Messages in this thread Patch in this message | | | Date | Sat, 5 Dec 2009 10:06:39 -0500 (EST) | From | Bart Oldeman <> | Subject | Re: [PATCH] x86, vm86: fix preemption bug for int3 breakpoint handlers. |
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Thomas,
On Mon, 25 May 2009, Thomas Gleixner wrote: > On Sun, 24 May 2009, Bart Oldeman wrote: >> --- a/arch/x86/kernel/vm86_32.c >> +++ b/arch/x86/kernel/vm86_32.c >> @@ -551,8 +551,12 @@ cannot_handle: >> int handle_vm86_trap(struct kernel_vm86_regs *regs, long error_code, int >> trapno) >> { >> if (VMPI.is_vm86pus) { >> - if ((trapno == 3) || (trapno == 1)) >> + if ((trapno == 3) || (trapno == 1)) { >> + /* re-enable preemption: return_to_32bit() >> + jumps straight to entry_32.S */ >> + dec_preempt_count(); > > This will break other callers of handle_vm86_trap().
sorry for the late reply, I was just pointed out by someone else about the existence of this bug again. You are right of course. Below is a new patch.
Note that on the linux-2.6-x86.git tree, commit 08d68323d1f0c34452e614263b212ca556dae47f ("hw-breakpoints: modifying generic debug exception to use thread-specific debug registers") broke vm86 debug exceptions as well again. The trouble is that handle_vm86_trap() may jump and change the stack to let the kernel return to 32 bit user space, so the handle_vm86_trap() call itself may not return.
-- Impact: fix kernel bug such as: May 22 16:47:47 localhost kernel: note: dosemu.bin[5281] exited with preempt_count 1
Commit be716615fe596ee117292dc615e95f707fb67fd1 ("x86, vm86: fix preemption bug"), fixed the problem for debug exceptions but not for breakpoints. This change also fixes breakpoints.
Cc: stable@kernel.org Signed-off-by: Bart Oldeman <bartoldeman@users.sourceforge.net> --- arch/x86/kernel/traps.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-)
--- a/arch/x86/kernel/traps.c +++ b/arch/x86/kernel/traps.c @@ -181,9 +181,14 @@ kernel_trap:
#ifdef CONFIG_X86_32 vm86_trap: + /* reenable preemption: handle_vm86_trap() might sleep */ + dec_preempt_count(); if (handle_vm86_trap((struct kernel_vm86_regs *) regs, - error_code, trapnr)) + error_code, trapnr)) { + inc_preempt_count(); goto trap_signal; + } + inc_preempt_count(); return; #endif }
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