linux-kernel - Enabling a disabled hw-breakpoint using modify_user_hw

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Date:	Wed, 16 Jun 2010 10:32:35 +0100
From:	Will Deacon <will.deacon@....com>
To:	fweisbec@...il.com
Cc:	linux-kernel@...r.kernel.org
Subject: Enabling a disabled hw-breakpoint using modify_user_hw_breakpoint

Hi Frederic,

In the ARM hw-breakpoint implementation, the ptrace API may create a new
breakpoint with attr->disabled = 1. At a later time, this may be updated
to 0 and modify_user_hw_breakpoint will be called to propagate the
changes.

As part of this callchain, the architecture-specific function
arch_validate_hwbkpt_settings is called with the breakpoint as a
parameter. The attr->disabled field is checked and the corresponding
`enabled' field in the ARM control register struct is set accordingly.

When arch_install_hw_breakpoint is called later on, the actual control
register is programmed with the values set in the struct. The problem is
that modify_user_hw_breakpoint doesn't update the bp->attr.disabled
field before calling the validate function:

	if (attr->disabled)
		goto end;

	/* bp->attr.disabled still has the old value */
	err = validate_hw_breakpoint(bp);
	if (!err)
		perf_event_enable(bp);

	if (err) {
		bp->attr.bp_addr = old_addr;
		bp->attr.bp_type = old_type;
		bp->attr.bp_len = old_len;
		if (!bp->attr.disabled)
			perf_event_enable(bp);

		return err;
	}

end:
	bp->attr.disabled = attr->disabled;

For the time being, I always write the control register enable bit in
arch_install_hw_breakpoint (because this function shouldn't be called if
the breakpoint is disabled). Is this a sensible workaround, or does the
generic code need changing?

Thanks,

Will

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