Date: Thu, 16 Feb 2006 05:58:15 -0500
From: Chuck Ebbert <76306.1226@compuserve.com>
Subject: i386 singlestep is borken
To: Linus Torvalds <torvalds@osdl.org>
Cc: Andrew Morton <akpm@osdl.org>, linux-kernel <linux-kernel@vger.kernel.org>
Message-ID: <200602160601_MC3-1-B882-BFB6@compuserve.com>
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Playing around with singlestep on i386, I found this:

1. User sets TF and starts to singlestep through code,
   handling SIGTRAPS as they occur.

2. Makes vsyscall; debug trap occurs in kernel mode and TF
   is cleared.  TIF_SINGLESTEP gets set so kernel will remember
   to re-enable TF on return to user.  But when user eflags
   is saved on the stack, TF has already been cleared.

3. When user gets control back, TF is not re-enabled.

4. Even after user clears TF, TIF_SINGLESTEP remains set
   for that thread.

None of this happens when using int80 because the flag
is cleared and re-enabled by the interrupt handler.
TIF_SINGLESTEP never gets set and doesn't need to be.

-- 
Chuck
"Equations are the Devil's sentences."  --Stephen Colbert

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