the SP value. When you first take an SA_ONSTACK signal and switch to the alternate signal stack, it sets the SS_ONSTACK flag in the thread's sigaltstack state in the kernel. Thereafter you are "on the signal stack" and don't switch SP before pushing a handler frame no matter what the SP value is. Only when you sigreturn from the original handler context do you clear the SS_ONSTACK flag so that a new handler frame will start over at the base of the alternate signal stack. The undesireable effect of the Linux behavior is that an overflow of the alternate signal stack can not only go undetected, but lead to a ring buffer effect of clobbering the original handler frame at the base of the signal stack for each successive signal that comes just after the overflow. This is what Shi Weihua's test case demonstrates. Normally this does not come up because of the signal mask, but the test case uses SA_NODEFER for its SIGSEGV handler. The other subtle part of the existing Linux semantics is that a simple longjmp out of a signal handler serves to take you off the signal stack in a safe and reliable fashion without having used sigreturn (nor having just returned from the handler normally, which means the same). After the longjmp (or even informal stack switching not via any proper libc or kernel interface), the alternate signal stack stands ready to be used again. A paranoid program would allocate a PROT_NONE red zone around its alternate signal stack. Then a small overflow would trigger a SIGSEGV in handler setup, and be fatal (core dump) whether or not SIGSEGV is blocked. As with thread stack red zones, that cannot catch all overflows (or underflows). e.g., a local array as large as page size allocated in a function called from a handler, but not actually touched before more calls push more stack, could cause an overflow that silently pushes into some unrelated allocated pages. The BSD behavior does not do anything in particular about overflow. But it does at least avoid the wraparound or "ring buffer effect", so you'll just get a straightforward all-out overflow down your address space past the low end of the alternate signal stack. I don't know what the BSD behavior is for longjmp out of an SA_ONSTACK handler. The POSIX wording relating to sigaltstack is pretty minimal. I don't think it speaks to this issue one way or another. (The program that overflows its stack is clearly in undefined behavior territory of one sort or another anyhow.) Given the longjmp issue and the potential for highly subtle complications in existing programs relying on this in arcane ways deep in their code, I am very dubious about changing the behavior to the BSD style persistent flag. I think Shi Weihua's patches have a similar effect by tracking the SP used in the last handler setup. I think it would be sensible for the signal handler setup code to detect when it would itself be causing a stack overflow. Maybe something like the following patch (untested). This issue exists in the same way on all machines, so ideally they would all do a similar check. When it's the handler function itself or its callees that cause the overflow, rather than the signal handler frame setup alone crossing the boundary, this still won't help. But I don't see any way to distinguish that from the valid longjmp case. Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> |
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