Re: [PATCH v4 3.0-rc2-tip 2/22] 2: uprobes: Breakground pagereplacement.

[Peter Zijlstra]

On Mon, 2011-06-13 at 14:29 +0530, Srikar Dronamraju wrote:
> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> [2011-06-10 01:03:29]:
> 
> > On Tue, 2011-06-07 at 18:28 +0530, Srikar Dronamraju wrote:
> > > +       vaddr_old = kmap_atomic(old_page, KM_USER0);
> > > +       vaddr_new = kmap_atomic(new_page, KM_USER1);
> > > +
> > > +       memcpy(vaddr_new, vaddr_old, PAGE_SIZE);
> > > +       /* poke the new insn in, ASSUMES we don't cross page boundary */
> > > +       addr = vaddr;
> > > +       vaddr &= ~PAGE_MASK;
> > > +       memcpy(vaddr_new + vaddr, &opcode, uprobe_opcode_sz);
> > > +
> > > +       kunmap_atomic(vaddr_new);
> > > +       kunmap_atomic(vaddr_old); 
> > 
> > 
> > > +       vaddr_new = kmap_atomic(page, KM_USER0);
> > > +       vaddr &= ~PAGE_MASK;
> > > +       memcpy(opcode, vaddr_new + vaddr, uprobe_opcode_sz);
> > > +       kunmap_atomic(vaddr_new);
> > > 
> 
> 
> > 
> > Both sequences in resp {write,read}_opcode() assume the opcode doesn't
> > cross page boundaries but don't in fact have any assertions validating
> > this assumption.
> > 
> 
> read_opcode and write_opcode reads/writes just one breakpoint instruction
> I had the below note just above the write_opcode definition.
> 
> /*
>  * NOTE:
>  * Expect the breakpoint instruction to be the smallest size instruction for
>  * the architecture. If an arch has variable length instruction and the
>  * breakpoint instruction is not of the smallest length instruction
>  * supported by that architecture then we need to modify read_opcode /
>  * write_opcode accordingly. This would never be a problem for archs that
>  * have fixed length instructions.
>  */

Whoever reads comments anyway? :-)

> Do we have archs which have a breakpoint instruction which isnt of the
> smallest instruction size for that arch. If we do have can we change the
> write_opcode/read_opcode while we support that architecture?

Why not put a simple WARN_ON_ONCE() in there that checks the assumption?
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