Subject: Re: sigaction's ucontext_t with incorrect stack reference when
	SA_SIGINFO is being used ?
From: Nicholas Miell <nmiell@comcast.net>
To: Xavier Roche <roche+kml2@exalead.com>
Cc: Linux Kernel <linux-kernel@vger.kernel.org>
In-Reply-To: <45B47C68.2000903@exalead.com>
References: <45B47C68.2000903@exalead.com>
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Date: Mon, 22 Jan 2007 21:37:57 -0800
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On Mon, 2007-01-22 at 09:57 +0100, Xavier Roche wrote:
> Hi folks,
> 
> I have a probably louzy question regarding sigaction() behaviour when an
> alternate signal stack is used: it seems that I can not get the user
> stack reference in the ucontext_t stack context ; ie. the uc_stack
> member contains reference of the alternate signal stack, not the stack
> that was used before the crash.
> 
> Is this is a normal behaviour ? Is there a way to retrieve the original
> user's stack inside the signal callback ?
> 
> The example given below demonstrates the issue:
> top of stack==0x7fffff3d7000, alternative_stack==0x501010
> SEGV==0x7fffff3d6ff8; sp==0x501010; current stack is the alternate stack
> 
> It is obvious that the SEGV was a stack overflow: the si_addr address is
> just on the page below the stack limit.

POSIX says:
"the third argument can be cast to a pointer to an object of type
ucontext_t to refer to the receiving thread's context that was
interrupted when the signal was delivered."

so if uc_stack doesn't point to the stack in use immediately prior to
signal generation, this is a bug.

(In theory I should be able to pass the ucontext_t supplied to the
signal handler to setcontext() and resume execution exactly where I left
off -- glibc's refusal to support kernel-generated ucontexts gets in the
way of this, but the point still stands.)

I have no idea who to bother about i386 signal delivery, though. (And I
suspect this bug has probably been copied to other architectures as
well.)

-- 
Nicholas Miell <nmiell@comcast.net>

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