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b42c66ed10
the kernel. They are not used atm, but having them in trunk allows them to be easily used when needed. To set a breakpoint that triggers when the variable foo is written to (the most common use case), one calls: breakpoint_set(vir2phys((vir_bytes) &foo), 0, BREAKPOINT_FLAG_MODE_GLOBAL | BREAKPOINT_FLAG_RW_WRITE | BREAKPOINT_FLAG_LEN_4); It can later be disabled using: breakpoint_set(vir2phys((vir_bytes) &foo), 0, BREAKPOINT_FLAG_MODE_OFF); There are some limitations: - There are at most four breakpoints (hardware limit); the index of the breakpoint (0-3) is specified as the second parameter of breakpoint_set. - The breakpoint exception in the kernel is not handled and causes a panic; it would be reasonably easy to change this by inspecing DR6, printing a message, disabling the breakpoint and continuing. However, in my experience even just a panic can be very useful. - Breakpoints can be set only in the part of the address space that is in every page table. It is useful for the kernel, but to use this for user processes would require saving and restoring the debug registers as part of the context switch. Although the CPU provides support for local breakpoints (I implemened this as BREAKPOINT_FLAG_LOCAL) they only work if task switching is used. |
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