linux-kernel - sigaction's ucontext_t with incorrect stack reference when SA

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Date:	Mon, 22 Jan 2007 09:57:12 +0100
From:	Xavier Roche <roche+kml2@...lead.com>
To:	linux-kernel@...r.kernel.org
Subject: sigaction's ucontext_t with incorrect stack reference when SA_SIGINFO
 is being used ?

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.

View attachment "stacktest.c" of type "text/x-csrc" (2145 bytes)