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From: Will Deacon on 16 Jun 2010 05:40
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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Prev: [PATCH -tip v3 3/5] x86: Introduce text_poke_smp_batch() for batch-code modifying
Next: i2c:i2c_core Fix warning: variable 'dummy' set but not used