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SubjectRe: [PATCH] Linux Kernel Markers
* Martin Bligh (mbligh@google.com) wrote:
> Mathieu Desnoyers wrote:
> >* Martin Bligh (mbligh@google.com) wrote:
> >
> >jump. I think it would be doable to overwrite a 5+ bytes instruction with
> >a NOP
> >non-atomically in all cases, but as the instructions not in the prologue
> >seems to
> >be smaller :
> >
> >prologue on x86
> > 0: 55 push %ebp
> > 1: 89 e5 mov %esp,%ebp
> >epilogue on x86
> > 3: 5d pop %ebp
> > 4: c3 ret
> >
> >Then is can be a problem. Ideas are welcome.
>
> Ugh, yes that's somewhat problematic. It does seem rather unlikely that
> there's a function call in the function prologue when we're busy
> offloading stuff onto the stack, but still ...
>
A function call is not the cause of the problem : an interrupt/trap is.

> For the cases where we're prepared to overwrite the call instruction in
> the caller, rather than insert an extra jump in the callee, can we not
> do that atomically by overwriting the address we're jumping to (the
> call is obviously there already)? Doesn't fix function pointers, etc,
> but might work well for the simple case at least.
>
I don't think we have any guarantee that the function pointer in the call is
aligned, so I guess it would not be an atomic replacement.

Mathieu

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